1. ATMATM PETE 406 UBD ATMATM ATMATMATMATM PETE 406 - Underbalanced Drilling, UBD Lesson 9 Benefits of Underbalanced Drilling UDM - Chapter 3

2. ATMATM PETE 406 UBD ATMATM ATMATMATMATM Harold Vance Department of Petroleum Engineering Benefits of Underbalanced Drilling Increased Penetration Rate Increased Bit Life Reduced Differential Sticking Minimize Lost Circulation Improved Formation Evaluation Reduced Formation Damage

3. ATMATM PETE 406 UBD ATMATM ATMATMATMATM Harold Vance Department of Petroleum Engineering Benefits of Underbalanced Drilling Reduced Probability of Differential Sticking Earlier Production Environmental Benefits Improved Safety Increased Well Productivity Less Need for Stimulation

4. ATMATM PETE 406 UBD ATMATM ATMATMATMATM Harold Vance Department of Petroleum Engineering Increased Penetration Rate In permeable rocks, a positive differential will decrease penetration because –increases the effective confining stress which increases the rocks shear strength Therefore increasing shear stress (by drilling UB) increases penetration rate –and increases the chip hold down effect

5. ATMATM PETE 406 UBD ATMATM ATMATMATMATM Harold Vance Department of Petroleum Engineering Chip hold down effect Bit tooth Crack in the formation As drilling fluid enters the fracture, the pressure differential across the rock fragment decreases, releasing the chip

6. ATMATM PETE 406 UBD ATMATM ATMATMATMATM Harold Vance Department of Petroleum Engineering Effect of Pressure Differential In permeable rocks penetration rate is a function of the differential pressure not the absolute pressure Micro-bit test

7. ATMATM PETE 406 UBD ATMATM ATMATMATMATM Harold Vance Department of Petroleum Engineering Gas drilling vs. mud drilling Mud Gas

8. ATMATM PETE 406 UBD ATMATM ATMATMATMATM Harold Vance Department of Petroleum Engineering Penetration rate as a function of the differential pressure across the workfront For permeable rocks

9. ATMATM PETE 406 UBD ATMATM ATMATMATMATM Harold Vance Department of Petroleum Engineering Penetration rate in impermeable rocks Bit tooth Crack in the formation In impermeable rock, the instantaneous initial pressure in the crack itself is close to zero, i.e. the penetration rate is now a function of absolute wellbore pressure.

10. ATMATM PETE 406 UBD ATMATM ATMATMATMATM Harold Vance Department of Petroleum Engineering Field example switching from air to mud Switch to mud

11. ATMATM PETE 406 UBD ATMATM ATMATMATMATM Harold Vance Department of Petroleum Engineering Increased Bit Life??? Increased vibration with air drilling may actually decrease bearing life Bit may drill fewer rotating hours but drill more footage - fewer bits

12. ATMATM PETE 406 UBD ATMATM ATMATMATMATM Harold Vance Department of Petroleum Engineering Effect of UBD on cutting structure of roller cone bits Mechanical Specific Energy, MSE, is defined as the mechanical work that must be done to excavate a unit volume of rock

13. ATMATM PETE 406 UBD ATMATM ATMATMATMATM Harold Vance Department of Petroleum Engineering The work done by the bit is:

14. ATMATM PETE 406 UBD ATMATM ATMATMATMATM Harold Vance Department of Petroleum Engineering The volume of rock excavated per revolution is:

15. ATMATM PETE 406 UBD ATMATM ATMATMATMATM Harold Vance Department of Petroleum Engineering The mechanical specific energy is give by:

16. ATMATM PETE 406 UBD ATMATM ATMATMATMATM Harold Vance Department of Petroleum Engineering What does this mean? 1. Bit torque is not a function of borehole pressures. 2. Penetration rates generally increase with decreasing borehole pressures. 3. MSE are therefore, usually lower at lower borehole pressures

17. ATMATM PETE 406 UBD ATMATM ATMATMATMATM Harold Vance Department of Petroleum Engineering What does this mean? 4. Therefore, cutting structure wear rates (in terms of distance drilled) should be inversely related to the MSE 5. If the bit has to do less work to remove a given volume of rock, its cutting elements should wear less. 6. A bit should be able to drill more footage, when drilling underbalanced.

18. ATMATM PETE 406 UBD ATMATM ATMATMATMATM Harold Vance Department of Petroleum Engineering Reduced Differential Sticking F s = A c  P  s *144 sq.in./sq.ft. F s = force required to free pipe (lbf) A c  = contact area (sq. ft)  P  = pressure differential across the mud cake (psid)  s = coefficient of friction

19. ATMATM PETE 406 UBD ATMATM ATMATMATMATM Harold Vance Department of Petroleum Engineering Example Contact area is 30 feet long and 0.25 ft wide Pressure differential is 300 psid The coefficient of friction is 0.3 The force to free the pipe (in excess of string weight) is 30 x 0.25 x 300 x 0.3 x 144 = 97,200 lbf Note equation 3.5 in text is incorrect

20. ATMATM PETE 406 UBD ATMATM ATMATMATMATM Harold Vance Department of Petroleum Engineering Minimized Lost Circulation If the pressure in the wellbore is less than the formation pressure in the entire open hole section, lost circulation will not occur.

