Are Horizontal Wells Right for Development of Groundwater Resources in the Garber-Wellington Aquifer

• Are Horizontal Wells Right for Development of Groundwater Resources in the Garber-Wellington Aquifer? •

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Horizontal drilling has revolutionized the development and production of oil and gas reserves in the United States. The impact has been so great that the U.S. is now number one in the world in oil reserves, surpassing Saudia Arabia and Russia. With this kind of success, public water providers in central Oklahoma, as well as other stakeholders are interested in adopting the use of horizontal drilling technology to optimize groundwater production from their well fields.

Application of horizontal directional drilling (HDD) technology to successful exploitation and development of groundwater resources presents technical challenges that are tied to geology of the aquifer. In central Oklahoma’s Garber-Wellington bedrock aquifer, depositional environment, spatial distribution of sedimentary facies, grain size and depth of targeted groundwater zones will influence decisions about where to site a horizontal water supply well and development of an appropriate well design. The discussion that follows is intended to provide insight into some of the “geo-technical” challenges that should be addressed before drilling and completing a horizontal well in the Garber-Wellington Aquifer.

(Photo reprinted from Water Well Journal with permission of the National Ground Water Association, copyright 2016).

What is Horizontal Directional Drilling?

When the topic of horizontal drilling arises the first thing most people think of is the oilfield. However, HDD has been employed for a range of environmental and engineering applications since the 1980s. HDD technology affords the drilling, placement and completion of a horizontal wellbore in three-dimensional space below the surface using specialized equipment and steering technology.

Fresh groundwater resources are typically encountered at depths that are much shallower than those of the petroleum reservoirs being developed today. Unlike horizontal drilling in the oilfield, the borehole of a horizontal water supply well enters the subsurface at an angle, because in most cases the depth of the targeted groundwater zone(s) is too shallow to permit the borehole to be taken from vertical to a horizontal attitude.

There are two drilling and completion methods that can be employed for drilling and construction of horizontal water supply wells.

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Continuous Entry-Exit Method The continuous entry-exit, or surface-to-surface method of horizontal drilling is routinely used for installation of utilities and pipelines, but has also been successfully applied to drilling and completion of horizontal groundwater supply wells. With the continuous entry-exit method, the well is drilled and completed in three stages. The first stage consists of drilling a pilot hole (Figure 1). The pilot hole enters the subsurface at an angle and is steered to the specified target depth. The hole is then drilled horizontally for a specified distance before being steered back to the surface.

Figure 1: Illustration showing first phase of the continuous entry-exit drilling method (Graphic reprinted with permission, Ground Effects Directional Drilling, Saginaw, MN).

During the second stage the pilot hole is enlarged using a hole-opener or reamer to increase the diameter of the borehole to permit installation of the well screen and casing. Depending on well diameter, more than one pass may be required to enlarge the borehole to the desired size. The third stage involves setting the screen and casing into the enlarged borehole.

One of the more recent horizontal water supply wells drilled using the continuous entry-exit method was drilled in July 2017 in Lamb County, Texas by Directed Technologies Drilling, Inc. (DTD). The well was drilled and completed in the Ogallala Aquifer in an area where the Ogallala only has about 40 feet of saturated thickness, and depth to the base of the aquifer is around 200 feet. The productive yield from shallow vertical wells in the area reportedly average around 100 gallons per minute (gpm), which is largely due to the limited saturated thickness and shallow depth to the base of the aquifer. To meet the groundwater volumetric needs for DTD’s client, a single horizontal well was drilled as an alternative to drilling multiple vertical wells (Bardsley, 2018).

Total length of the Lamb County well is 2,330 feet, and maximum true vertical depth (TVD) is 205 feet. The screened section of the well was landed at a TVD of 190 feet, just above the contact between the Ogallala formation and underlying Permian rocks. The completed well is 12.0 inches in diameter and was constructed with 500 feet of screen. Natural formation materials were used as the filter pack around the screen (Bardsley, 2018).

Immediately following completion, the well was developed extensively and tested at an initial rate of approximately 1,000 gallons per minute (gpm). The well is currently producing at an average rate of around 700 gpm (Bardsley, 2018), and produces a volume that is approximately equal to that of seven vertical wells.

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Blind Single-Ended Method Blind holes (Figure 2) have one entry point and are drilled with the drill head never resurfacing. Like the continuous entry-exit method, the well is drilled and installed in three phases. In the first phase a pilot hole is advanced downward and steered to the desired horizontal depth and location. In the second phase the drill head is retrieved from the borehole and a specialized reamer is attached to enlarge the borehole.

During the third phase the well screen and casing are pushed into the wellbore to the desired position.

Figure 2: Illustration showing blind single-ended borehole. Installation and completion of a water supply well requires the borehole be reamed to a diameter sufficient to push the well screen and casing into the wellbore (graphic reprinted from National Driller with permission, David Bardsley, Directed Technologies Drilling, Inc.).

Geology of the Garber-Wellington Aquifer When evaluating the feasibility of drilling a horizontal water supply well in the Garber-Wellington Aquifer, it is necessary to understand how geology of the aquifer will impact decisions related to identifying and characterizing target zones, selecting a well location and developing an appropriate well design.

The Garber-Wellington bedrock aquifer (Figure 3) is part of a larger hydrologic unit locally referred to as the Central Oklahoma Aquifer. Garber-Wellington rocks are Permian (Leonardian) age and comprised of a series of stacked, very fine to fine-grained sandstones, occasional minor carbonate conglomerate beds, siltstone and mudstone (shale).

