Effects of Steam/Liquid Counterflow on Pressure-Transient Data From Two-Phase Geothermal Reservoirs

Pressure Transient Analysis for Two-Phase Pressure Transient Analysis for Two-Phase (Water/Steam) Geothermal Reservoirs Abstract A new diffusivity equation for two-phase (water/steam) flow in geothermal reservoirs is derived. The geothermal reservoir may be initially two-phase or may evolve into a two-phase system during production. Solutions of the diffusivity equation for a continuous line source are presented; the solutions imply that the plot of bottomhole pressure vs. loglot (t=time) should be a straight pressure vs. loglot (t=time) should be a straight line. The slope of the straight line is inversely proportional to the total kinematic mobility. proportional to the total kinematic mobility. Comparison of the theory with a limited number of computer-simulated drawdown histories shows excellent agreement. Introduction In petroleum engineering and groundwater hydrology, well tests are conducted routinely to diagnose the well's condition and to estimate formation properties. Well test data may be analyzed to yield quantitative information regarding (1) formation permeability, storativity, and porosity, (2) the presence of barriers and leaky boundaries, (3) the condition of the well (i.e., damaged or stimulated), (4) the presence of major fractures close to the well, and (5) the mean formation pressure. Well testing procedures (and the quality of information obtained) procedures (and the quality of information obtained) depend on the age of the well. During temporary completion, testing involves producing the reservoir using a temporary plumbing system (e.g., drillstem testing), and the estimates obtained for the formation parameters are not very accurate. After completion, parameters are not very accurate. After completion, testing usually is performed in the hydraulic mode. In hydraulic testing, one or more wells are produced at controlled rates, and pressure changes within the producing well itself or nearby observation wells producing well itself or nearby observation wells (interference tests) are monitored.A major concern of well testing is the interpretation of pressure transient data. Much of the existing literature deals with isothermal single-phase (water/oil) and isothermal two-phase (oil with gas in solution, free gas) systems. In general, there is a lack of methodology for analyzing nonisothermal reservoir systems, either single- or two-phase (water/steam). Geothermal reservoirs commonly involve nonisothermal two-phase flow during well testing. This paper presents a theoretical framework for analyzing multiphase pressure transient data in geothermal systems. Two-Phase Flow in Geothermal Systems Consider a fully penetrating well located in an infinite reservoir of thickness h. We neglect any variations in either formation or fluid properties in the vertical direction. (This is a common assumption in pressure transient analysis.) The geothermal system may be two-phase before production or may evolve into a two-phase system as a result of fluid production. In the latter case, a boiling front will production. In the latter case, a boiling front will propagate outward from the wellbore. The boiling propagate outward from the wellbore. The boiling front may be treated as a constant-pressure boundary (p=saturation pressure corresponding to the local reservoir temperature).For the sake of simplicity, consider a reservoir that is initially two-phase everywhere. Furthermore, it is convenient to assume that the pressure (and, hence, temperature) is uniform throughout the system. In radial geometry, the pressure response is governed by the following diffusivity equation (see Appendix for a derivation of Eq. 1). (1) SPEJ P. 206

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Wellbore Effects in the Analysis of Two-Phase Geothermal Well Tests

Society of Petroleum Engineers Journal ◽

10.2118/9922-pa ◽

1982 ◽

Vol 22 (03) ◽

pp. 309-320 ◽

Cited By ~ 3

Author(s):

Constance W. Miller ◽

Sally M. Benson ◽

Michael J. O'Sullivan ◽

Karsten Pruess

Keyword(s):

Water Reservoir ◽

Hot Water ◽

Well Test ◽

Two Phase ◽

Pressure Transient ◽

Analysis Techniques ◽

Wellbore Storage ◽

Storage Effects ◽

Transient Data ◽

Well Tests

Abstract A method of designing and analyzing pressure transient well tests of two-phase (steam/water) reservoirs is given. Wellbore storage is taken into account, and the duration of it is estimated. It is shown that the wellbore flow can dominate the downhole pressure signal completely such that large changes in the downhole pressure that might be expected because of changes in kinematic mobility are not seen. Changes in the flowing enthalpy from the reservoir can interact with the wellbore flow so that a temporary plateau in the downhole transient curve is measured. Application of graphical and nongraphical methods to determine reservoir parameters from drawdown tests is demonstrated. Introduction Pressure transient data analysis is the most common method of obtaining estimates of the in-situ reservoir properties and the wellbore condition. Conventional graphical analysis techniques require that. for a constant flowrate well test in an infinite aquifer, a plot of the downhole pressure vs. log time yields a straight line after wellbore storage effects are over. The slope of that line is inversely proportional to the transmissivity (kh/u) of the reservoir. The extrapolated intercept of this line with the pressure axis at a specified time (1 hour or 1 second depending on the units used) gives the factor 0 Cth(re2), which is used to calculate the skin value of a well. In this study, the effects of a two-phase steam/water mixture in the reservoir and/or the wellbore on pressure transient data have been investigated. There have been a number of attempts to extend conventional testing and analysis techniques to two-phase geothermal reservoirs including drawdown analysis by Garg and Pritchett, Garg, Grant, and Moench and Atkinson. Pressure buildup analysis has been investigated by Sorey et al. To solve the diffusion equation that governs the pressure change in a two-phase reservoir analytically, it is necessary to make a number of simplifying assumptions. One assumption is that the fluid compressibility in the reservoir is initially uniform and remains uniform throughout the test. With this approach, it can be shown that a straight line on a pressure vs. log time plot will be obtained, the slope being inversely proportional to the total kinematic mobility When conducting a field test it is rarely possible to maintain the uniform saturation distribution in the reservoir required for that type of analysis to be applicable. In addition, the very high compressibility of the two-phase fluid creates wellbore storage of very long duration. Since most of the available instrumentation for hot geothermal wells (greater than 200C) can withstand geothermal environments for only limited periods, long-duration wellbore storage further complicates data analysis. Thus numerical simulation techniques must be used to study well tests to determine the best method of testing two-phase reservoirs. This work investigates and defines more thoroughly the well/reservoir system when the reservoir or wellbore is filled with a two-phase fluid. Four examples are considered:a single-phase hot water reservoir connected to a partially two-phase wellbore,a hot water reservoir that becomes two-phase during the test,a two-phase liquid-dominated reservoir, anda two-phase vapor-dominated reservoir. State-of-the-art analysis techniques are applied to pressure transient data after wellbore storage effects have ended. In the first example, a nongraphical method of analysis is discussed, which is applicable at early times when wellbore storage effects still dominate the pressure response. Note that our analysis has been done for a two-phase homogeneous, nonfractured reservoir. Previous studies of well test methods for two-phase reservoirs have been restricted to this case. SPEJ P. 309^

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