Well Test Analysis for Wells Producing Layered Reservoirs With Crossflow

Abstract

Abstract The pressure response of a well producing a two-layer reservoir with crossflow is examined. Virtually all studies on the response of a well in multilayered systems with crossflow claim that after a few hours of production these systems behave as if they are single-layer systems. A careful examination of the early-time performance of a well in a reservoir with crossflow indicates that its behavior is remarkably different from that of an equivalent single-layer system and is influenced significantly by the degree of communication. It is important to understand short-time behavior, since the time span of virtually all pressure buildup tests encompasses the duration in which a layered reservoir with crossflow may not behave as if it were a single-layer system. Thus, interpretations of pressure buildup data based on single-layer theory can be erroneous. In this study, we show that the flowing pressure response of a well at early times can be divided into three flow periods. The first period is one in which the reservoir behaves as if it were a stratified (no-crossflow) system. This period is followed by a transitional period. The response of the well during this period depends on the contrast in horizontal permeabilities and on the degree of communication between the layers. During the third period, the reservoir can be described by an equivalent single-layer system. An examination of the time ranges of the various flow periods indicates that, unless tests are designed properly, most of the interpretable pressure buildup data would be measured during the time the well response is influenced by the transitional period. The influence of the skin regions on the well response is examined. The significance of the estimate of the skin factor obtained from a pressure test is discussed. We show that the nature and the magnitude of the skin regions and the size of the reservoir determine the applicability of conventional semilog procedures to systems with interlayer communication.