Everything you always wanted to know about well test analysis but were afraid to ask Alain C. Gringarten Imperial College London Content • WTA: what, why, how • Challenges • Milestones • Other applications • Conclusion: a very powerful tool - use it or lose it 2/44 What is well test analysis (WTA)? • It is the extraction of information from pressure and rate data measured in a producing well 10000 6000 q BUILD UP 9000 PIPELINE 8000 ) 5000 7000 6000 (psia) (STB/D p WELL 4000 5000 q 4000 DRAWDOWN 3000 Pressure 3000 Oil Rate RESERVOIR 2000 1000 p 2000 0 0 200 400 600 800 1000 1200 1400 1600 1800 2000 2200 2400 2600 2800 Time from the start of production (hrs) 3/44 Pressure History Why do we do well test analysis? • To obtain information on the well • Permeability • Well damage or stimulation (skin effect) • To obtain information on the reservoir • Fluid • Average reservoir pressure • Reservoir heterogeneities • Reservoir hydraulic connectivity • Distances to boundaries 4/44 How do we do well test analysis? • We select a period at constant rate (usually, a build up) 6000 10000 FP4 FP76 8000 5000 6000 4000 4000 3000 2000 Pressure (psia) Pressure Oil Rate (STB/D) Rate Oil 2000 0 0 200 400 600 800 1000 1200 1400 1600 1800 2000 2200 2400 2600 2800 Time from the start of production (hrs) • We plot some function of pressure vs. some function of time • We try to identify flow regimes (radial, linear, spherical,…) • We include these flow regimes into an interpretation model which can reproduce the pressure given the rate (or vice-versa) • We verify that the interpretation model is consistent with all other information (geology, seismic, cores, logs, completion, etc…). 5/44 WTA: the Good • WTA allows to assess well condition and to estimate reservoir parameters • Over the last 50 years, new WTA techniques have been developed which give more and better results, and more confidence in those results • Nowadays, WTA potential contribution to reservoir knowledge has never been greater • Only WTA provides reservoir hydraulic connectivity • Well test analysis is the technique of choice in arbitrations 6/44 WTA: the Bad • Interest and knowledge in WTA seems to be fading • One reason may be that the latest new techniques (deconvolution) are perceived as too complex • This fear of complexity is compounded by WTA being often taught, wrongly, as “basic” and “advanced” which is mistaken for “practical (for everybody)” and “esoteric (for experts only)” • Reservoir engineers tend to believe they know how to interpret well tests as a side benefit of knowing how to do simulation • The Big Crew Change: WTA experts educated by Ramey at Texas A&M and Stanford, or trained in Flopetrol/Schlumberger during the heyday of WTA have retired or are about to retire, with no systematic replacements 7/44 WTA: the Ugly • Buttonology instead of domain expertise: engineers often believe software will do the interpretation for them • Resistance to changes: commercial software vendors are often forced by their clients to include techniques which are obsolete or have been proven wrong over 40 years ago • Pressure from operators: Regular well testing is no longer mandatory in many oil provinces • Short term focus: In Unconventionals, data acquisition in general (and well testing) is considered unnecessary cost • Economic constraints: Formal WTA teams have disappeared in many oil companies following the latest oil price drop 8/44 The challenge “Partially, we (the experts) are to be blamed for the decline of the importance of testing. The SPE testing literature is so much polluted that it is difficult to find relevant papers. Many interpretations (80%) even in published papers are incorrect. And MDT has replaced the Testing fluid sampling.” Expert in WTA, personal communication 9/44 The challenge “The picture actually looks very gloomy. It is not because inexperienced hands ask the same questions about the same old problems as 30 years ago, but because they get the same answers as 30 years ago, even though our industry has advanced tremendously and gotten much better answers and solutions, but this knowledge that resides in repositories and in the heads of the experienced and knowledgeable older experts is not being transmitted effectively.” The Big Crew Change: Knowledge Loss or Management of Change? Robert Mathes http://oilpro.com/post/22284/big-crew-change-knowledge-loss-management-change 10/44 Publication inflation 800 1960 2012 Well Test papers SPE 700 Well Test papers OnePetro 600 Rig Count U.S./20 500 Rig Count Total World/40 Oil price, 2015 $ /Bbl 400 300 1972 200 100 Number of publications with testing”“well 0 1927 1941 1968 1995 2023 1954 1982 2009 11/44 Publication inflation & WTA undersold 104 PowerPoint 47002012 (13 years) Word SPE