Outcrop and subsurface studies suggest that Garber-Wellington rocks were deposited in a predominantly fluvial-deltaic setting under conditions that were perhaps not too dissimilar from those of the present day Colorado River delta in the Gulf of California (Figure 4). Outcrop and core studies indicate that a variety of interrelated constructive (sediment accumulation) and destructive (erosional) depositional processes were operational during Garber-Wellington time. Observed features have been interpreted to include low relief bars, longitudinal bars, mud and sand-filled channels, point bars, over bank deposits and interdistributary mud deposits. The diversity of sedimentary features have contributed to physical and chemical heterogeneity of the aquifer.

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Figure 3: Map showing the location of the Garber-Wellington Aquifer in central Oklahoma.

Figure 4: Colorado River Delta, Gulf of California, Baja, Mexico. Note the presence of overlapping, prograding delta lobes and meandering and anastomosing stream channels. These features are indicative of both constructive and destructive sedimentary processes. Depositional features observed in outcrop suggest that Garber-Wellington “deltas” may have had similar geometries to those shown above (Photo reprinted with permission, copyright Kevin Ebi).

Garber-Wellington rocks outcrop at the surface along the contact with the overlying Hennessey Group. The contact strikes roughly north-northwest to south-southeast and is located (from north to south) near the centers of Logan, Oklahoma and Cleveland Counties in central Oklahoma. Dip is to the west-southwest toward the Anadarko Basin at a rate of approximately 50 feet per mile. Maximum thickness of the entire GarberWellington sequence is approximately 1,000 feet. The percentage of porous sandstone comprising the entire section ranges between about 15% and 65%, with individual Garber-Wellington sand accumulations ranging from less than 10 feet to more than 120 feet in thickness. The thickest sand accumulations are associated with a diversity of depositional processes that resulted in juxtapositioning and stacking of sandstone bodies where prograding, meandering or anastomosing channel deposits and their associated lithofacies combined. In the subsurface, local erosional truncation or complete removal of sandstone layers, interpreted to be associated with channel and/or delta lobe avulsion during Permian time, also occurred. Owing to these processes, lateral changes in aquifer thickness ranging from a few feet to more than 100 feet across short horizontal distances are common..

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Mapping Garber-Wellington Sandstones The ideal target for placement of the lateral section of a horizontal water supply well in the GarberWellington Aquifer would be in a laterally extensive accumulation of thick porous and permeable sandstone. Such targets may be found along and parallel to the axis of thick channel sand, point bar and channel mouth bar accumulations. Accurate placement of a horizontal wellbore in these sandstone bodies requires they be classified and mapped with a reasonable degree of certainty.

The best opportunities for encountering thick accumulations of sandstone are along a 6-mile to 10-mile wide corridor that trends northwest to southeast beginning just east of the Oklahoma State Capital in Oklahoma City southeastward to Thunderbird reservoir east of Norman, Oklahoma. Along this corridor aquifer thickness ranges between about 800 feet and 1,000 feet, with depths to the base of fresh water ranging between approximately 600 feet and 800 feet. Here thick collections of sandstone are oriented roughly perpendicular to oblique to the trend of the corridor. The aquifer becomes increasing shaley and thins rapidly up-dip to the east and northeast of the corridor trend where depths to the base of fresh groundwater along the eastern margins of Oklahoma and Cleveland Counties range between about 350 feet and 400 feet below the surface. West of the corridor, total thickness of the aquifer remains about the same, but salinity gradually increases toward the Anadarko Basin.

To facilitate identification of multiple drilling targets, geological and hydrochemical studies should be conducted to characterize subsurface conditions and expedite the identification of aquifer “sweet spots” in the GarberWellington groundwater basin. A “sweet spot” is an area where geologic and hydrochemical conditions exhibit an overlap or coincidence of circumstances that are more or less ideal for siting and drilling fresh groundwater supply wells. An ideal Garber-Wellington “sweet spot” can be described as an area where laterally-extensive accumulations of thick, porous and permeable sandstone are present, and where geochemical conditions are known or indicated to be favorable to the production of high quality groundwater (i.e. groundwater quality that meets all primary drinking water regulatory limits).

Higher yielding groundwater zones (i.e. higher transmissivity) are associated with coarser grained lithofacies, such as near the base of channel sand and point bar deposits, or near the top of progradational sandstone bodies such as distributary mouth bars. Finer-grained lithofacies, such as channel margin or interdistributary deposits exhibit lower transmissivities. With respect to water quality, the proportions of coarse-grained to finegrained lithofacies have been shown to be related to the presence of dissolved-phase concentrations of arsenic, chromium, selenium and uranium in the deeper portions of the aquifer where higher pH (> 8.3) conditions exist.

Ideally, subsurface mapping studies begin with development of cross sections to characterize stratigraphic and lithofacies controls on the distribution of porous and permeable strata. For example, in petroleum work studies typically begin with preparation of basin-scale cross sections with later refinement down to sub-regional and local scales. For groundwater work in the Garber-Wellington, efforts should at a minimum begin with development of sub-regional and local-scale cross sections to identify and characterize the spatial distribution of stacked sandstone, siltstone and mudstone lithofacies.

Sandstone, siltstone and mudstone packages representing contemporaneous deposition in a basin are typically bounded by regional to sub-regional time-stratigraphic surfaces. Time-stratigraphic surfaces are typically characterized by regional shales or limestones, or an unconformity surface. Upper and lower time-stratigraphic bounding surfaces are used to constrain, group and map genetically-related strata. Mapping genetically related strata are key to defining aquifer “sweet spots”, because all of the lithofacies contained therein are interrelated (Paxton, 2001).

