GREEN WELL TESTING A REPORT SUBMITTED TO THE DEPARTMENT OF PETROLEUM ENGINEERING OF STANFORD UNIVERSITY IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE By Ashish Dabral June 2003 I certify that I have read this report and that in my opinion it is fully adequate, in scope and in quality, as partial fulfillment of the degree of Master of Science in Petroleum Engineering. __________________________________ Prof. Roland Horne (Principal Advisor) iii Abstract Conventional well testing usually involves production of reservoir fluid at the surface. For newly completed wells, surface equipment to store reservoir fluid may not be present, so the fluid may be discharged and flared. Discharge of such fluids can be an environmental hazard. In this study, testing of wells without surface production is investigated. Three possible testing methods are suggested and compared. Three tests suggested are (1) closed chamber test, (2) controlled reinjection of reservoir fluid, and (3) modification of closed chamber test for longer duration. The radii of investigation for these tests were studied. The modified test and controlled reinjection tests are shown to have the large radius of investigation. A composite reservoir model was used to test the investigation radius. v Acknowledgments I wish to express my sincere thanks to Dr. Roland Horne for his guidance, patience, and encouragement. His insights were invaluable during the research. I would like to express my gratitude to SUPRI-D for funding this research. I must mention the help I received from my friends and colleagues at Stanford. I would especially like to thank Sunderrajan Krishnan. vii Contents Abstract v Acknowledgments vii Contents ix List of Tables xi List of Figures xiii Introduction 1 Types of Green Well Tests 4 2.1 Closed Chamber Test 4 2.2 Controlled reinjection 7 2.3 Choked Closed Chamber test 8 Results of Green Well Tests 11 Applicability and Comparison of Green Well tests 21 Conclusions and Recommendations 31 Nomenclature 32 References 33 A. Generation of data for CCT 35 ix List of Tables Table 1: Properties of example reservoir………………………………………………11 Table 2: Properties obtained from the inverse model (CCT)…………………………..16 Table 3: Properties obtained from the inverse model (choked CCT)…………………..20 . xi List of Figures Figure 1. Mechanical configuration of closed chamber test (CCT)……………………….4 Figure 2. Mechanical configuration of controlled reinjection…………………………….8 Figure 3. Mechanical configuration of choked closed chamber test………………………9 Figure 4. Rate and pressure in high permeability reservoir (CCT)………………………12 Figure 5. Rate and pressure in medium permeability reservoir (CCT)…………………..12 Figure 6. Rate and pressure in low permeability reservoir (CCT)……………………….13 Figure 7. Change in duration of CCT with change in chamber radius…………………..13 Figure 8. Change in duration of CCT with cushion……………………………………...14 Figure 9. Inertial pressure varying with cushion level…………………………………...15 Figure 10. Inverse model CCT in high permeability…………………………………….15 Figure 11. Inverse model CCT in medium permeability………………………………...16 Figure 12. Inverse model CCT in low permeability……………………………………..16 Figure 13. Rate and pressure in high permeability reservoir(Controlled Reinjection)…..17 Figure 14. Rate and pressure in medium permeability reservoir (Controlled reinjection)17 Figure 15. Rate and pressure in low permeability reservoir (Controlled reinjection )…..17 Figure 16. Rate and pressure in high permeability reservoir (Choked CCT)……………18 Figure 17. Rate and pressure in medium permeability reservoir (Choked CCT)………..18 Figure 18. Rate and pressure in low permeability reservoir (Choked CCT)…………….19 Figure 20. Change in duration of test with Choke size…………………………………..19 Figure 21. Inverse Model -Match for high permeability reservoir (Choked CCT)………21 Figure 22. Inverse Model -Match for medium permeability reservoir (Choked CCT)…..21 Figure 23. Inverse Model -Match for low permeability reservoir (Choked CCT)……….21 Figure 24. Two ring composite model…………………………………………………...22 Figure 25. Closed Chamber Test response in high permeability reservoir………………23 Figure 26. Closed Chamber test response with fault at 100 ft.(high permeability)……...23 Figure 27.Choked CCT response in high permeability reservoir………………………...24 Figure 28.Choked CCT response in high permeability reservoir with fault at 750’……..24 xiii Figure 29. Controlled reinjection response with fault at 1000’in high permeability reservoir……...……………………………………………………………25 Figure 30. Derivative plot for Controlled reinjection with fault at 1000’ in high permeability reservoir……...………………………………………………25 Figure 31. Closed Chamber Test response in medium permeability reservoir…….....26 Figure 32. Derivative plot for choked CCT with fault at 150’ in medium permeability reservoir…………..