Analysis and Interpretation of Interwell Connectivity From Production and Injection Rate Fluctuations Using a Capacitance Model

Abstract

Abstract This paper presents new applications of a capacitance model to characterize reservoirs based on temporal fluctuations in injection and production rates. In a previous report (SPE 95322), two coefficients are determined for each injector-producer pair; one parameter, ?, quantifies the connectivity and another, t, quantifies the fluid storage in the interwell region. This report describes the development of this method into a diagnostic tool to enhance inference about preferential transmissibility trends and the presence of flow barriers. Applying the capacitance model to results from numerical flow simulation, we found that complex geological conditions are often not easily identified using ? and t values separately. However, combining them in certain representations enhances the inference about geological features. Two different representations are used: one representation is a log-log plot of the ?'s versus the t's for a producer and nearby injectors, and another representation is the flow capacity plot where the ?'s and the t's are combined as in Lorenz plots. These techniques were tested using a numerical simulator and then applied to data sets from the South Wasson Clearfork and the North Buck Draw fields. The simulation results and field applications show that the relation between ?'s and corresponding t's are consistent with the known heterogeneity, the distance between wells, and their relative positions. The flow capacity plots and the log-log plots are capable of identifying whether the connectivity of an injector-producer well pair is through fractures, a high-permeability layer, multiple-layers or through partially completed wells. Finally, the approach, based on Lorenz plots, can make use of the extensive literature already available on the interpretation of such plots. Introduction Most real reservoirs, if not all, are heterogeneous. Different methods have been used to quantitatively determine the interwell connectivity during waterflood and thereby map reservoir heterogeneity. Albertoni and Lake[1] estimated interwell connectivity based on a linear model with coefficients estimated by multiple linear regression. The linear model coefficients quantitatively indicate the communication between a producer and the injectors in a waterflood. Yousef et al.[2] used a more complete model that includes capacitance (compressibility) as well as resistive (transmissibility) effects. For each injector-producer pair, two coefficients are determined; one parameter (the weight) quantifies the connectivity, and another (the time constant) quantifies the degree of fluid storage between the wells. All the reservoir simulation models used in Yousef et al.[2] were homogeneous; the emphasis was on validation of the new aspects that the capacitance model (CM) has over the method proposed by Albertoni and Lake.[1]