Well bore Stability & MW issues (1).pptx

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1. Wellbore Instability & MW Issues

2. Wellbore instability problems arise when stresses around wellbore exceed the rock strength Cost: 10% of drilling costs, $500-$1000MM/year to industry Causes: High in situ stress, low rock strength, drilling fluid/shale interaction, incorrect mud weight, surge/swab Consequences: Hole collapse, tight hole, stuck pipe, hole cleaning problems, torque/drag, washouts, lost circulation Solutions: Planning adjustments: Well path, mud system and mud weight, drill string, casing, cementing Operational adjustments: pipe movement and trip speed, wiper trips, mud weight and rheology, ROP, torque/drag monitoring Wellbore Instability Issues

3. Pre-drill Planning As the wells become more challenging and expensive, the importance of good planning has become increasingly critical. A good modern drill plan has more than just formation tops and casing points. It should include faults, fractured zones, areas of high bedding dip or other structural complexity, weak or fissile zones, and other known drilling hazard. This type of information comes mainly from integrating offset-well drilling and logging data with a seismic interpretation of the subsurface. In addition to this description of the qualitative risks, the predrill plan also specifies the MW window.

4. Planning Mission Identify potential drilling problems in well planning stage to wellbore instability. Reduce NPT Deduct costs Reduce risk During Exploration Reduces exploration risk with Fault Leakage Analysis During Drilling Provides more accurate Safe Operating Mud Window Reduces kicks and lost circulation Improves wellbore stability Reduces stuck pipe, sidetracks, washing and reaming Reveals feasibility of Underbalanced Drilling During Production Improves production from Natural Fractures Predicts and manages Sand Production Optimizes Hydraulic Fracturing operation Reduces Casing Shear and Collapse

5. Lost Circulation Fractures that do not propagate accept relatively small quantities of fluid and do this slowly, as the hole advances; they do not generally lead to lost-circulation incidents. Lost circulation occurs when: A significant fracture propagates away from the wellbore When the wellbore intersects a conductive natural fracture When a drilling-induced fracture (figure: image logs) connects with a conductive natural fracture. Lost circulation may be caused due to streaks of extremely high permeability, or vugular porosity, but these are not influenced by geomechanics.

6. Th e concept of hydraulic fracturing tells us that the fracture plane along which fracture or parting is first possible is the one perpendicular to the least principal stress. This is usually the minimum horizontal stress. In an intact rock, the drilling induced fracture growth occurs in two phases: fracture initiation and fracture propagation Lost Circulation

7. F racture initiation occurs when the borehole pressure exceeds the FIP (minimum horizontal stress) and results in formation of small DIFs near the borehole. Fracture propagation occurs when the borehole pressure is maintained above the formation propagation pressure. During the fracture propagation phase, the formed DIF grows significantly in size, and a large amounts of fluid will suddenly be lost. Enlargement of the DIFs result in formation of large fracture networks, which can create lost circulation problems. Fracture Initiation & Propagation

8. U sing strain and acoustic emission measurements, Lau and Gorski (1992) determined the crack initiation, onset of strain localization and peak strengths for each confining pressure in a series of tri-axial tests. The results of these tests are in figure. It is clear from figure that fracturing in laboratory samples is a complex process and that simply measuring the peak stress does not capture this fracturing process. However, it is also clear that we can define the boundaries for this process, i.e. fracture initiation, onset of fracture localization and collapse peak stress. Lab data on FIP & FPP

9. O ne of the core tasks of the geomechanics engineer is to define this MW window. The bulk of the work of defining the MW window is in calculating MWmin. This involves determining the rock strength, pore pressure and stress magnitudes (closely related to fracture gradient) and how these vary with depth. In the predrill planning stage, the MW is based on seismic data and offset- well data. The accuracy of the MW is therefore highly dependent on the quality and applicability of these data. MW Window

10. O n the high side, the MW should not exceed the fracture gradient There are numerous definitions of fracture gradient, but in practice this is the MW at which excessive lost circulation occurs. MW window is bounded on the low side by either the minimum mud pressure required to prevent excessive shear failure of the wellbore wall (MWmin also sometimes called collapse pressure) or the pore pressure of any permeable intervals, whichever of these is higher. MW Window

11. Borehole stability by Drilling mud The drilling mud creates an impermeable barrier (filter cake) between the soil and itself. The bentonite platelets in the mud shingle off against the formation to create the filter-cake which serves as the barrier. Similar barriers are created by any other polymer based slurry where a polymer gel membrane serves the purpose. Once filter cake is formed, a positive pressure must be applied evenly against it in order to stabilize the hole and prevent cave-ins. Hydrostatic pressure is sufficient for this pressure. The positive hydrostatic pressure against the filter cake or polymer gel membrane stabilizes the loose unconsolidated formations.