21. ATMATM PETE 406 UBD ATMATM ATMATMATMATM Harold Vance Department of Petroleum Engineering Improved Formation Evaluation Production rates while drilling UB can be measured with no filtrate invasion occurring No filtrate invasion can mean more accurate LWD measurements.

22. ATMATM PETE 406 UBD ATMATM ATMATMATMATM Reduces formation damage

23. ATMATM PETE 406 UBD ATMATM ATMATMATMATM Harold Vance Department of Petroleum Engineering Formation damage mechanisms during drilling (overbalanced) Scales, sludges or emulsions due to interaction between filtrates and pore fluids Interaction between aqueous mud filtrate and clay particles in the formation Solids invasion

24. ATMATM PETE 406 UBD ATMATM ATMATMATMATM Harold Vance Department of Petroleum Engineering Formation damage mechanisms during drilling (overbalanced): Phase trapping or blocking Adsorption of drilling fluid additives, leading to permeability reductions or changes in wettability Migration of fines Generation of pore-blocking organic byproducts from bacteria entering the formation from the drilling fluid

25. ATMATM PETE 406 UBD ATMATM ATMATMATMATM Harold Vance Department of Petroleum Engineering Formation damage mechanisms during drilling (underbalanced): Temporary overbalance Spontaneous imbibition Gravity-induced invasion Wellbore glazing Post-drilling damage Mechanical degradation

26. ATMATM PETE 406 UBD ATMATM ATMATMATMATM Harold Vance Department of Petroleum Engineering Temporary overbalance Can be intentional to: –kill well for trips, –transmit MWD surveys, –log the well, –completion and WO operations

27. ATMATM PETE 406 UBD ATMATM ATMATMATMATM Harold Vance Department of Petroleum Engineering Temporary overbalance Can be unintentional: –Slug flow or liquid holdup causing fluctuations in annular pressure –High fluid pressures across the face of diamond and TSP bits –Near wellbore production reduces the formation pressure near the face of the wellbore

28. ATMATM PETE 406 UBD ATMATM ATMATMATMATM Harold Vance Department of Petroleum Engineering Temporary overbalance Can be unintentional: –Varying pore pressure along the wellbore –Excessive surge pressures –Equipment malfunctions or procedural errors

29. ATMATM PETE 406 UBD ATMATM ATMATMATMATM Harold Vance Department of Petroleum Engineering Spontaneous Imbibition Due to capillary effects - even if drilling underbalanced The underbalance pressure necessary to prevent water from being drawn from an aqueous drilling fluid into the formation will depend on the initial formation water saturation and the pore sizes

30. ATMATM PETE 406 UBD ATMATM ATMATMATMATM Harold Vance Department of Petroleum Engineering Gravity-induced invasion Can occur during UBD in the formation produces from natural fractures or vugs

31. ATMATM PETE 406 UBD ATMATM ATMATMATMATM Harold Vance Department of Petroleum Engineering Wellbore glazing UBD can result in high wellbore temperatures due to the friction between the rotating drillstring and the borehole wall. This can cause a thin low permeability “glazed” zone

32. ATMATM PETE 406 UBD ATMATM ATMATMATMATM Harold Vance Department of Petroleum Engineering Post-drilling damage Due to: –Killing the well for completion –Cementing –Mobilization of “fines” during production –Liquid coning in gas reservoir

33. ATMATM PETE 406 UBD ATMATM ATMATMATMATM Harold Vance Department of Petroleum Engineering Mechanical degradation Rock around the wellbore experiences a concentration of in-situ stresses due to drilling the well. As the wellbore pressure is lowered, the effective stresses increase, resulting in a decrease in porosity and available flow channels leading to reduced permeability

34. ATMATM PETE 406 UBD ATMATM ATMATMATMATM Harold Vance Department of Petroleum Engineering Earlier Production With the necessary equipment on location during UBD operations, produced fluids can go to sales. Open-hole completions are sometimes performed. If the well is drilled and completed underbalanced, wells from depleated reservoirs will not need swabbing.

35. ATMATM PETE 406 UBD ATMATM ATMATMATMATM Harold Vance Department of Petroleum Engineering Environmental Benefits Closed loop systems produce less wasted drilling fluids

36. ATMATM PETE 406 UBD ATMATM ATMATMATMATM Harold Vance Department of Petroleum Engineering Less Need for Stimulation If the formation is not damaged during drilling and completion, stimulation to remove the damage will not be needed

37. ATMATM PETE 406 UBD ATMATM ATMATMATMATM Harold Vance Department of Petroleum Engineering