Lesson 3 Benefits of UBD - PETE 689 ­ Underbalanced PETE...

Info iconThis preview shows page 1. Sign up to view the full content.

View Full Document Right Arrow Icon
This is the end of the preview. Sign up to access the rest of the document.

Unformatted text preview: PETE 689 ­ Underbalanced PETE 689 ­ Underbalanced Drilling, UBD Lesson 3 Benefits of Underbalanced Drilling Read: UDM ­ Chapter 3 Harold Vance Department of Petroleum Engineering Benefits of Underbalanced Benefits of Underbalanced Drilling Increased penetration rate. Increased bit life. Minimize lost circulation. Improved formation evaluation. Reduced formation damage. Harold Vance Department of Petroleum Engineering Benefits of Underbalanced Benefits of Underbalanced Drilling Reduced probability of differential sticking. Earlier production. Environmental benefits. Improved safety. Increased well productivity. Less need for stimulation treatments. Harold Vance Department of Petroleum Engineering Increased Penetration Rate 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. Harold Vance Department of Petroleum Engineering Chip Hold Down Effect 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. Harold Vance Department of Petroleum Engineering Effect of Pressure Differential Effect of Pressure Differential 12 Indiana Limestone Confining Pressure= 6000 psi 10 Drilling Rate (ft/hr) In permeable permeable rocks rocks penetration rate is a function of the differential pressure not the absolute pressure. Micro­bit test Bit weight= 1000 lbm 50 rpm 8 6 4 2 0 0 1000 2000 3000 4000 5000 Overbalanced Differential Pressure (psl) Harold Vance Department of Petroleum Engineering Drilling Days 0 20 40 60 80 100 120 0 Drilled With Mud 1000 Drilled With Gas 2000 3000 Depth (feet) 4000 Mud 5000 6000 Gas Drilling Gas Drilling Vs. Mud Drilling 7000 8000 Gas 9000 10000 Harold Vance Department of Petroleum Engineering Penetration Rate As A Function Of The Penetration Rate As A Function Of The Differential Pressure Across The Workfront 100 90 For permeable rocks 80 15000 lbm 30000 lbm ROP (ft/hr) 70 60 50 40 30 20 10 0 0 100 200 300 400 500 600 700 800 900 1000 Pressure Drop Through Filter Cake (psi) Harold Vance Department of Petroleum Engineering Bit tooth Penetration Rate in Penetration Rate in Impermeable Rocks 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. Harold Vance Department of Petroleum Engineering Borehole pressure = 440 psi Borehole pressure = 440 psi 150 125 X Pore Pressure … 87 psi O Pore Pressure … 508 psi Rate of Penetration (ft/hr) 100 75 50 25 0 0 5000 10000 15000 20000 25000 30000 35000 40000 45000 50000 Downhole Weight on Bit (lbf) Harold Vance Department of Petroleum Engineering Borehole pressure = 1.450 psi 150 O Pore Pressure … 870 psi + Pore Pressure … 116 psi Rate of Penetration (ft/hr) 125 X Pore Pressure … 580 psi 100 75 50 25 0 0 5000 10000 15000 20000 25000 30000 35000 40000 45000 50000 Downhole Weight on Bit (lbf) Harold Vance Department of Petroleum Engineering Borehole pressure = 4.800 psi 150 + Pore Pressure … 4930 psi 125 Rate of Penetration (ft/hr) X Pore Pressure… 2320 psi 100 75 50 25 0 0 5000 10000 15000 20000 25000 30000 35000 40000 45000 50000 Downhole Weight on Bit (lbf) Harold Vance Department of Petroleum Engineering Normalized Drilling Strength WOB x RPM ROP Pb DS n= WOB x RPM ROP Po DSn = Normalized Drilling Strength Index. WOB = Weight on Bit (lbf). RPM = Rotary speed (rpm). ROP = Rate of penetration (ft/hr). P = Pressure (psia). Subscript b Indicates borehole conditions. Subscript o Indicates atmospheric conditions. Harold Vance Department of Petroleum Engineering Influence Of BHP On Normalized Drilling Strength In Hard Shales Drilling Normalized Rock Drilling Strength, DSn 10 9 8 7 DTM/Jurassic­2 Shale SDM/Welsh Shale Results from Cheatham et al. SDM/Jurassic­2 Shale 6 5 4 3 2 1 0 A value of 5 means that the penetration rate at an specific BHP will be 1/5 of the penetration rate at atmospheric pressure. 0 500 1000 1500 2000 2500 3000 3500 4000 4500 5000 Bottomhole Pressure (psi) Harold Vance Department of Petroleum Engineering Normalized Shale Strength Example 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%. Harold Vance Department of Petroleum Engineering Normalized Shale Strength Normalized Example Example 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. Harold Vance Department of Petroleum Engineering Normalized Shale Strength Normalized Example Example 10 Normalized Rock Drilling Strength, DSn 9 8 7 DTM/Jurassic­2 Shale SDM/Welsh Shale Results from Cheatham et al. SDM/Jurassic­2 Shale 6 5 4 3 2 1 0 0 500 1000 1500 2000 2500 3000 3500 4000 4500 5000 Bottomhole Pressure (psi) Harold Vance Department of Petroleum Engineering Field Example Field Example Switching From Air To Mud 3000 Well 1 Well 2 4000 Well 3 DRY AIR 5000 Depth (feet) SWITCH TO MUD 6000 7000 8000 9000 5 10 15 20 25 30 Days Harold Vance Department of Petroleum Engineering Increased Bit Life??? 