PETE 689 - Underbalanced Drilling, UBD

1. PETE 689 - Underbalanced Drilling, UBD Lesson 3 Benefits of Underbalanced Drilling Read: UDM - Chapter 3

2. Benefits of Underbalanced Drilling Increased penetration rate. Increased bit life. Minimize lost circulation. Improved formation evaluation. Reduced formation damage.

3. Benefits of Underbalanced Drilling Reduced probability of differential sticking. Earlier production. Environmental benefits. Improved safety. Increased well productivity. Less need for stimulation treatments.

4. Increased Penetration Rate In permeable rocks, a positive differential pressure 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. Chip Hold Down Effect

6. Effect of Pressure Differential In permeable rocks penetration rate is a function of the differential pressure not the absolute pressure.

7. Gas Drilling Vs. Mud Drilling

8. Penetration Rate As A Function Of The Differential Pressure Across The Workfront

9. Penetration Rate in Impermeable Rocks

10. Borehole pressure = 440 psi

13. Normalized Drilling Strength

14. Influence Of BHP On Normalized Drilling Strength In Hard Shales

15. Normalized Shale Strength Example A well drilled with an unweighted (8.5 ppg) mud at a depth of 6000�. BHP ~ 2900 psi. Reducing the effective MW to 7 ppg reduces BHP to 2400 psi. Decreases the drilling strength, i.e., increase the penetration rate by less than 15%.

16. To double the penetration rate the BHP would have to be dropped to ~ 1500 psi. A BHP of 100 psi might be expected if drilling with air and would increase the penetration rate approximately 5 times. Note: This assumes equal WOB and RPM. Normalized Shale Strength Example

17. Normalized Shale Strength Example

18. Field Example Switching From Air To Mud

19. Increased Bit Life??? Increased vibration with air drilling may actually decrease bearing life. Bit may drill fewer rotating hours but drill more footage. The number of bits required to drill an interval will be inversely proportional to the footage drilled by each bit.

20. 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.

21. The Work Done By The Bit Is:

22. The Volume Of Rock Excavated Per Revolution Is:

23. The Mechanical Specific Energy Is Give By:

24. What Does This Mean? Bit torque is not a function of borehole pressures. Penetration rates generally increase with decreasing borehole pressures. MSE are therefore, usually lower at lower borehole pressures.

25. Therefore, cutting structure wear rates (in terms of distance drilled) should be inversely related to the MSE. If the bit has to do less work to remove a given volume of rock, its cutting elements should wear less. A bit should be able to drill more footage, when drilling underbalanced.

26. Reduced Differential Sticking Fs = Ac * DPms *144 sq.in./sq.ft. Fs = Force required to free pipe (lbf) Ac = Contact area (sq. ft) DP = Pressure differential across the mud cake (psid) ms = Coefficient of friction between the string and the mud cake.

27. 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.

28. 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.

29. 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.

30. Reduces Formation Damage

31. 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.

32. Phase trapping or blocking. Adsorption of drilling fluid additives, leading to permeability reductions or changes in wettability. Migration of fines in the formation. Generation of pore-blocking organic byproducts from bacteria entering the formation from the drilling fluid. Formation Damage Mechanisms During Drilling (Overbalanced)

33. Temporary overbalance. Spontaneous imbibition. Gravity-induced invasion. Wellbore glazing. Post-drilling damage. Mechanical degradation. Formation Damage Mechanisms During Drilling (Overbalanced)

34. Temporary Overbalance Can be intentional to: Kill well for trips. Transmit MWD surveys. Log the well. Completion and WO operations.

35. Can be unintentional: Slug flow or liquid holdup causing fluctuations in downhole 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. Temporary Overbalance

36. Can be unintentional: Varying pore pressure along the wellbore. Drill string running too fast after a bit is changed. Equipment malfunctions or procedural errors. Temporary Overbalance

37. 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.

39. Gravity-induced Invasion Can occur during UBD in the formation produces from natural fractures or vugs.

40. 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.

41. Post-drilling Damage Due to: Killing the well for completion. Cementing. Mobilization of �fines� during production. Liquid coning in gas reservoir.

42. 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.

43. 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.

44. Environmental Benefits Closed loop systems produce less wasted drilling fluids.

45. Less Need for Stimulation If the formation is not damaged during drilling and completion, stimulation to remove the damage will not be needed.