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A traditional approach to mapping spatial extent and variations in thickness of sandstone bodies is through preparation of sand isopach maps using wireline logs. Preparation of isopach maps requires the geologist have access to a wireline log database and be able to map targeted sand bodies based on recognition of classes of “electrofacies” (Figure 5) and their associated depositional environments. The term “electrofacies” refers to a set of wireline log responses that are used to classify the depositional environment(s) represented in an interval of strata. Classification of “electrofacies” must be done through integration of well logs with core and/or outcrop data.

Figure 5: Five examples of classes of gamma ray log electrofacies associated with deltaic, channel, point bar, crevasse splay, marginal marine and eolian depositional environments (Modified after Cant, 1992).

Development of sand isopach maps is also dependent upon the geologist being able to identify timestratigraphic markers that bound genetically related increments of strata so that targeted sandstone sequences and their corresponding electrofacies can be classified and delineated.

The upper and lower bounding surfaces that define the top and bottom of the Garber-Wellington sequence are relatively easy to identify from geophysical logs. The top of the Garber-Wellington sequence is recognized as the contact between the overlying Hennessey Group (shale) and underlying Garber sandstone. The base of the sequence is easily identified from logs in some areas, but can be challenging in others. For example, on the central Oklahoma platform the base of the Wellington formation is marked by its contact with the underlying Herington limestone. However, west of the platform along the eastern Anadarko Basin shelf, the bottom of the Wellington formation occurs at the base of the Wellington evaporites. Between these two extremes the contact between the base of the Wellington and underlying Permian Chase Group (Wolfcampian) tends to be more gradational.

The contact between the Garber and Wellington formations is not clearly defined and can be erosional or gradational in nature. Consequently, the contact between the two formations is most often inferred. There are a couple of distinguishing characteristics that are helpful for differentiating between Garber and Wellington rocks. Outcrop studies indicate that Garber sandstones are generally coarser-grained than Wellington sandstones. Additionally, study of open-hole resistivity logs indicate that resistivities of Garber mudstones (shales) are slightly higher than those of the Wellington, indicating that particle sizes of Garber mudstones are also larger than those of the Wellington.

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Outcrop studies performed by Gromadski (2004) revealed that massive sandstone accumulations within the Garber-Wellington sequence are often comprised of a variety of lithofacies and thus represent a diversity of depositional environments. While facies-specific sand accumulations are recognizable in outcrop, their identification, classification and delineation in the subsurface using geophysical logs can be problematic. As shown in Figure 6, correlation of log profiles in the Garber-Wellington, even over short distances, can be challenging. The reason for this is that each vertical profile represents a collection of stacked lithofacies, which in turn correspond to a particular electrofacies. To provide some idea of the complexity represented in wireline log profiles, consider the following lithofacies that have been observed in Garber-Wellington outcrops (Paxton, 2009): • Massive sandstone

• Mudstone with paleosol features

• Cross-bedded sandstone

• Shale (mudstone)

• Horizontal planar sandstone

• Carbonate clast conglomerate

• Ripple-laminated sandstone

• Mud flake conglomerate

• Muddy ripple-laminated sandstone

• Ironstone

Outcrop Gamma-ray Response of the Permian Garber-Wellington Aquifer, Central Oklahoma Behind Best Western Edmond Inn, 2700 East 2nd Street, Edmond Oklahoma

Figure 6: Panoramic view of Garber sandstone exposure in Edmond, Oklahoma. The gamma ray profiles shown above were recorded using a handheld spectral gamma ray device at locations along the length of the outcrop and demonstrate the diversity of log signatures, and difficulty correlating sands over short distances. The profile shown on the far left is a composite profile developed from two gamma ray profiles. The end-to-end distance across the outcrop shown above is approximately 175 feet (Photo and profiles reprinted with permission, Stan Paxton, USGS).

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The diversity of lithofacies suggests that development of sand isopach maps may be of limited value in GarberWellington work because log (gamma ray) profiles recorded across thick accumulations of sand represent a collection of stacked lithofacies, rather than just one or two lithofacies. The development of isopach maps is further complicated by the fact that there are an insufficient number of closely-spaced well log (gamma ray) profiles of sufficient resolution to permit identification and tracking of specific electrofacies in the subsurface for distances of much more than several tens of feet.

Observations made by the author as the result of studying outcrops, photos of outcrops and correlating hundreds of wireline logs suggest that Garber-Wellington sediments represent a succession of “stacked depositional packages” (Figure 7), or “delta lobes”. Each depositional package is interpreted to represent a series preserved catastrophic events or sediment pulses that occurred in response to rapid influxes of terrigenous sand, silt and mud. Each depositional package is then comprised of a variety of internal sedimentary features that serve as a record of the internal constructive and destructive depositional processes at work during deposition. Regional studies conducted by the author using wireline logs indicate the entire GarberWellington sequence can be comprised of five to seven stacked depositional packages depending on location.

The absence of one or more depositional packages is interpreted to be associated with delta lobe avulsion and/ or prograding channel erosional processes.

