Injection-Well Testing in a Light-Oil Steamflood, Buena Vista Hills Field, California
- Victor M. Ziegler (Chevron Oil Field Research Co.)
- Document ID
- Society of Petroleum Engineers
- SPE Production Engineering
- Publication Date
- November 1990
- Document Type
- Journal Paper
- 394 - 402
- 1990. Society of Petroleum Engineers
- 3 Production and Well Operations, 3.3.1 Production Logging, 1.8 Formation Damage, 5.4.1 Waterflooding, 2.4.3 Sand/Solids Control, 4.1.2 Separation and Treating, 5.1.2 Faults and Fracture Characterisation, 5.6.4 Drillstem/Well Testing, 5.6.3 Pressure Transient Testing, 2.2.2 Perforating, 1.2.3 Rock properties, 5.4.6 Thermal Methods
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Pressure-falloff and step-rate injectivity tests were performed during a light-oil steamflood at the Buena Vista Hills performed during a light-oil steamflood at the Buena Vista Hills field, CA. Data obtained from these tests allowed determination of steam-zone properties and formation fracture pressure at an injection well. Information obtained from an offset temperature-observation well and from a heat balance corroborated the pressure-analysis results and allowed estimation of steam-zone thickness. An effective method for monitoring steamfloods, pressure-transient testing of steam-injection wells may be used pressure-transient testing of steam-injection wells may be used with other surveillance techniques to improve reservoir management.
The recent application of pressure-transient analysis to steam- injection wells provides useful information for evaluating steam- flood projects. A steam well test gives the condition of the near-wellbore region (i.e., stimulated or damaged) and steam-zone properties (e.g., transmissivity and volume). Repeated measurement of properties (e.g., transmissivity and volume). Repeated measurement of these parameters may be used (1) to monitor well status, (2) to assess the need for remedial treatments, (3) to determine the growth rate of the steam zone, and (4) to monitor the thermal efficiency of the steamflood process.
Estimation of steam-zone properties from well-test data is based on Eggenschwiler et al.'s theory, which applies to the simple, composite reservoir model shown in Fig. 1. In the Eggenschwiler et al. method, the reservoir is divided into two regions having highly contrasting fluid mobilities. Region 1, adjacent to the wellbore, represents the steam zone with an extremely high fluid mobility. Region 2 represents that portion of the reservoir that is unaffected by steam injection and contains lower-mobility fluid. Because of the high contrast in fluid mobilities, the boundary between the swept (Region 1) and unswept (Region 2) areas acts as a "no-flow boundary" for a short time. Consequently, the computed pressure response exhibits pseudo-steady-state behavior (i.e., dp/dt=constant) during pseudo-steady-state behavior (i.e., dp/dt=constant) during the no-flow-boundary time period.
The Eggenschwiler et al. theory indicates that the pseudo-steady-state period is bordered by two time intervals where pseudo-steady-state period is bordered by two time intervals where the pressure behavior corresponds to that expected for a well in an infinite system. This behavior is illustrated in Fig. 2 for conditions corresponding to an increase in injection-well pressure (i.e., pressure buildup). The initial infinite-acting period in Fig. 2 is influenced entirely by the steam zone. Steam-zone flow properties can be estimated from conventional, semilog properties can be estimated from conventional, semilog pressure-analysis procedure. The volume of the steam-swept zone pressure-analysis procedure. The volume of the steam-swept zone can be-calculated from the pressure response during the pseudo-steady-state period with the concepts of reservoir-limit pseudo-steady-state period with the concepts of reservoir-limit testing. The final infinite-acting pressure regime is influenced by the properties of the unswept region. Accurate determination of the flow properties of the unswept region may be difficult, however, because of the uncertain effects of variable storage capacity (between Regions 1 and 2) and system boundaries on the pressure response. pressure response. Prior tests on steam injectors were pressure-falloff tests. Pressure-falloff response is analogous to that shown in Fig. 2, Pressure-falloff response is analogous to that shown in Fig. 2, except that the injection-well pressure decreases rather am increases with time. Using Eggenschwiler et al's theory, Walsh et al. presented a detailed procedure for quantitatively interpreting presented a detailed procedure for quantitatively interpreting pressure-falloff tests in steamflood and in-situ combustion pressure-falloff tests in steamflood and in-situ combustion projects. Their procedure is modified in this study by projects. Their procedure is modified in this study by incorporating the steam pseudopressure (or real-gas potential), , in the analysis. Substitution of pseudopressure for pressure should enable greater accuracy in the estimation of steam-zone properties. Fig. 3 plots steam pseudopressure (calculated from properties. Fig. 3 plots steam pseudopressure (calculated from published correlations of steam properties) vs. steam saturation published correlations of steam properties) vs. steam saturation pressure. pressure. Steam confinement within the target reservoir is critical to the success of a steamflood project. Determination of reservoir fracture pressure during steam injection would therefore be useful for establishing the limiting injection pressure. Step-rate injectivity tests historically have been conducted in waterfloods to determine reservoir parting pressure. These tests involve injecting water at a series of increasing rates, with each rate lasting the same length of time. A plot of bottomhole pressure (BHP) vs. injection rate win consist of two straight-line segments. The break in the line indicates formation fracture pressure. For steam injection, the step-rate plot must be modified to account for the gaseous nature of the injected steam. Thus, steam pseudopressure must be plotted vs. vapor injection rate. For pseudopressure must be plotted vs. vapor injection rate. For projects injecting high-quality steam >80 mass%), such as the projects injecting high-quality steam >80 mass%), such as the Buena Vista Hills field trial, the volume of the liquid phase is insignificant ( < 3 vol%) and can be ignored.
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