A Critical Review of Capacitance-Resistance Models

Abstract

A Capacitance Resistance Model (CRM) is an analytical model that only requires production and injection rates to predict reservoir performance. The CRM input is the injection rates and the output is the production rate. The input and output are related by the CRM parameters. The first parameter is the time delay (also called time constant) and is a function of pore volume, total compressibility, and productivity indices. The second parameter is the connectivity (also called gain, or weight), which quantifies the connectivity between producers and injectors (i.e. how much of the input is supporting the output). The CRM was developed for fields with minimum reservoir data, or for small fields not requiring a full reservoir simulation model, which can be time-consuming and expensive. The CRM is a quick, powerful analytical tool that is simple to use and requires readily available data. Most of the time, the injection and production rates are measured accurately and frequently, either weekly or bi-weekly. By solving the continuity equation for a homogenous reservoir (i.e. constant reservoir and fluid properties throughout the reservoir) the solution of the continuity equation can be indicative of the injection and production relation and therefore can be used to optimize injection schemes for higher ultimate hydrocarbon recovery. It is important to recognize that the CRM is not supposed to replace numerical reservoir simulators, which, in essence, are the most accurate means of reservoir performance prediction. Instead, the CRM aims to be a quick and easy way to infer reservoir performance in the absence of full-fledged simulation. The CRM has been used for several purposes as seen in the literature. First, as a tool to optimize waterflooding in oil reservoirs. The CRM can infer inter-well connectivity which will allow the engineer to adjust water injection rates to ensure uniform sweep in the reservoir and reduce the chance of early water breakthrough. The CRM was also used to optimize CO2 sequestration, whereby CO¬2 is captured from the atmosphere and stored in subsurface formations. The main hypothesis in CRM is that the characteristics of the reservoir can be inferred from analyzing production and injection data only. CRM does not require core data, logs, seismic, or any rock or fluids properties. This hypothesis can be challenged easily since most reservoirs have gradients of fluid properties, multi-porosity systems, and heterogeneous formations with different wettability presences. Albeit, several publications have shown that CRM can result in high certainty output. The objective of this report is to explain the concept of the CRM, conduct a critical review of the main CRM publications, compare CRM to other reservoir characterization tools and finally demonstrate some applications of the CRM.