The Relative Importance of Wellbore Pressure Drop and Formation Damage in Horizontal WellsA. D. Hill and Ding ZhuTexas A&M UniversitySPE 100207Background• Previous studies have identifiedparticular conditions for whichwellbore pressure drop effects areimportant.• In some cases, pressure drop willrestrict, limit, or even eliminatewell flow rate at the surface.• No quantitative general guidelineshave been developed to aid wellperformance predictionBackground• Difference between vertical and horizontal/multilateral wells: complex trajectory ofwellboreObjectives• Present simple, analytical expressionsto determine the relative effects ofwellbore pressure drop and formationdamage on horizontal well inflow.• Use the equations to show howhorizontal wells or laterals can bedesigned so that neither of theseeffects causes significant flowrestrictions.Concept Dpwellaffects flow rate distribution alonga horizontal lateral Dpwelldepends on flow rate Non-uniform flow distribution Higher drawdown at the heel possiblycauses early coningpwfpeApproach Segment wellbore Calculate reservoir flow by an inflowequation Calculate wellbore flow by a wellbore/pipeflow equation Update drawdown at each segmentPwf,ipeFurui et. al. ModelInflow Equation for Steady State Flowzrw2ybLhyzrw2ybLhyDRanibaniwaniorsshIyIrhIkLqBp)1(294.0ln2.141• s formation damage skin• sRpartial penetration skin• Ianianisotropy ratioFrictional pressure drop in horizontal pipeWellbore Pressure DropDgLufpcw22D• u flow velocity• f friction factor• L length of pipe• D diameter of pipeLDRelative Importance of Pressure DropWellbore pressure drop is important only when it is large relative to the reservoir pressure drop (drawdown)Relative Importance of Wellbore Pressure DropDpwDprHfrfNNfppRe4DDRanibaniwaniwHsshIyIrhIDkLN)1(294.0ln42Relative Importance of Wellbore Pressure DropHfrfNNfppRe4DDRanibaniwaniwHsshIyIrhIDkLN)1(294.0ln42DqN4ReRelative Importance of Wellbore Pressure DropDpwDprHfrfNNfppRe4DDImportant parameters:• Reservoir permeability• Formation damage conditions• Wellbore length• Wellbore diameter• Wellbore trajectoryRelative Importance of Wellbore Pressure DropHfrfNNfppRe4DDWellbore pressure drop is high when• Wellbore flow rate is high (high NRe)• Reservoir drawdown is low (high NH)Relative Importance of Wellbore Pressure DropHfrfNNfppRe4DD• We have assumed that we can approximate the wellbore pressure drop by calculating the frictional pressure gradient using the average flow rate (q/2)• Is this good enough?The Comparison of Analytical Modal with Numerical Simulation5970597559805985599059956000600502004006008001000length [ft]Pressure [psi]ouyangYuanno inflowRelative Importance of Wellbore Pressure DropHfrfNNfppRe4DD• High permeability• Small wellbore diameter• Large wellbore lengthExamplekxDprqDpratio50 500 4630 0.0150 1000 9270 0.02100 500 9266 0.04100 1000 18500 0.0811000 50 9270 0.411000 100 18500 0.81500 50 4630 0.1500 100 9270 0.2500 200 18500 0.4Conditions for significant wellbore pressure drop2000 3000 4000 5000100120140160180200well length (ft)k (md)0.06-0.080.04-0.060.02-0.040.00-0.02Formation damage – Is it important? Fig. 4 Effect of formation damage on horizontal well production in an isotropic reservoir. 0.0 0.2 0.4 0.6 0.8 1.0 0.0 0.2 0.4 0.6 0.8 1.0 h / y b Jd/Jo s = 5 s = 10 s = 20Foramtion damage effects – influence of anisotropy Fig. 5 Effect of formation damage on horizontal well production in an anisotropic reservoir. 0.0 0.2 0.4 0.6 0.8 1.0 0.0 0.2 0.4 0.6 0.8 1.0 h / y b Jd/Jo, s = 10, Iani =1 s = 10, Iani = 5 s = 10, Iani = 10• The importance of the wellbore pressure drop in a horizontal well is determined by 3 dimensionless numbers:– The Reynold’s Number for pipe flow – The horizontal well number, which is related to the reservoir pressure drop– The friction factor for pipe flowConclusions• The pressure drop in a horizontal well can be estimated in most cases by assuming a constant pressure gradient along the well, and calculating the pressure gradient using the total flow rate divided by 2Conclusions• Results show that the wellbore pressure drop is important only for long horizontal wells in high permeability formationsConclusionsExample: input dataSegment Average flow rate (b/d)Dp (psi)1 (toe) 1000 0.12 3000 0.43 5000 1.14 7000 2.05 (heel) 9000 3.3ExampleSegment Average flow rate (b/d)Dp (psi)1 (toe) 1000 0.12 3000 0.43 5000 1.14 7000 2.05 (heel) 9000