Well Test Analysis for Wells Producing Layered Reservoirs With Crossflow

Well Test Analysis for Wells Producing Layered Reservoirs Producing Layered Reservoirs With Crossflow Abstract The pressure response of a well producing a two-layer reservoir with crossflow is examined. Virtually all studieson the response of a well in multilayered systems with crossflow claim that after a few hours of production theses ystems 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 allpressure buildup tests encompasses the duration in which pressure buildup tests encompasses the duration in which a layered reservoir with crossflow may not behave as ifit were a single-layer system. Thus, interpretations of pressure buildup data based on single-layer theory can be 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 threeflow 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 onthe contrast in horizontal permeabilities and on the degreeof communication between the layers. During the third period, the reservoir can he described by an equivalent period, the reservoir can he described by an equivalent single-layer system. An examination of the time ranges of the various flowperiods indicates that, unless tests are designed periods indicates that, unless tests are designed properly, most of the interpretable pressure buildup data would 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 responseis examined. The significance of the estimate of the skin factor obtained from a pressure test is discussed. We showthat the nature and the magnitude of the skin regions andthe size of the reservoir determine the applicability of conventional semilog procedures to systems with interlayer communication. Introduction The economic consequences of interlayer crossflow arewell established in the literature. Several studies have examined the well response in a reservoir with interlayer communication. However, most of these studies have been concerned primarily with the long-term performance of the well. A reservoir with crossflow can be representedby a single-layer reservoir of equal volume if the flow capacity of the single-layer system is equal to the arithmetic sum of the flow capacities of all layers. Some of these studies also have shown that the early-time response of a well draining a reservoir with interlayer crossflow is similar to the response of a well in a stratified(no-crossflow, commingled) reservoir. Undoubtedly, a transitional period must exist between these two extremes. None of the works cited previously discuss the duration of or the characteristics of the transitional period. If oneis interested in short-time testing, such as pressure builduptests, then it is imperative that the duration of the transitional period and the characteristics of the well responseduring this period be known. For example, if the duration of the test period is such that the well behaves as ifit drains a stratified system or a homogeneous system, then classical well test theories should be applicable. On the other hand, if the test period is such that the transitional period governs the well response, then important questions need to be answered. First, what are the magnitudes of the errors that would result if data during this period are analyzed by conventional procedures? Second, what are the parameters that control the duration of the transitional flow period? Third, is it possibleto obtain reservoir characteristics from a pressure buil duptest? None of the studies in the literature considers the influence of the skin regions on the well response. The skin regions have a significant influence on interlayer crossflow. In this study we show that the skin regions can havea dramatic influence on the well response, particularly during early times. We also show that conventional interpretations of flow behavior in the skin region are inadequate if the layers are in communication. The objective of this paper is to present a thorough examination of the performance of a well in a reservoir with intelayer crossflow. We intend to address the questions raised in the preceding paragraphs. The determination of formation parameters will be discussed. The results obtained here are applicable to both pressure transient testsand production logging. SPEJ P. 380