membership/25 103 265 (9 months) 400 (13 months) SPE membership/50 102 WT/Total SPE (19%) SPE Total 10%30 (1 month) Number of publications 10 7% DL % WTA (~7%) SPE Well Test 1 1941 1995 1954 1927 1982 1968 2009 2023 12/44 Well test interpretation milestones 1960 Interpretation methods Hardware/Completion Mathematical tools DEVELOPERS GROUNDWATER IOC UNIVERSITIES SERVICE ??????? UNIVERSITIES Straight Type Derivatives Single well lines Curve Deconvolution Analysis IKVF Electronic Horizontal Multiple-fraced Multiwell gauges wells horizontal wells Deconvolution MHF MDT Permanent gauges Laplace transform Stehfest Green’s functions 1935 1940 1945 1950 1955 1960 1965 1970 1975 1980 1985 1990 2010 1995 2000 2005 2015 http://www.spe.org/industry/history/oral_archives.php 13/44 Straight line methods IOC Shell, Horner Analysis - Flow Period 4 Gulf Oil Corp, 5100 ARCO… 5000 Straight 4900 Line 4800 p*= 5000 psia Methods 4700 4600 k = 485mD Horner 4500 S = 102 MDH MBH (psia) Pressure 4400 4300 0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000 11000 12000 13000 14000 Superposition Function (STB/D) 1935 1940 1945 1950 1955 1960 1965 1970 1975 1980 1985 1990 2010 1995 2000 2005 2015 Year Horner, D. R.: "Pressure Build-ups in Wells", Proc., Third World Pet. Cong., E. J. Brill, Leiden (1951) II, 503-521. 14/44 Straight line methods IOC Shell, Horner Analysis - Flow Period 76 Gulf Oil Corp, 5100 ARCO… 5000 Straight 4900 Line 4800 FP 4 Methods 4700 Horner 4600 p*= 4953 psia p*= 5000 psia 4500 MDH MBH (psia) Pressure k = 1060 mD k = 485 mD 4400 S = 232 S = 102 4300 0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000 11000 12000 13000 14000 Superposition Function (STB/D) 15 1935 1940 1945 1950 1955 1960 1965 1970 1975 1980 1985 1990 2010 1995 2000 2005 2015 Year 15/44 Log-log pressure analysis GROUNDWATER IOC UNIVERSITIES Theis (1935) , Shell, Texas A&M, Stanford Jacob (1947), Gulf Oil Corp, Henry J. Ramey, et al. Hantush (>1947) ARCO… 5100 8000 Straight Type 5000 7000 Line Curve 4900 6000 Dp 5000 Methods Analysis 4800 4000 4700 Horner 3000 4600 Dt 2000 Pressure (psia) Pressure FP3 FP4 Oil Rate(STB/D) MDH MBH 4500 1000 4400 0 25 26 27 28 Production time (hrs) 19 years 1935 1940 1945 1950 1955 1960 1965 1970 1975 1980 1985 1990 2010 1995 2000 2005 2015 Year Ramey, H. J., Jr.: "Short-Time Well Test Data Interpretation in The Presence of Skin Effect and Wellbore Storage," J. Pet. Tech. ( Jan., 1970) 97. 16/44 Pressure log-log plot 1000 FP4 p (psi) p D FP76 100 INTERPRETATION MODEL Radial flow ? 10 Wellbore storage and Skin Homogeneous behaviour Infinite reservoir 1 0.0001 0.001 0.01 0.1 1 10 100 1000 Elapsed time Dt (hrs) Rate Normalised Pressure Rate Normalised Change 17/44 Wellbore storage and skin type curves 104 2 D 10 50 500 250 100 3 5,000 2,500 1,000 25,000 10,000 50,000 500,000 250,000 100,000 cp cp ft md ft psi 103 , C 10 m kh / 5.6146 t, t, MINUTES ‘’’’’’’’’’’’’’’’’’ D 102 10-1 SHUT-IN TIME, SHUT-IN TIME, 10 Dimensionless Pressure, p DimensionlessPressure, H. J. Ramey, Jr (1970) -2 McKinley (Exxon) 1971 10 102 103 104 105 106 107 108 1 10-4 10-3 10-2 10-1 1 10 102 5.6146 Dp C ft3 day Dimensionless time, t PRESSURE BUILDUP GROUP , D qB RBbl 1 qB 24C p t -1 D Same interpretation model D 10 = D D D t C D D p Different type curves Earlougher and Kersch (Marathon) 1974 10-2 Different results 103 104 105 106 1 t kh Dt md.ft hr D = , . 0.000295 CD m C cp bbl/psi Type curve analysis GROUNDWATER IOC UNIVERSITIES SERVICE Theis (1935) , Shell, Texas A&M, Stanford Flopetrol Jacob (1947), Gulf Oil Corp, Henry J. Ramey, et al. Schlumberger Hantush (>1947) ARCO… Straight Type TC with Line Curve Independent Methods Analysis parameters 102 2S D C e Approximate start of semi-log radial flow D10 30 Horner p 1015 106 10 103 102 MDH MBH 5 1 Approximate 0.5 end of 10-1 10-2 wellbore 10-3 storage 1 19 years 9 years 1979 Dimensionless pressure, Dimensionless 10-1 10-1 1 10 102 103 104 105 Dimensionless time, tD /CD 1935 1940 1945 1950 1955 1960 1965 1970 1975 1979 1985 1990 2010 1995 2000 2005 2015 Year 19/44 SPE 8205, Gringarten, A. C. et al.: "A Comparison between Different Skin and Wellbore Storage Type-Curves for Early-Time Transient Analysis” Pressure type curve matching Elapsed time Dt (hrs) 102 10-3 10-2 10-1 1 10 102 103 103 p (psi) p Approximate start of semi-log radial flow 10103030 D D p FP4 FP76 1015 106 10 103 102 102 5 Approximate 1 0.5 end of 10-1 wellbore 10-2 storage 10-3 2S 1 CD e 10 k = 175 mD C = 0.01 Bbl/psi Dimensionless pressure, Dimensionless S = 32 ? -1 10 1 10-1 1 10 102 103 104 105 Dimensionless time, tD /CD Pressure Rate Normalised Change 20/44 Methodology (workflow) GROUNDWATER IOC UNIVERSITIES SERVICE Theis (1935) , Shell, Texas A&M, Stanford Flopetrol Jacob (1947), Gulf Oil Corp, Henry J.