Extended Well Test for Identifying Connectivity Between Adjacent Compartments

Abstract

This article, written by Technology Editor Dennis Denney, contains highlights of paper SPE 93988, "Analysis of an Extended Well Test To Identify Connectivity Between Adjacent Compartments in a North Sea Reservoir," by A.C. Gringarten, SPE, Imperial College London, prepared for the 2005 SPE Europec/EAGE Annual Conference, Madrid, Spain, 13–16 June. The paper demonstrates the usefulness of extended well tests and advanced interpretation techniques to complement seismic information with an example from a North Sea reservoir. This reservoir is heavily faulted, but seismics indicated that the faults were discontinuous, thus suggesting good communication between the various parts of the reservoir. The combination of the interpretations of the tests on both wells allowed delineating compartments in the reservoir, assessing their connectivity, and correcting the seismic information. Introduction Well-test analysis has been used for many years to obtain reservoir parameters. Early interpretation methods (using straight lines or log-log pressure graphs) were limited, and consequently, well-test analysis was used mostly for the estimation of well performance. With the introduction of pressure-derivative analysis and the development of complex interpretation models that are able to account for detailed geological features, well-test analysis has become a very powerful tool for reservoir characterization. The amount of information obtained and the reservoir volume investigated, however, are direct functions of the testing time, which can make well testing potentially very expensive, especially offshore. Consequently, operators have been looking for alternatives to obtain the same information. There is a trend to replace drillstem tests (DSTs) with wireline formation testing and production tests with permanent-pressure-gauge measurements, and to rely more on seismics than on well testing to identify reservoir boundaries. However, these alternatives have their own limitations. Wireline formation testing has a very limited radius of investigation and provides mostly spherical-flow-related information, which must be scaled up. Permanent pressure gauges provide well-test data that are controlled by operation constraints and, therefore, are not optimized for analysis and often are difficult to interpret. And seismics cannot assess hydraulic continuity between different reservoir compartments, which requires dynamic data that can be obtained only from an extended well test. Background The schematic map of the reservoir in Fig. 1 shows faults identified from seismics and the traces of Wells V and H. Well V is a vertical well and penetrates, from top to bottom, 260 ft of a high-porosity zone, 300 ft of a medium-porosity zone, and 700 ft of a calcite-cemented zone. The high-porosity zone is divided into three layers on the basis of core permeability. Well V is perforated in the high-porosity zone only. Well H is a horizontal well with a perforated length of 900 ft in the same high-porosity zone as Well V. It is approximately 10 ft from the bottom of that zone, which in that region of the reservoir, comprises only two layers with different permeabilities and thickness. In Well V, the high-porosity zone is on top of a 300-ft-thick medium-porosity zone and is thought to be in communication with it.