Figure 7: Outcrop of Permian-age Garber sandstone and finer-grained mudstones, siltstones and sandstones. End-to-end distance across the photo is approximately 70 feet. Photo illustrates aquifer heterogeneity that arises in response to rapid vertical and lateral transitions from sand-rich lithofacies to mud-rich lithofacies. Note the contact between the relatively flat-topped thicker sand body, pictured in the lower left portion of the photo (partially covered channel sand?), with the overlying, finer grained channel margin sediments above. This feature is evidence for migration of channel axis on right side of the photo (Photo reprinted with permission, Stan Paxton, USGS).

Recognizing that a variety of depositional processes were at work during Garber-Wellington time, development of sand-shale ratio maps has been found to be a useful method for delineating the spatial distribution of sandrich transmissivity trends across large areas (i.e. one or more townships where 1 township = 36 mi2). Sandshale ratio maps use contours to show the ratio of sand to shale, or percentage of sand within a bounded vertical sequence and are a practical alternative to isopach mapping. In most cases the upper and lower boundaries of each vertical sequence or depositional package appear to be erosional contacts (disconformities) that are best identified through correlation of regional to sub-regional accumulations of mudstone (shale). Using this technique it is possible to generate a sand-shale ratio map for discrete depositional packages. Advantages to the use of this mapping technique are that the effects of heterogeneity have less impact on the delineation of sand-rich depositional trends, and that these trends can be mapped consistently over relatively large areas (i.e. several townships).

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Well yields from wells completed in the Garber-Wellington are largely a function of grain size (Figure 8) and well design. The Garber-Wellington Aquifer is considered to be a “tight aquifer”, owing to the fine-grain nature of the rocks that result in low permeability (hydraulic conductivity and transmissivity). Porosities of sandstone accumulations typically range between 27% and 35%.

Particle size analyses indicate sandstones are very fine grain (0.002 – 0.004 inches) to fine grain (0.004 – 0.007 inches). Sandstone grains are subrounded to round and moderately-well to well-sorted (moderately-poor to poorly-graded). The sandstones are friable and loosely cemented with hematite, and often contain a depositional matrix of clay and silt which degrades the intergranular pore system and impacts aquifer hydraulic conductivity and transmissivity (Paxton, 2009). Hydraulic conductivity values of sandstone layers in the GarberWellington range from about 17.9 gallons per day per square foot (gpd/ft2) to 24.6 gpd/ft2 (Mashburn, et al, 2013). Particle sizes of silty mudstone are less than 0.002 inches in diameter and have an estimated hydraulic conductivity of around 2.5 gpd/ft2 (Mashburn, et al, 2013).

Figure 8: Particle size distribution of Garber-Wellington sediments (Source: Gromadski, 2004).

Well Completion Methods To select specific water-bearing sandstone intervals for completion in a water supply well an exploratory boring (“test hole”) is typically drilled to expedite collection of water chemistry and grain size data. When drilling is finished wireline logging surveys consisting of induction resistivity, gamma-ray, and spontaneous potential profiles are typically recorded in the open borehole to identify the depth and thickness of water-bearing sandstone intervals and select samples collected from the borehole during drilling operations for submittal to a geotechnical laboratory for grain size analyses. The open-hole resistivity profile is used as a qualitative measure of general water quality conditions (i.e. total dissolved solids or TDS) and aid in identifying zones for packer testing. Groundwater zones exhibiting higher formation resistivities in water-bearing sandstone intervals have comparatively lower TDS concentrations than groundwater zones having lower formation resistivities. Lower formations resistivities of water-bearing zones are indicative higher TDS concentrations.

Based on interpretation of wireline logging data, specific sandstone intervals are targeted for the performance of packer testing. During packer testing, individual selected sandstone intervals are isolated to facilitate the collection of “zone-specific” groundwater samples and evaluation of groundwater chemistry. For example, in the Garber-Wellington this practice is used to identify groundwater zones penetrated in the borehole that produce high concentrations of dissolved-phase trace metals, as well as characterize other zone-specific groundwater chemical parameters. Groundwater zones containing dissolved-phase concentrations of arsenic, chromium, selenium, uranium and/or other chemical analytes that exceed drinking water regulatory standards are avoided, while zones exhibiting chemical characteristics that meet regulatory standards are preferentially targeted for completion in the new supply well.

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Gun-Perforated Wells Historically, most of the wells completed in the Garber-Wellington Aquifer have been completed as “gun-perforated wells”. This practice is a carryover from the oilfield that dates back to the 1940s.

Oddly, this practice is still used by engineers and some hydrogeologists for completion of municipal water supply wells.

Gun-perforated wells have low specific capacities (Q/s) that range between 0.4 gallons per minute per foot of drawdown (gpm/ft) and 1.2 gpm/ft.

These wells pump inefficiently because the well loss component of the total drawdown is relatively large. Gun-perforated wells also have problems with sand entering the well, and generally require rehabilitation more often than screened wells due to buildup of scale and iron bacteria caused by cascading water, high entrance velocities and turbulent flow.

Screened and Filter-Packed Wells Vertical screened and filter-packed wells completed in the Garber-Wellington are more efficient and have superior performance, because there is less total drawdown which results in specific capacities that are 3.5 to 4 times that of gun-perforated wells.

Other benefits to completing screened and filterpacked wells are higher well yields, laminar flow, lower pumping costs and fewer well rehabilitation events.

