Analysis of Pressure Fall-Off Curves Obtained in Water Injection Wells To Determine Injective Capacity and Formation Damage

Abstract

Published in Petroleum Transactions, AIME, Volume 204, 1955, pages 96–102. Abstract The paper presents a practical method of utilizing pressure fall-off data obtained when a water injection well is shut in for determination of:the static reservoir pressure,the potential water injective capacity (md-ft) of the formation, andthe formation damage (skin effect) surrounding the wellbore. In the development of the method, equations are presented which give the theoretical background for predicting the behavior of water injection wells. Certain physical factors which limit the application of the method are discussed. The procedure and the relevant precautions in obtaining the data are discussed. A sample analysis and computation from an actual pressure fall-off curve is illustrated. Finally, general characteristics of pressure fall-off curves obtained in several reservoirs are presented. Introduction Theoretically, in an injection well the flow of water into the formation will continue as long as the sand-face pressure is greater than the reservoir pressure. However, in practice, engineers have observed that upon stopping injection and opening the wellhead to atmospheric pressure, some wells backflow water for time intervals ranging from several seconds to several hours although the sand-face pressure, as calculated from the hydrostatic head, is considerably greater than the static reservoir pressure. Further, field observations indicated that upon shutting in a water input well, the wellhead pressure does not drop off immediately, but a pressure decline occurs lasting from several seconds to several days. These observations make it evident that when an input well is shut in, a residual pressure lingers in the vicinity of the wellbore which is greater than the average reservoir pressure. This residual pressure declines first rapidly, then more slowly, to the static reservoir pressure; therefore, it is called transient back-pressure or pressure fall-off. The first step in investigating the mechanism of pressure fall-off is to establish what general conditions are necessary for its occurrence, and to this end, Dickey and Andresen have presented a qualitative explanation describing the mechanism. They point out that the formation pressure gradient in the vicinity of the well does not disappear immediately when the input well is shut in, as if the system were filled with a totally incompressible fluid, but it is maintained by the expansion of the compressible fluids and by the elasticity of the formation. In such a compressible system the decrease in pressure is possible only to the extent that the water front continues to advance until the pressure is equalized. The principal aim of this paper is three-fold; namely,to present a theoretical quantitative analysis of pressure fall-off data,to present an application of the developed theory to field data, andto discuss the general characteristics of actual pressure fall-off curves.