12. Wh at is the criterion used to establish the minimum safe mud weight? Clearly, it is one that will minimize the risk of complete hole collapse. But additional factors can influence this value, including: The volume of cutting The inclination of the well The position around the well of the breakouts. . Establishing a minimum safe mud weight

13. Th e cuttings volume are important because of hole-cleaning issues. The larger the cuttings volume per unit hole length, more hole cleaning is required. Increasing pumping rate and carrying capacity, or reduced penetration rates, can mitigate the risk associated with excessive cuttings volumes Cuttings Volume

14. What impacts cuttings removal? With Mechanical removal of cuttings from the wellbore, there are many different parameters that work together to clean the hole, in specific: Drill string rotation / RPM Flowrate Fluid Rheology Bottoms Up

15. Drill String Rotation Pipe rotation in high angle ERD wells supports cuttings reduction and hole cleaning. Drill-string rotation helps in reduction of drag and application of weight to the bit. The pipe and cuttings lie on low side of the hole. Without rotation, active flow area is at top of the hole over the pipe and cuttings bed. Regardless of the fluid rheology or flowrate, it is almost impossible to move this cuttings bed without mechanical agitation. Rotation provides desired agitation, pulling the cuttings up into the active flow area with a mechanical and hydraulic action.

16. Rotary Speeds It has been shown that for high angle ERD wellbores, higher volume of cuttings come over shakers for higher RPM used. There are at least two distinct hurdle rotary speeds at which step improvements in cuttings will occur in high angle wellbore. These occur at 100-120 rpm, and at 150-180 rpm. These speeds have proven to be quite consistent for different hole sizes (9-7/8" through to 17-1/2" hole size), drill pipe sizes and mud types. Lower rpm can be used in 8” or smaller hole sizes to effectively clean the hole.

17. Pipe RPM & Hole Cleaning Numbers in table are rule of thumb. Several Operators have experimented with rotary speeds of up to 220rpm, but little benefit has been seen over 180rpm.

18. Flow Rate Like rotary speed, a "hurdle" exists on the low side of the flow rate numbers, and that a point of diminishing returns exists on the high side of the numbers As long as cuttings are coming over the shakers, the hole is being cleaned. Higher hole cleaning "rate" means higher flowrates coupled with ample rotary speed, ie faster cuttings to surface.

19. Circulation / Flow rate & ROP Hole Size Desirable Flow Rate (gpm) Minimum Workable Flow Rate 17½” 900 – 1200 800 gpm, with ROP @ 20 m/hr (65’/hr) 12 ¼” 800 ‐ 1100 650 – 700 gpm, ROP @ 10‐15 m/hr (30‐50’ /hr) 9 7 / 8 ” 700 – 900 500 gpm, ROP @ 10‐20 m/hr (33‐65’ /hr) 8½” 450 – 600 350‐400 gpm, ROP @ 10‐20 m/hr (33‐65’ /hr) Flowrate alone is ineffective unless the pipe is being rotated fast enough to stir the cuttings into the flow regime. Pressure drop at bit nozzle must be optimized with flowrate limitations for the given well.

20. Well Inclination Hole cleaning is easier in vertical wells than in deviated wells, vertical wells can accommodate larger amounts of failure. In near vertical wells (0-45 deg), cutting beds do not form because the annular fluid velocity acts to overcome the cuttings settling force and there is a net upward movement of the cuttings. In high angle wells, AV has little significance as fluid essentially moves above drillpipe where there are no cuttings.

21. Cuttings Transport in high angle wells For 45-65 degree, moving cuttings bed is unstable as cuttings move up the hole mostly on the low side and when the pumps are shut‐off, the cuttings will begin to slide (or avalanche) downhole. For greater than 65 degree, cuttings fall to the low side of the hole and form a long, continuous cuttings bed. The drilling fluid moves above drill-pipe requiring mechanical agitation to move cuttings, regardless of the flowrate or mud viscosity.

22. In deviated wells there is considerable pipe contact with the top and bottom of the well, breakouts in these locations are likely to be more problematic than breakouts on the sides of a vertical hole. If the well needs to be steered, breakouts on its sides may adversely affect directional control. We ordinarily choose the breakout width as the criterion to establish the appropriate minimum mud weight for following reasons: Breakout width is a relatively easy measurement that is directly related to cuttings volume, Breakout depth increases with time Breakouts & Well Collapse

23. Data Typically Used in MW Determination