Design of screened and filter-packed wells requires sieve analyses be performed on formation samples collected from groundwater zones penetrated in the “test hole to facilitate selection of the appropriate filter pack and screen aperture (slot size). When designing Garber-Wellington wells, emphasis is placed on selecting a filter pack capable of filtering out the smallest particles of the zones to be screened. The filter pack should be chosen such that it will hold back 70% (D70 of the retained particle size) of the natural formation particles, while the well screen should be capable of holding back 90% or more of the filter pack material (Driscoll, 1986). For the case presented in Figure 9, the water-bearing sandstone intervals were very fine to fine-grain and required selection of a graded U.S. Standard Sieve No. 20 X No. 40 filter pack (dashed line). The screen slot opening size selected for this case had an aperture of 0.015 inches.

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Figure 9: Sieve plots developed from formation samples collected from an exploratory boring (test hole) drilled in the Garber-Wellington indicates water-bearing sands penetrated in the borehole were very fine to fine-grain. Selected filter pack (dashed line) = U.S. Standard Sieves No. 20 X No. 40.

The screen slot opening size selected for this case has aperture of 0.015 inches.

Successful design of a vertical screened and filter-packed Garber-Wellington well requires that screens and filter pack materials only be placed opposite water-bearing sandstone intervals penetrated by the wellbore. This is because the rock units that occur between water-bearing sand units are predominantly comprised of loosely cemented mudstone (shale) and/or siltstone, which have particles sizes less than 0.002 inches in diameter. In the presence of water clay and silt-size rock particles will completely break down and separate, and become suspended in water. If a continuous filter-pack is placed in the annulus, say from the bottom of the well to the top of the upper screened zone, silt and shale-sized particles that become suspended in the groundwater traveling up and down the back side of the casing will pass through the filter pack and screens into the well.

Isolation of mudstone and siltstone intervals that lie between the water-bearing sands is achieved by placing a slurry of bentonite or a blended slurry of bentonite and cement in the annulus across from these intervals using a tremie pipe or injection tube.

Isolation of mud-rich zones penetrated in the borehole provides an additional benefit, as these fine-grained rocks contain higher relative concentrations of iron oxides than do the sandstones, and in turn are richer in adsorbed arsenic, chromium (chromium species III and VI), selenium and uranium. In deeper portions of the aquifer pH levels are higher than in shallower portions, which is a condition that develops as a function of residence time in the aquifer and distance away from the recharge zone. At pH levels above about 8.3 desorption of arsenic, chromium, selenium and uranium from iron oxide surfaces begins to occur, and as a consequence can cause dissolved-phase metal concentrations to exceed drinking water Maximum Contaminant Levels (MCLs).

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Horizontal Well Technical Considerations The goal of good well design is to maximize yield and optimize performance. Design of a horizontal well requires much of the same thought that goes into design of a vertical well. However, special attention must be given to anticipated total dynamic head pressure, estimated well yield, and range of grain sizes associated with the target formation, as each will have an impact on, or be impacted by the following design components: • Well diameter • Depth of the target formation

• Selection and installation of screen and filter pack

• Casing and screen durability

LL Well Diameter The diameter of a well (i.e. diameter of the casing and screen) is dictated by pump size, specifically the horsepower required to move the anticipated volume of water to be produced from the well to a specified height at the surface (total dynamic head). Well diameter will in turn dictate rig size and influence decisions related to durability and selection of casing and screen materials.

Vertical municipal and industrial wells completed in the Garber-Wellington Aquifer are typically equipped with submersible pumps. Yields from vertical Garber-Wellington wells generally range between about 150 gallons per minute (gpm) and 300 gpm, with pump setting depths ranging between roughly 400 feet and 650 feet below the surface. Pump sizes generally range from 20 horsepower (hp) up to 75 hp, with nominal pump and motor diameters ranging between 6 inches and 8 inches. In the event the pump motor must be fitted with a shroud to promote cooling of the motor, an additional 2.0 inches of diameter is added to the diameter of the pump and motor assembly. Thus, well diameters are typically either 10.0 inches or 12.0 inches.

LL Depth of the Target Formation Unlike horizontal drilling in the oil field, there will be insufficient vertical distance between the surface and the target zone to begin drilling a horizontal Garber-Wellington well from a vertical position. For example, in Oklahoma’s STACK an SCOOP resource plays, the target formations are at TVDs ranging from about 6,280 feet to 16,000 feet (Oil and Gas Investor, 2017, and Natural Gas Intel, 2017). At these depths there is ample room to take the wellbore from vertical to horizontal. In most cases TVD of a Garber-Wellington horizontal section will likely be less than 600 feet below the surface, which will require the entry hole begin at an angle.

TVD of the target formation will dictate the radius of curvature and lateral distance required to take the wellbore from its entry point at the surface to a horizontal position in the subsurface. This in turn will impact decisions related to rig size and the selection of casing and screen materials. In the event the continuous entry-exit method is used to drill and complete the well, the setback distance required to take the borehole from the surface to the target depth, drill horizontally, then drill back to the surface will greatly impact the size of the surface location.

Vertical municipal and industrial supply wells drilled in the Garber-Wellington typically target groundwater zones below 200 feet. The reason for this is that in Oklahoma groundwater stored in the upper 200 feet of the subsurface is typically reserved solely for domestic use. Domestic use, as defined by the Oklahoma Water Resources Board (OWRB), refers to groundwater used by individuals and families for household purposes, for irrigating gardens and for watering farm or domestic animals. To minimize interference with this use, groundwater zones within the upper 200 feet are not targeted for completion. This condition places a restriction on the minimum TVD of the lateral section of the well.