……………………………………………………….26 Figure 33. Choked CCT response in medium permeability reservoir…………..……27 Figure 34. Derivative plot for choked CCT with fault at 300’in medium permeability reservoir……………..…………………………………………………….27 Figure 35. Controlled reinjection response in medium permeability reservoir…..…..27 Figure 36. Derivative plot for controlled reinjection with fault at 150’ in medium permeability reservoir…………..…………………………………………28 Figure 37. CCT response in low permeability reservoir…...……………………...….28 Figure 38. Derivative plot for CCT with fault at 150’ in low permeability reservoir...29 Figure 39.Choked CCT response in low permeability reservoir…………………...…29 Figure 40. Derivative plot for choked CCT with fault at 150’ in low permeability reservoir………………..……………………………………………….29 Figure 41. Controlled Reinjection response in low permeability reservoir………......30 Figure 42. Derivative plot for controlled reinjection with fault at 250’ in low permeability reservoir…………………..……………………………………………….30 Figure 43. Inertial pressure in the three tests………………………………………….31 Figure 44. Frictional pressure in the three tests………………………………..……...31 xiv Chapter 1 Introduction Transient well testing is used to determine physical properties that are required to evaluating reserves and to estimate dynamic flow properties that are used to forecast production behavior from the reservoir. In transient analysis, a disturbance is created in the reservoir and the response of the reservoir is recorded and then matched with a mathematical model to obtain the unknown parameters. An assumption inherent in this process is that the tested reservoir is represented by the selected model. Disturbances in the reservoir can be created by changing wellbore conditions. Different ways of creating disturbances define different well tests. Common examples are the flow rate changes giving rise to drawdown and buildup tests. Mathematical models used to match reservoir responses are usually solutions of the diffusivity equation under specific boundary conditions. Common examples are constant wellhead flow rate or constant pressure boundary conditions. Conventional well testing methods usually involve surface production of fluid or changing rate at the surface. Sometimes surface facilities to store the reservoir fluid is not available and hence the fluid is discharged or flared. This is an environmental concern. The present study, focuses on Green Well Test procedures that do not involve surface production of reservoir fluid. One example is a closed chamber test (CCT). Fluid is produced inside the wellbore until the wellbore pressure equates to the reservoir pressure. Rapid changes in rates give rise to inertial and frictional forces and hence there is a dynamic relationship between the rate and wellbore pressure. Saldana (1983) derived relationships between inertial, frictional, gravitational forces acting on the fluid and 1 wellbore pressure. These relationships were used for the wellbore pressure calculations in the present study. 1.1 Overview of Existing Work Several methods have been proposed in order to achieve the objective of an emission free well test. All the proposed methods satisfy the condition that no fluid reach the wellhead and flow is confined within the wellbore. This has led to three possible methods: 1. Closed chamber test: Alexander (1977) proposed this method. This test was further studied in detail by Saldana (1983), who derived a wellbore equation and coupled that to reservoir equation for infinite acting reservoir model. Advantage of the test is that it is very simple. However the test has several limitations: I) a small radius of investigation, II) non repeatability of the test as it ends when the wellbore is filled with reservoir fluid and III) the well may not be ‘clean’ prior to the test and hence the results may be affected. 2 Harmonic testing: The principle of harmonic testing is to alternate periodically between production and injection sequences. Using the periodic input signal (rate) and susbsequent periodic output (pressure), the data are interpreted with regard to frequency (Rosa, 1991). The test has a larger radius of investigation compared to the closed chamber test. The limitation of the test is that it is difficult to achieve control over rate with downhole equipment. The main issue with harmonic testing is its duration. Performing a harmonic test requires an impractical test time, longer than conventional tests by more than one order of magnitude (Hollaender et al.,2002). 3 Downhole production/reinjection: This test involves simultaneous production from a pay zone and reinjection into other. The test is almost like a conventional well test and can achieve a