8 minute read
Falling Into Place
Dr. Alban Duriez, and Dr. Nigel Clegg, Halliburton, USA & UK, explain how azimuthally sensitive electromagnetic LWD tools can be deployed as part of a proactive approach to well placement operations.
omplex geology requires innovative solutions to optimise well placement and maximise production. Deposition of a reservoir unit can result in a complex distribution of productive and nonproductive formations, which can be further deformed by later erosional and tectonic events. The ability to intersect as much productive reservoir as possible requires early identification of formation and fluid boundaries to allow for corrections to the well path while limiting tortuosity. Reactive geosteering logging-while-drilling (LWD) technologies require wells to intersect a unit to identify their lithology. In a simple three-layer case, this can result in steering a well by oscillating between the top and bottom boundaries. In complex cases where formations may pinch out, are subjected to deformation, or show lateral lithological variation, reactive geosteering can result in multiple exits from the target zone and a tortuous well path that will increase drag on the completion assembly.
Azimuthally sensitive electromagnetic (EM) LWD tools are deployed to provide a more proactive approach to the well placement operation. The technology investigates the
resistivity properties of formations away from the wellbore. Combined with advanced processing, the measurements resolve the location of the bed boundaries around the borehole and anticipate trajectory adjustments to remain in the targeted layer. Ultra-deep EM tools, such as the EarthStar® ultra-deep resistivity service, have shown the advantages of this approach in thick reservoirs. However, many wells require a more focused approach that provides early warning of resistivity and can map very thin units to understand multiple layers or thinner target zones in more detail.
With the release of the StrataStarTM deep azimuthal resistivity service, Halliburton has added another technology to its iStarTM intelligent logging and drilling platform. The service expands the portfolio of LWD solutions to precisely place wells where they most benefit the operators to maximise their asset value. The StrataStar service provides an early warning of changes in resistivity and the ability to map thin units, which allows proactive geosteering in difficult environments where a strategic understanding of a well’s position can be refined to maximise reservoir exposure.
Multiple simultaneous measurements
Figure 3. 1D inversion canvas of the complete lateral section showing the distribution of high (red) and low (blue) resistivity 25 ft (± 9 m) above and below the wellbore. Resistivity colour scale is in ohm-metres, and the vertical scale is exaggerated with a horizontal to vertical distance ratio of 10.
The proactive element of deploying an EM tool is the result of EM energy penetrating the formation away from the wellbore. Put simply, this is a function of the distance between transmitter and reciever antenna, the frequency of transmission, and formation resistivities. Higher depths of investigation can be achieved with longer spacings and lower frequencies. Combining a long spacing deep transmitter with shallow spacing transmitters allows the tool to provide both the bigger picture of the reservoir for early warnings and the higher detail to provide a better understanding of the reservoir (Figure 1). The ability to optimise how the formation is investigated for depth and detail makes the service a useful component for well placement operations in complex wells. The service uses accurate deep azimuthal resistivity measurements and an inversion algorithm to provide a fast, real-time visualisation of the geology up to 30 ft (9 m) away from the wellbore (Figure 2). Representing the position of formation and fluid boundaries in this way provides an easy-to-understand picture of the position of the well and simplifies decision making. Its ability to delineate the positions, thicknesses and resistivities of the surrounding rock and fluid layers while drilling allows geosteering experts to precisely
place the borehole in the reservoir, even in very thin and demanding formations. Uninterrupted contact with the productive zone is increased, which simplifies the completion design and leads to higher production.
Evaluating reserves with accuracy
In addition to accurate well placement, understanding the reservoir in detail is vital for evaluating its quality. The StrataStar service provides high-accuracy formation insight, driving advanced petrophysical analyses of a reservoir. The tool provides resistivity at four different spacings, using two frequencies that are all corrected for borehole effect. The service also measures the formation anisotropy, and its dip and azimuth at any well angle through its antenna design. Real-time access to the apparent resistivity and Rv and Rh in a relatively undisturbed environment enables a more accurate calculation of the hydrocarbons in place.
The service uses highly sensitive resistivity antennas mounted on a tool design that enables the deepest single-collar resistivity reading without adding length to the bottom hole assembly. With this approach, the deep azimuthal resistivity service provides better data interpretation, widens the volume investigated, and increases the amount of information collected in real time to precisely steer wells where they will most benefit operators.
The challenges of field deployment in the face of the pandemic
First deployed in North America as a prototype, the service demonstrated its high-resolution mapping capability over a 10 000 ft long section (Figure 3). Its resistivity measurements were also compared with another deep resistivity service in the same run and validated the accuracy and the quality of the data for petrophysical analysis. The complex environment (Figure 3) demonstrates the need for high-accuracy geosteering and geomapping solutions as the target units show considerable variability in thickness and resistivity. It is not possible to track a single boundary for the length of the entire well: units pinch out, are dislocated by faults and tectonic deformation, and overall, the formation shows lateral variability. Attempting optimal well placement in this environment with reactive geosteering methods would be extremely difficult if not impossible. Displaying the resistivity distribution in the simple format of an inversion canvas shows the position of the well as it is being drilled, the position of the units of interest and how the well path could be changed to maintain its position within the desired zone. During well placement, successful operations are driven by good communications. Presenting data in this way provides a depiction of the reservoir that all the stakeholders can understand from the subsurface team to the drillers, allowing for better decisions to be made quickly.
Deep EM LWD tools can be applied to an array of subsurface environments. Provided there is a resistivity contrast to detect, the tools can be deployed to identify it. For example, in Canada the StrataStar service endurance was later tested in
Figure 4. Overlay of the high-definition 1D inversion canvas of the horizontal injector-producer pair. The vertical scale is exaggerated with a horizontal to vertical distance ratio of 20. steam-assisted-gravity drainage wells, where wells are drilled very quickly in an abrasive sand. The service was used on five consecutive wells reliably. In this field, the resistivity mapping sheds a new light on the heavy oil distribution and the locations of natural production barriers. When steam is injected into the formation to mobilise the oil in place, these production barriers can limit the penetration of the steam. Understanding their distribution allows optimisation of the steam injection and production strategy. In this scenario, mapping the units is as critical as changing the well position during drilling. Deep azimuthal resistivity data acquired in a pair of injector–producer wells showed a perfect match of the resistivity maps, delivering a complete picture of the heavy oil distribution in the reservoir (Figure 4). This provides critical information to the operator and is also a valuable validation of new technology. Two parallel wells in which inversions are run independently on overlap provides an opportunity to test the repeatability of the inversion. The results show continuity of the steam barriers across the two inversions when they are superimposed. This allows the operator to adapt the completion design to maximise production and save on steam injection costs.
Preparation is key
With tools capable of working in multiple diverse geological environments, preparation is important to the success of any operation. Extensive pre-well modelling is conducted before the commencement of any operation to both identify how the tool will respond in any given situation and how to use it best. Offset data is used to generate multiple scenarios anticipated during the operation to give operators the confidence in new technology and enable them to become familiar with the data, how it will be displayed, and how best to use it. As a result, well placement decisions become easier, and operations more successful.
Integrated approach to well placement
As these tools are deployed in multiple geographies, the challenges can be significant. The ability to identify early warnings, combined with an understanding of the subsurface geology and the high level of detail in thin units, provides improved reservoir understanding and accelerated educated decisions while drilling. The StrataStar service can be combined with other LWD technologies, such as gamma rays, density/neutron, and pressure in order to data resolve various reservoir complexities.
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