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Another point of consideration will be to determine the maximum TVD for placing the lateral section, because in deeper portions of the aquifer arsenic, chromium, selenium and uranium become mobilized once pH levels begin to rise above about 8.3. While there is no specific depth at which pH levels begin to rise above 8.3, in the central and western parts of the aquifer where the Garber-Wellington is thickest, pH can begin to increase above 8.0 at depths approaching 350 feet to 400 feet and continue to increase as TVD increases.

The reservation of “shallow groundwater” for domestic use and mobilization of trace metals in deeper portions of the Garber-Wellington implies that TVD of the lateral section be somewhere between 200 feet and 400 feet. Optimum placement of the lateral will depend on aquifer stratigraphy, as well as aquifer and well hydraulic conditions and whether pumping might induce the flow of groundwater from zones above or below the targeted landing zone.

LL Casing and Screen Durability In horizontal well completions the casing and screen are subjected to greater stresses and increased potential for damage. Designing a horizontal well for maximum strength is a relatively straight forward process, which is largely dictated by casing diameter, material type(s) and grade, and screen selection (Directed Technologies Drilling, 2004). Extremely stiff casing can be installed, but the borehole must be drilled using a smooth, gradual transition from the surface launch angle to the horizontal section. In some cases the transition from the surface to target depth must be longer to facilitate development of a workable bend radius for the pipe. For large diameter wells the borehole will need to be enlarged to a diameter that can accommodate placement of the casing and screen. As a general rule, large diameter boreholes will also require more well development after construction.

Tensile strength of the well screen is also of particular importance, as practice has shown that wire-wrapped, rod-based screens are easily damaged during installation in horizontal wells, with screen separation most often occurring at joint ends where the rods are welded to the coupling. To minimize the potential for damaging the screen during installation, a carrier casing or pipe-based screen can be used. Stiffer pipe-based screen, such as louvered screen can be used, but is typically only suitable for construction of small diameter wells (i.e. 4.0 inches or less). In large diameter wells the heavy gauge metal used in fabrication of louvered screen results in greater stiffness, which actually increases the likelihood of failure caused by buckling during the installation process (Directed Technologies Drilling, 2004).

Compressive forces can become an issue, particularly in blind or “single-ended” completions where the casing and screen must be pushed hundreds or thousands of feet into the wellbore (Directed Technologies Drilling, 2004). The casing and screen will also need to be capable of bending and conforming to the curved portion of the wellbore.

LL Selection and Installation of Screen and Filter Pack Installation of the screen and filter pack in a vertical well is a relatively straight forward process, however installation in a horizontal well poses challenges, as selection of the screen and filter pack must take into account the range of particle sizes that will be encountered along the length of the horizontal screened section of the well. Given heterogeneity of the Garber-Wellington Formation, the likelihood of landing the lateral in a continuous, horizontally extensive, massive sandstone unit alone is improbable. A more likely scenario is that the horizontal section of the well will be landed in a zone having a range of grain sizes resulting from lateral penetration of at least some blend of mudstone (shale), siltstone and sandstone.

Ideally, to prevent entry of silt and clay-sized particles (< 0.0025 inches) into the horizontal section of the well, selection of a fine, well graded filter pack and screen having a small aperture would be necessary. To prevent entry of shale and silt-size particles it would be necessary to install something like a U.S. Sieve No. 40 X No. 70 graded filter pack and screen having a slot size of 0.005 inches. This design would probably be Partners for a Better Quality of Life

• Are Horizontal Wells Right for Development of Groundwater Resources in the Garber-Wellington Aquifer? •

neither practical nor cost effective, as the transmitting capacity of the screen would be so low that it would negate the purpose of the horizontal well. For example, if it were possible to place 2,500 feet of 12.0-inch diameter, 0.005-inch screen surrounded with the appropriate filter pack into a horizontal well, the total transmitting capacity of the screen would only be around 470 gpm. For that kind of result, drilling vertical screened and filter-packed wells would appear to be more practical and cost-effective.

Placement of the filter pack opposite the screen in a horizontal well also poses challenges, as the filter pack would need to be emplaced using a sand injection device or tremie. There would also be the challenge of ensuring the well screen is centralized along its entire length. This would require the use of centralizers, which in a large diameter, steel-cased horizontal well would be prone to collapse under the weight of the pipe. Further, if the well is a blind, single-ended completion, difficulties could arise from the need to push the injection tubes or tremie pipe hundreds of feet, or perhaps more into the annulus to emplace the filter pack. The entire process would be an extremely high risk and difficult operation to perform.

LL Natural Filter Pack Under the right conditions the natural formation may be used as the filter pack. This requires that the screen to be placed in the well be compatible with the range of particle sizes penetrated along the length of the horizontal section of the wellbore. For example, the Lamb County, Texas horizontal well completed in 2017

by Directed Technologies Drilling was completed using the coarse sands and fine gravels of the Ogallala Formation as a natural filter pack.

Given heterogeneity of the Garber-Wellington aquifer and the range of particle sizes that characterize these two formations, use of a natural filter pack does not appear to be a practical design solution, thus warranting selection of an alternative filter pack or well completion design.

LL Pre-Packed Screens Pre-packed well screens, sometimes referred to as a “completed well on a pipe”, come in a variety of pipe materials, including PVC, HDPE and stainless steel, and can be customized to accommodate the needs of the project (Directed Technologies Drilling, 2004). However, pre-packed screens are costly on a per foot basis, and can be more difficult to install due to their weight, inherent stiffness and resistance to bending.

Installation of pre-packed screen is best achieved through the use of carrier casing, as they are not designed to withstand the rigors of horizontal well construction (Directed Technologies Drilling, 2004).

