PETE 689 - Underbalanced Drilling, UBDLesson 3Benefits of Underbalanced DrillingRead: UDM - Chapter 3

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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 DrillingReduced 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 RateIn 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 EffectBit 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 DifferentialIn permeable rocks penetration rate is a function of the differential pressure not the absolute pressure.Micro-bit testHarold Vance Department of Petroleum EngineeringDrilling Rate (ft/hr)Overbalanced Differential Pressure (psl)1210864020 1000 2000 3000 4000 5000 Indiana LimestoneConfining Pressure= 6000 psiBit weight= 1000 lbm50 rpm

Gas Drilling Vs. Mud DrillingMudGasHarold Vance Department of Petroleum EngineeringDrilling DaysDepth (feet)0 20 40 60 80 100 120 010002000300040005000600070008000900010000 Drilled With Mud Drilled With Gas

Penetration Rate As A Function Of The Differential Pressure Across The WorkfrontHarold Vance Department of Petroleum Engineering

Penetration Rate in Impermeable RocksBit toothCrack 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 psiHarold Vance Department of Petroleum EngineeringRate of Penetration (ft/hr)Downhole Weight on Bit (lbf)150 125100 75 50 25 00 5000 10000 15000 20000 25000 30000 35000 40000 45000 50000 X Pore Pressure 87 psiO Pore Pressure 508 psi

Harold Vance Department of Petroleum EngineeringBorehole pressure = 1.450 psiDownhole Weight on Bit (lbf)Rate of Penetration (ft/hr)150 125100 75 50 25 0 0 5000 10000 15000 20000 25000 30000 35000 40000 45000 50000 X Pore Pressure 580 psiO Pore Pressure 870 psi+ Pore Pressure 116 psi

Harold Vance Department of Petroleum EngineeringBorehole pressure = 4.800 psiDownhole Weight on Bit (lbf)Rate of Penetration (ft/hr)150 125100 75 50 25 0 0 5000 10000 15000 20000 25000 30000 35000 40000 45000 50000 X Pore Pressure 2320 psi+ Pore Pressure 4930 psi

Normalized Drilling Strength

WOB x RPM ROP Pb WOB x RPM ROP PoDSn=Normalized Drilling Strength Index.WOB=Weight on Bit (lbf).RPM= Rotary speed (rpm).ROP=Rate of penetration (ft/hr).P=Pressure (psia).Subscript bIndicates borehole conditions.Subscript oIndicates atmospheric conditions. DSn=Harold Vance Department of Petroleum Engineering

Influence Of BHP On Normalized Drilling Strength In Hard ShalesA value of 5 means that the penetration rate at an specific BHP will be 1/5 of the penetration rate at atmospheric pressure.Harold Vance Department of Petroleum EngineeringNormalized Rock Drilling Strength, DSnBottomhole Pressure (psi)0 500 1000 1500 2000 2500 3000 3500 4000 4500 5000

Normalized Shale Strength ExampleA 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 ExampleTo 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 ExampleHarold Vance Department of Petroleum EngineeringNormalized Rock Drilling Strength, DSnBottomhole Pressure (psi)1098 7 6 5 4 3 2 1 00 500 1000 1500 2000 2500 3000 3500 4000 4500 5000

Field Example Switching From Air To MudHarold Vance Department of Petroleum EngineeringDepth (feet)DaysSWITCH TO MUD30004000500060007000800090005 10 15 20 25 30 Well 1 Well 2 Well 3DRY AIR

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.

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Effect Of UBD On Cutting Structure Of Roller Cone BitsMechanical 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:Harold Vance Department of Petroleum EngineeringWOB x ROP 60 x RPMW= 2 +

Where

W=work done by the bit (ft/ lbf/ revolution)=torque (ft- lbf)WOB = weight on bit (lbf)ROP = rate of penetration (ft/hr)RPM = revolutions per minute

The Volume Of Rock Excavated Per Revolution Is:Harold Vance Department of Petroleum EngineeringV=volume of rock excavated per revolution (ft 3)db = bit diameter (feet)

The Mechanical Specific Energy Is Give By:Harold Vance Department of Petroleum Engineering

480 x RPM 4WOB d b2 x ROP d b2

MSE=+MSE = mechanical specific energy (psi)

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.Harold Vance Department of Petroleum Engineering

480 x RPM 4WOB d b2 x ROP d b2

MSE=+

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.Harold Vance Department of Petroleum EngineeringWhat Does This Mean?

Reduced Differential StickingFs = 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.

Harold Vance Department of Petroleum Engineering

ExampleContact area is 30 feet long and 0.25 ft wide.Pressure differential is 300 psid.The coefficient of friction is 0.3The 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 CirculationIf 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 EvaluationProduction 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 Formation DamageHarold 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.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 pore-blocking 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.Gravity-induced invasion.Wellbore glazing.Post-drilling damage.Mechanical degradation.Harold Vance Department of Petroleum Engineering

Temporary OverbalanceCan be intentional to:Kill well for trips. Transmit MWD surveys. Log the well. Completion and WO operations.Harold Vance Department of Petroleum Engineering

Temporary OverbalanceCan 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 OverbalanceCan 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 ImbibitionDue 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

Harold Vance Department of Petroleum EngineeringCapillary Pressure (psi)800600500400300200100 0700Spontaneous ImbibitionWetting Phase, , Saturation (%)0 20 40 60 80 100 Countercurrent Imbibition is Possible for Initial Wetting Phase Saurations Between 20 and 47% for the Underbalance Pressure Shown in this Example (200 psi).Zone of Potential Spontaneous ImbibitionExample Underbalance PressureS i = 20%S irr= 40%S c = 47% (Equilibrium)

Gravity-induced InvasionCan occur during UBD in the formation produces from natural fractures or vugs.Harold Vance Department of Petroleum Engineering

Wellbore GlazingUBD 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

Post-drilling DamageDue to:Killing the well for completion.Cementing.Mobilization of fines during production.Liquid coning in gas reservoir.Harold Vance Department of Petroleum Engineering

Mechanical DegradationRock 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.Harold Vance Department of Petroleum Engineering

Earlier ProductionWith 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.Harold Vance Department of Petroleum Engineering

Environmental BenefitsClosed loop systems produce less wasted drilling fluids.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.Harold Vance Department of Petroleum Engineering

Harold Vance Department of Petroleum Engineering