Average Reservoir Pressure Estimation of a Layered Commingled Reservoir
- Her-Yuan Chen (New Mexico Inst. of Mining) | Rajagopal Raghavan (Phillips Petroleum Co.) | S.W. Poston (Consultant)
- Document ID
- Society of Petroleum Engineers
- SPE Journal
- Publication Date
- March 1997
- Document Type
- Journal Paper
- 3 - 15
- 1997. Society of Petroleum Engineers
- 5.6.4 Drillstem/Well Testing
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- 557 since 2007
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Late-transient buildup responses at a well that produces a two-layer reservoir with no-crossflow are studied. The well is located at the center of a circle and the outer boundaries are assumed to be impermeable. The initial pressure is identical in each layer. Our emphasis is on the determination of average reservoir pressure. This study provides (i) a complete description of the late-time buildup trace by obtaining analytical expressions that highlight the influence of the variables that affect the pressure trace, (ii) a physical explanation for the value of pressure that corresponds to the level portion of the buildup curve, (iii) an interpretation of average pressure estimate obtained by using the Matthews- Brons-Hazebroek functions for single-layer systems, (iv) a theoretical justification for the application of the Muskat and the Arps-Smith methods to estimate average reservoir pressure, and (v) a direct method to compute average reservoir pressure. Results are extended to multilayer systems.
Wells producing commingled reservoirs are characterized by a long late- transient period. This work presents a rigorous examination of this flow period with a view to estimate average reservoir pressure from buildup tests. For single-layer reservoirs, the buildup trace following shut in is unaffected by the skin factor. Thus, the methods suggested by Matthews, Brons, and Hazebroek and Dietz can be used without regard to skin factor. In commingled (no-crossflow) systems, however, the layer skin factors do affect the pressure buildup trace and thus the Horner false pressure is a function of layer skin factors. This fact makes it difficult to use procedures that readily apply to single-layer systems to calculate average reservoir pressure. Larsen and Prijainbodo have documented the difficulties involved in estimating average reservoir pressures by conventional methods. Considering the myriad difficult issues the are involved, Raghavan noted that, as recommended by Lefkovits et al., the Muskat method, despite the fact that longer buildup times are required, is probably the best avenue for computing average reservoir pressure.
The contributions of this study are as follows: (i) it presents a rigorous examination of the late-transient period and identifies the variables of interest that govern pressure behavior, (ii) it provides a complete description of the shape of the pressure buildup trace following the transient period (or following the semilog straight line should one exist), (iii) it provides a direct method to compute average reservoir pressure, (iv) it explains the consequences of using the single-layer Matthews-Brons- Hazebroek functions (in this regard a number of insights are provided), and (v) it provides a "physical interpretation" for the "leveling-off" of the pressure buildup trace following the initial transient period.
At the outset, two issues of a generic kind should be noted. First, unlike single-layer systems, the Muskat straight line begins rather early in commingled reservoir systems, and second, the use of the term the Muskat method in this paper is not synonymous with the "trial-and-error" procedure associated with the said method. Our method is a "direct" procedure to calculate average reservoir pressure.
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