LL Well Development Well development is probably the most neglected component of the well completion process, because a significant amount of time can be required to fully develop the well. The purpose of the development process is to facilitate the extraction of mud filtrate which invaded the formation during drilling, as well as remove drill solids and formation particles that can clog the formation, filter pack and screen. Time spent on development depends on a variety of factors including but not limited to borehole diameter, drilling mud properties, partial or complete losses in circulation during drilling operations, grain size, and well design. It is not at all unusual for the development process to take as long or longer that the time required to drill the well. There is no substitute for good well development, and the process is absolutely critical to ensuring good well performance once the well is constructed. There are a variety of well development methods that are applicable to development of horizontal wells, including jetting, over-pressuring, pumping, surging and swabbing.

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• Are Horizontal Wells Right for Development of Groundwater Resources in the Garber-Wellington Aquifer? •

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The feasibility of drilling and completing horizontal water supply wells in the Garber-Wellington aquifer will require the following questions and considerations be evaluated: LL Is there sufficient subsurface wireline log data available to identify and map water-bearing sandstone targets with reasonable certainty? If not, what method(s) or alternatives will be used to delineate the spatial extent and thickness of candidate drilling targets and how would the alternative delineation method(s) impact total cost of a project?

LL Can the horizontal portion of the well be landed in a laterally continuous, thick section of porous and permeable sandstone with a relatively high degree of confidence?

LL What will be the total measured depth (length) of the wellbore, how much well screen will be emplaced in the well, and how will these conditions impact well design and construction methods?

LL What is the true vertical depth of the target formation, and how will that impact drilling the “curved portion” of the wellbore, size of the well location and selection of casing and screen materials?

LL How much well yield (production) is considered sufficient to justify drilling a horizontal water supply well in the Garber-Wellington Aquifer?

LL What is the feasibility of using the natural formation as the filter pack?

LL Could hydrologic conditions in the aquifer and well hydraulics induce drainage of overlying “domestic use” groundwater or the upward migration of elevated dissolved-phase concentrations of trace metals as a result of pumping horizontal wells?

LL What is the optimum well spacing between horizontal wells to minimize the potential for localized groundwater depletion due to interference drawdown with other horizontal and/or vertical wells?

LL Has a cost-benefit analysis been performed to compare the costs and benefits of drilling horizontal wells versus vertical wells?

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Selected References 1. Bardsley, David, Directed Technologies Drilling, Inc., July 23, 2018, personal communication 2. Cant, D.J., 1992, Subsurface Facies Analysis, In Facies Models: Response to Sea Level Change (Walker, R.G. and James, N.P., editors), Geological Association of Canada, p. 195-218 3. Directed Technologies Drilling, Inc., 2004, Horizontal Environmental Well Design an Installation, http:// horizontaldrill.com/wp-content/uploads/2016/04/DTD_Horizontal_EnvWell_Handbook.pdf , downloaded 8/7/2018 4. Driscoll, F. G., 1986, Groundwater and Wells, Second Edition, Johnson Filtration Systems, Inc., St. Paul, MN 5. Gromadski, G. A., 2004, Outcrop-based gamma-ray characterization of arsenic-bearing lithofacies in the Garber-Wellington Formation, Central Oklahoma Aquifer (COA), Cleveland County, OK, unpublished M.S. thesis, Oklahoma State University, Stillwater, OK 6. Mashburn, S. L., Ryter, D. W., Neel, C. R., Smith, S. J., and Magers, J. S., 2013, Hydrogeology and simulation of ground-water flow in the Central Oklahoma (Garber-Wellington) Aquifer, Oklahoma, 1987 to 2009, and simulation of available water in storage, 2010–2059 (ver. 1.1, April 2018): U.S. Geological Survey Scientific Investigations Report 2013–5219, 92 p.

7. Natural Gas Intel, 2017, Information on the Cana-Woodford Shale, http://www.naturalgasintel.com/ canawoodfordinfo, accessed 8/14/2018 8. Oil and Gas Investor, 2017, Scoop/Stack Activity Highlights: July 2017, https://www.ugcenter.com/scoopstack-activity-highlights-1646066#p=full, accessed 8/14/2018 9. Paxton, Stan, 2015, 135 Years of USGS Activity: Why Data Matter – Perspective on USGS Data Collection, Analysis, Application and Reporting, USGS Oklahoma Water Science Center, PowerPoint presentation 10. Paxton, Stan, 2009, Lithofacies, stratigraphy, and depositional setting of the Garber-Wellington aquifer, central Oklahoma, field trip guide, U.S. Geological Survey 11. Paxton, Stan, 2001, Defining “Sweet Spots” in Sedimentary Basins through Practical Application of Reservoir-quality Technology, unpublished technical paper, abstract http://www.searchanddiscovery. com/abstracts/html/2001/DL/index.htm , accessed 8/27/2018

â&#x20AC;˘ Are Horizontal Wells Right for Development of Groundwater Resources in the Garber-Wellington Aquifer? â&#x20AC;˘

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About CP&Yâ&#x20AC;&#x2122;s Groundwater and Hydrogeology Services Water supply planning and sourcing Reconnaissance studies Hydrogeologic investigations and aquifer characterization studies Siting of water supply wells for optimum production and water quality Well field siting, expansion design, rehabilitation Well field optimization and management Pump/aquifer test design and analysis Exploratory drilling and testing Hydraulic profiling and aquifer hydrostratigraphy Groundwater resource exploitation and development Borehole and suface geophysics Groundwater chemistry Groundwater rights Regulatory permitting Feasibility studies Groundwater salinization studies Groundwater modeling

At CP&Y, we provide municipalities, rural water districts, industrial, and commercial clients with tailored, state-of-the-art, cost-effective solutions for meeting a variety of groundwater, water supply and regulatory challenges.

