A Method for Analyzing Pressures Measured During Drillstem-Test Flow Periods

Abstract

Kohlhaas, Charles A., SPE-AIME, Colorado School of Mines This procedure, previously used in water-well analysis, can be applied to oil wells to determine transmissibility and storage coefficients from commonly available but generally neglected data. Despite some limitations, this method used in conjunction with results of buildup analysis yields an improved reservoir evaluation. A curve-matching procedure makes it suitable for field use. procedure makes it suitable for field use. Introduction The determination of reservoir characteristics from drillstem tests (DST's) generally consists in calculating reservoir transmissibility from the buildup portion of the test after a flow period. This portion of the test after a flow period. This transmissibility can be used to obtain permeability when formation thickness and fluid viscosity are known. Permeability thus calculated is considered to be Permeability thus calculated is considered to be flowing-fluid permeability, free of wellbore effects such as mud damage. These buildups are commonly analyzed with Horner or Miller-Dyes-Hutchinson techniques. In a DST the shut-in and flow controls are at the bottom of the well rather than at the surface as in a conventional production-well test. Wellbore effects such as afterflow and fluid segregation are therefore minimized, providing a great advantage for interpreting DST results. Unless the test interval is sufficiently productive that fluid will reach the surface and flow, productive that fluid will reach the surface and flow, however, analysis of pressure behavior is hampered by a lack of knowledge of production rates during the flow period. Most DST's do not have production to the surface, either because of low formation productivity or because of the limited duration of the productivity or because of the limited duration of the flow period. Odeh and Selig presented a modification of Horner's technique in which corrected flow-rate and well-life terms are used for determining the initial pressure and transmissibility of the formation. The pressure and transmissibility of the formation. The corrections account for flow-rate variations during the flow period and the brevity of the flow period compared with the shut-in period. Regardless of whether flow can be measured on the surface, the flow period is commonly used only to establish a pressure drop from which a buildup can be recorded and analyzed and to permit collection of a fluid sample. Analysis of the flow period is generally restricted to an attempt to determine period is generally restricted to an attempt to determine producing rate. Bottom-hole pressures are recorded producing rate. Bottom-hole pressures are recorded during flow, however, and analysis of flow-period pressure behavior is a valuable supplement to analysis pressure behavior is a valuable supplement to analysis of buildup characteristics. With these data and the method of analysis described below, one has an independent determination of some of the reservoir properties obtained from buildup analysis and can more properties obtained from buildup analysis and can more effectively use DST results for reservoir evaluation. Cooper et al. and Blankennagel described the use of flow-period analysis in water wells and presented examples. Van Poollen and Weber discussed the same technique. Ramey and Agarwal studied the effect of wellbore storage on wellbore loading and unloading rates and discussed DST's as an application of their results. Ramey and Agarwal included skin effect in their solutions and presented charts showing the effect of skin factor on the analysis. Mathematical Solution It is assumed that flow in the reservoir is described with a two-dimensional diffusivity equation in cylindrical coordinates: (1) p. 1278