The interpretation of pressure data recorded during a well test has been used for many years to evaluate reservoir characteristics. The pressure derivative has been applied as a powerful diagnostic tool in interpretation of pressure tests for a single well with wellbore storage and skin in a homogeneous reservoir. The objective of this study is to illustrate the applications of the pressure derivative plot, a) as a powerful diagnostic tool for reservoir model identification; b) to perform type-curve analysis; and, c) as a stand-alone specialized plot for evaluating basic reservoir parameters for single-well tests. The application of the pressure derivative has significantly made well test interpretation easier to perform. Also, the pressure derivative allows greater accuracy in reservoir flow parameter determination eliminating the need to perform the complementary specialized analysis. The cases from Southern Iraq reservoirs as compared with theoretical and published examples highlight their practical application in this area. This paper illustrates the advantages of the derivative approach, in obtaining a better solution to a problem, in comparison to the more traditional dimensionless pressure change type-curve and semi-log methods.

1. INTRODUCTION

The analysis of pressure data recorded during a well test has traditionally been based upon the determination of straight lines drawn on plots with specific scales (Al-Rbeawi, 2018). A type curve analysis approach was introduced in the petroleum industry by Agarwal et al. (1970) as a valuable tool when used in conjunction with conventional semi-log plots. The type curve is a graphic representation of the theoretical response during a test of an interpretation model that represents the well and the reservoir being tested. For a constant-pressure test, the response is the change in production rate; for a constant-rate test, the response is the change in pressure at the bottom of the well. The type curves are derived from solutions to the flow equations under specific initial and boundary conditions. For the sake of generality, type curves are usually presented in dimensionless terms, such as a dimensionless pressure vs. a dimensionless time. A given interpretation model may yield a single type curve or one or more families of type curve, depending on the complexity of the model. The type curve analysis consists of finding a type curve that "matches" the actual response of the well and the reservoir during the test. The reservoir and well parameters, such as permeability and skin, can then be calculated from the dimensionless parameters defining that type curve. Type curves are advantageous because they may allow test interpretation even when wellbore storage distorts most or all of the test data; in that case, conventional methods fail.

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