Wellbore stability by Gama Eldien

WellBore Stability

SELF STUDY MATERIAL On well N2_st Agiba company

WELLBORE STABILITY

WellBore Stability INTRODUCTION

“Wellbore stability problems cost industry between $1- 6 Billion per year world-wide”  Mechanical instabilities account for the many of these problems Instabilities result when the stresses around a well exceed the rock strength (often in the formations above the reservoir)  Mitigating these problems requires accurate determination of stress magnitudes and reasonable rock strength 1.1 ed.

Holes drilled in the ground tend to collapse or fracture unless they are support-

1.2 Some rocks are very strong and will support themselves better than weaker rocks. 1.3 In most rocks we drill the forces acting in the formation push the wall of the hole inward. This may, if not stabilised with mud weight cause collapse. 2.1 At least, this stability problem leads to reduced drilling performance and/or stuck pipe. 2.2

At worst, it can lead to total collapse and loss of the hole.

CAUSES 3.1

The causes of wellbore stability problems can be either - MECHANICAL - CHEMICAL - Combination of both

WellBore Stability

MECHANICAL EFFECTS 4.1 By mechanical effects, we mean pressure acting against the hole wall to prevent collapse. 4.2 This ‘mechanical effect’ (see 4.1 above) is given by the mud weight. To be effective it must at least equal the forces that are pushing the formation into the wellbore (Fig 3). 4.3 Mud hydrostatic is used to prevent formation fluids from entering the wellbore, but it is also used to prevent collapse. The pressure required to prevent collapse may be a lot greater than the pressure required to prevent formation fluids flowing into the well. 4.4 The pressure at which formation collapses is called ‘collapse pressure’ or ‘collapse gradient’. 4.5 It is important to keep the mud hydrostatic pressure above this value to prevent such hole collapse problems from occurring.

WellBore Stability

WellBore Stability 4.6 Another mechanical effect is Erosion due to high annular velocity. The mud is being pumped up the hole too fast causing erosion of the hole wall (Fig.4).

4.7 The effect of high annular velocity may not be so important as previously thought but will increase problems where the hole wall has already been weakened 4.8 Mechanical damage can occur due to pipe movement and tools hitting the sides of the hole. 4.9 Pressure surges in the hole due to tripping or circulating effects will gradually cause the wall to weaken. (See Fig. 5) Surges can also occur due to the ECD effect when â&#x20AC;&#x2DC;thickâ&#x20AC;&#x2122; muds are used.

WellBore Stability

Mechanical Effect on Unconsolidated Formation 4.10 This is typically in top hole sections where certain formation cannot support their our weight. The most common unconsolidated formation is sand. (See Fig 6) 4.11

Preventable and remedial action would be to:

4.11.1 Use mud that will quickly create a filter cake. This helps to protect the hole wall. 4.11.2 Use a minimum flow rate that will clean the hole but prevent erosion. You may need to use mud that will clean the hole at lower flow rates (a mud with an improved cutting carrying capacity at lower flow rates). 4.11.3 Minimise number of trips and pressure surges, reaming and backreaming.

WellBore Stability Mechanical effects in competent (consolidated) formation 4.12 2 types of instability i) Compressive failure ii) Formation breakdown 4.13

Compressive failures occur when mud weight is too low resulting in the

hole closing up causing tight hole or brittle formations to break up and fall into the hole. (Fig 7)

WellBore Stability 4.13 Formation breakdown occurs when mud weight is too high resulting in a fracture and losses. (Fig 8.)

4.14 When drilling a well with different rock types, you may have formations that try to collapse into the well and formations that are easily fractured. This can cause problems in deciding on correct mud weight. 4.15 Too high a mud weight may fracture sand or limestone but too low a mud weight may cause some shales or salts to squeeze into the hole. 4.16 The difference between a minimum weight and maximum weight is called the ‘Mud Weight Window’. (Fig 9) 4.17 The wider the window, the easier it is to drill the well. 4.18 The narrower the window, the more difficult it can be to maintain a stable wellbore. 4.19 The length of time the wellbore is open also affects stability. Even stable formations will become unstable over time. Certain formations are known for their instability after a certain period of time. This is common in shales, giving them the name - ‘5 day shale or 10 day shale’. This means they need to be drilled and cased off before a certain number of days (before pressure surges or chemical attack from the mud weakens the formation). Any delays will lead to hole Problems and possible loss of the well. 4.20 Increases in hole angle increase the collapse forces (see fig 10). Higher mud weight is needed. In general the mud weight will need to be raised 0.5 - 1 ppg for every 30 degrees of hole angle in shale formation. This makes the window between collapse gradient and fracture gradient smaller (refer back to Fig 9) Local experience will determine exact requirements.

WellBore Stability

4.21 Preventive and Remedial action

4.21.1 Always check reports and logs from offset well for any evidence of stability problem. 4.21.2 Allow for increase in hole angle by raising mud weight 4.21.3 Are the stresses in the formation acting in one particular direction e.g. east West? This may affect stability between different well drilled from a platform. 4.21.4 Will casing program provide a wide enough ‘ Mud Weight Window’? 4.21.5 Consider using oil-based mud, which reduces chemical attack on shales. 4.21.6 Watch out for cavings, although the prescience of cavings may mean it’s too late. Raising mud weight may help. 4.21.7 Control water phase salinity of the mud when drilling shale 4.21.8 Minimise any pressure surges such as swab/surge and ECD; monitor for torque and drag 4.21.9 Do not reduce mud weight unless absolutely necessary. If mud weight need to be changed, then change slowly. 13

WellBore Stability 5.0 Chemical effects on wellbore stability 5.1 Due to chemical reactions between mud and formations. Occur mainly in shale and salts. 5.2 Reaction due to water. Therefore this can be minimised by using oil-based muds. 5.3 When shale reacts with water they tend to swell and soften causing tight hole, hole enlargement, ledging, balling and caving. See fig 11 5.4 Salt formations are soluble in water. Any mud must therefore be salt saturated. 5.5 Preventive action 5.5.1 Study offset well information 5.5.2 Minimise time that a formation is exposed. 5.5.3 Use oil-based mud if possible. 5.5.4 Match mud salinity to formation salinity. This applies to both water and oil-based muds

WellBore Stability 5.5 Preventive action 5.5.1 Study offset well information 5.5.2 Minimise time that a formation is exposed. 5.5.3 Use oil-based mud if possible. 5.5.4 Match mud salinity to formation salinity. This applies to both water and oil-based muds

5.6 Symptoms and remedial action 5.6.1 Monitor torque and drag condition. High values may mean a lack of mud inhibition or that the mud weight needs increasing. 5.6.2 Monitor quality of shale cuttings. Soft cuttings tell you that the inhibiting qualities of the mud are not correct. The salinity is not properly matched to formation salinity. 5.6.3 Increase in Funnel Viscosity may mean that the mud is breaking down Shale into very fine particles (Low gravity solids - LGS) 5.6.4 Problems running in or pulling out may be due to swelling shale, ledges, build up of cavings or balled bit and stabilizers. 5.6.5 Caliper logs are sometime run to check hole size if problems are suspected. 5.6.6 Appearance of Cavings or a large amount of cutting may indicate an unstable borehole

WellBore Stability Stability assessment

WellBore Stability