Analysis of Fluid Input Wells by Shut-In Pressures

The need for a simple and economical way of determining the operating efficiency of input wells has been evident for many years. Considerable time and money is spent each year in the maintenance of injection wells. Often the results obtained by work-over operations and various treatments do not pay back the extra cost. A method is needed to determine which wells can be expected to respond favorably to treatment. The application of build-up analysis to input wells, both water and gas, is helping greatly ill solving the problem. The purpose of this paper is to show how the conventional build-up equations can be applied to fluid input wells, how the results are used to evaluate the wells' input efficiency, and to give examples of field tests. Build-Up Equation The use of pressure build-up curves in evaluating producing wells has been a valuable tool in the oil industry, particularly in the last few years. The mathematical analysis of these pressure vs time curves has been treated by several authors, some of whom are listed in the references. More recently it has been confirmed that these same equations are applicable to water injection wells and also to gas injection wells, with some restrictions. The equations used for this study are given in Fig. 1. Application To Input Wells The application of these build-up analysis equations to input wells can disclose many useful things about the well and the formation. These include the relative amount of damage or improvement existing in the formation immediately at the wellbore. A comparison of these tests before and after workovers will indicate the degree of success accomplished by the well treatment. These analysis also furnish a yardstick to measure the effectiveness of different well completion methods. They provide a guide to use in a selective plugging program as well as determining the extent of formation fractures. When the effective permeability is considered in conjunction with the skin effect, the injection rate, and the injection pressure, a better understanding of the well's performance is usually reached. One requirement for the use of these equations is that enough of the injected fluid must be in the formation to insure that the measured pressure drop occurs in that phase only. Minor adjustments may be necessary in some gas input wells for the compressibility factor as well as the weight of the gas column.