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. Harold Vance Department of Petroleum Engineering Effect Of UBD On Cutting 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. Harold Vance Department of Petroleum Engineering The Work Done By The Bit Is: The Work Done By The Bit Is: W= 2 πτ+ WOB x ROP 60 x RPM Where W τ = work done by the bit (ft/ lbf/ revolution) = torque (ft­ lbf) weight on bit (lbf) rate of penetration (ft/hr) revolutions per minute WOB = ROP = RPM = Harold Vance Department of Petroleum Engineering The Volume Of Rock Excavated The Volume Of Rock Excavated Per Revolution Is: V= πd b2 x ROP 240 x RPM V = volume of rock excavated per revolution (ft 3) db = bit diameter (feet) Harold Vance Department of Petroleum Engineering The Mechanical Specific The Mechanical Specific Energy Is Give By: 480 τ x RPM 4WOB + MSE= d 2 x ROP πd 2 b b MSE = mechanical specific energy (psi) Harold Vance Department of Petroleum Engineering What Does This Mean? What Does This Mean? 480 τ x RPM 4WOB + MSE= d 2 x ROP πd 2 b b 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. Harold Vance Department of Petroleum Engineering What Does This Mean? 4. Therefore, cutting structure wear rates 4. (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. Harold Vance Department of Petroleum Engineering Reduced Differential Sticking Reduced Differential Sticking Fs = Ac * ∆ Pµ s *144 sq.in./sq.ft. Fs = Force required to free pipe (lbf) Ac = Contact area (sq. ft) Pressure differential across the mud cake (psid) ∆ P = µ s = Coefficient of friction between the string and the mud cake. Harold Vance Department of Petroleum Engineering Example 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. Harold Vance Department of Petroleum Engineering Minimized Lost Circulation 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. Harold Vance Department of Petroleum Engineering Improved Formation Evaluation 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. Harold Vance Department of Petroleum Engineering Reduces Reduces Formation Damage Harold Vance Department of Petroleum Engineering Formation Damage 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. 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 in the formation. Generation of pore­blocking organic byproducts from bacteria entering the formation from the drilling fluid. Harold Vance Department of Petroleum Engineering Formation Damage Mechanisms During Drilling (Overbalanced) Temporary overbalance. Spontaneous imbibition. Gravity­induced invasion. Wellbore glazing. Post­drilling damage. Mechanical degradation. Harold Vance Department of Petroleum Engineering Temporary Overbalance Temporary Overbalance Can be intentional to: Kill well for trips. Transmit MWD surveys. Log the well. Completion and WO operations. Harold Vance Department of Petroleum Engineering Temporary Overbalance 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. Harold Vance Department of Petroleum Engineering Temporary Overbalance Can be unintentional: Varying pore pressure along the wellbore. Drill string running too fast after a bit is changed. Equipment malfunctions or procedural errors. Harold Vance Department of Petroleum Engineering Spontaneous Imbibition 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. Harold Vance Department of Petroleum Engineering Spontaneous Imbibition 800 Zone of Potential Spontaneous Imbibition 700 600 Capillary Pressure (psi) Countercurrent Imbibition is Possible for Initial Wetting Phase Saurations Between 20 and 47% for the Underbalance Pressure Shown in this Example (200 psi). 500 400 300 Example Underbalance Pressure 200 100 Sα i = 20% Sα c = 47% (Equilibrium S α irr= 40% ) 0 0 20 40 60 80 100 α, Saturation (%) Wetting Phase, Harold Vance Department of Petroleum Engineering Gravity­induced Invasion Gravity­induced Invasion Can occur during UBD in the formation produces from natural fractures or vugs. Harold Vance Department of Petroleum Engineering Wellbore Glazing 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. Harold Vance Department of Petroleum Engineering Post­drilling Damage Post­drilling Damage Due to: Killing the well for completion. Cementing. Mobilization of “fines” during production. Liquid coning in gas reservoir. Harold Vance Department of Petroleum Engineering Mechanical Degradation Mechanical Degradation Rock around the wellbore experiences a concentration of in­situ 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. Harold Vance Department of Petroleum Engineering Earlier Production Earlier Production With the necessary equipment on location during UBD operations, produced fluids can go to sales. Open­hole completions are sometimes performed. If the well is drilled and completed underbalanced, wells from depleated reservoirs will not need swabbing. Harold Vance Department of Petroleum Engineering Environmental Benefits Environmental Benefits Closed loop systems produce less wasted drilling fluids. Harold Vance Department of Petroleum Engineering Less Need for Stimulation Less Need for Stimulation If the formation is not damaged during drilling and completion, stimulation to remove the damage will not be needed. Harold Vance Department of Petroleum Engineering Harold Vance Department of Petroleum Engineering ...
View Full Document

This note was uploaded on 10/30/2011 for the course PETROLEUM 689 taught by Professor Jeromej.schubert during the Fall '11 term at Texas A&M University-Galveston.

Ask a homework question - tutors are online