We have access to a world class collection of subsurface geologic data, including thousands of geophysical logs that were recorded through fresh water-bearing formations by the petroleum industry. In many cases, access to and evaluation of this data allows us to determine the extent and thickness of the aquifer, as well as estimate the productive yield before a well is ever drilled. This capability translates into significant project cost reductions and savings.

Our hydrogeological and engineering staff use an interdisciplinary approach to exploitation and development of groundwater resources that combines hydrogeologic data analysis, hydrologic modeling, and utility operation and maintenance considerations. We routinely conduct site-specific field studies to fully characterize aquifer conditions and design well fields that will provide maximum sustainable groundwater yields at the lowest possible cost.

Our staff also performs hydrogeological investigations for water supply development, to meet regulatory requirements, and for water supply planning. Investigations are used to evaluate the feasibility of developing potential sources, assess groundwater resource sustainability, design source water systems, and evaluate the impact of development on the groundwater system.

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• Are Horizontal Wells Right for Development of Groundwater Resources in the Garber-Wellington Aquifer? •

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About the Author James “Jim” Roberts has 31 years of combined professional geological experience working in the groundwater, environmental and petroleum industries, including supervision, project management, and performance of interdisciplinary geological, hydrogeological and environmental investigations and research with emphasis on groundwater resources. He has extensive regulatory liaison, permitting and compliance experience, as well as knowledge of Oklahoma groundwater law with particular emphasis on acquisition of groundwater rights.

Jim’s experience also includes managing and coordinating groundwater investigations and research, writing technical reports and preparing detailed cost estimates. He is a technicalarea expert in the hydrogeology, geochemistry, occurrence and distribution of trace metals in Oklahoma’s Permian redbeds as well as in the characterization and evaluation of physical, geochemical and hydrologic parameters in alluvial and bedrock aquifers.

Recent Project Experience

James Roberts, PE Senior Hydrogeologist

Delineation of Brackish to Saline Sources of Groundwater for use in Exploration and Production Operations in the STACK, Devon Energy Corporation, Oklahoma Project Lead and Principal Investigator. The project focused on identifying and delineating potential sources of brackish to saline groundwater for Devon Energy Corporation within Devon’s area of operations in the STACK play. Emphasis was placed on identifying and mapping sources of brackish to saline groundwater.

Groundwater Sourcing, City of Duncan, Oklahoma Project Hydrogeologist. This project focused on identification and delineation of groundwater resource options for the City of Duncan in May 2015. At the time of this study, Duncan was under mandatory Stage IV emergency water rationing restrictions.

The need for strict rationing was brought about by the severe drought which, at that time was beginning its fifth year, and because the City’s primary source of water, Waurika Lake, held only about 28% of the reservoir’s normal pool storage volume. A primary project objective was to identify groundwater resource options for developing a drought-proof source of groundwater that could be used to augment Duncan’s surface water supply. The methods employed to evaluate groundwater supply alternatives included the gathering, collection of geological and groundwater hydrological data. The evaluation led to successful delineation of an area where a 3.0 to 7.0 MGD well field can be developed. The prospective area identified is approximately 27,200 surface acres or 42.5 square miles in size.

Well Field Expansion, City of Purcell, Oklahoma Project Lead and Principal Investigator. The City had expressed a desire to become a regional water provider, and therefore requested that a well field evaluation study be completed prior to initiating further development or expansion. The study focused on identification of areas within and outside the limits of the city’s existing well field where aquifer conditions for development of groundwater resources appeared most favorable. This task was accomplished through analysis of existing wireline geophysical logs, published and unpublished literature and evaluation of groundwater hydrologic data. Other considerations focused on the unique geochemical conditions associated with the natural occurrence of trace metals such as arsenic and hexavalent chromium, as well as spatial distribution of porous and permeable sandstone packages.

Well Field Expansion, City of Bristow, Oklahoma Project Hydrogeologist. The City of Bristow Oklahoma needed assistance perfecting existing groundwater rights, identifying areas favorable to acquisition of additional groundwater rights, and identifying areas favorable for drilling and completing up to five new water supply wells. Challenges required that precise identification and delineation of two different bedrock aquifers (Ada-Vamoosa major aquifer and Barnsdall aquifer) be made using existing open-hole wireline logs recorded in oil and gas wells and avoid commingling groundwater from two different aquifers. Additional challenges required identification and characterization of aquifer conditions related to the natural presence of elevated radionuclide concentrations. Gathered, interpreted and correlated petrophysical logs recorded in 123 wells. The majority of the logs used in the Study were recorded in wells drilled for oil and gas. Other logs were recorded in exploratory borings (test holes) drilled by the City or existing Cityowned water supply wells. Successfully identified, characterized and delineated an area comprising several sections of land overlying the Barnsdall aquifer where the city is currently engaged in acquiring groundwater rights and plans to drill three new high capacity, high efficiency water supply wells. A successful confirmation drilling and testing program was completed in the summer of 2015.

Education Bachelor of Science, Geology, University of Oklahoma, 1987 Post Graduate Studies, Groundwater Hydrogeology, Oklahoma State University, 1992

Registrations Licensed Professional Geologist: Texas #12639 Kansas #854

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