Interference Testing of the Naturally Fractured Cozzette Sandstone: A Case Study at the DOE MWX Site

Abstract

Abstract The DOE Multi-Well Experiment (MWX) is a research oriented field laboratory whose objective is to improve the technology and provide additional insight into the processes required to develop low permeability lenticular gas sandstone reservoirs.1 The site of the MWX is located in the Rulison Field in the Piceance Basin of Colorado, as shown in Figure 1. The MWX consists of three closely spaced wells drilled through the Marine Mesaverde Formation to depths of about 8,000 ft. A detailed geological and geophysical assessment is a continuous effort being conducted throughout the Piceance Basin and other areas as a means to characterize the salient features of this site. Very extensive core and logging programs were carried out in all three MWX wells with emphasis on the zones of interest. Seismic surveys and sedimentological investigations were conducted to provide an assessment of morphology, areal extent of lenses and others. As a prelude to the intensive reservoir investigations that were scheduled for the lenticular sandstones, several well tests were performed in the blanket Cozzette sandstone. Some of the experiments conducted within the Cozzette included in situ stress testing, several short drawdown and buildup tests as well as a more sustained interference and pulse test that utilized two of the available MWX wells. The marine blanket Cozzette sandstone characteristically exhibits gas permeabilities on the order of 10 microdarcies or less. However, the production and test data from the MWX site along with subsequent analytic methods require an average reservoir permeability on the order of 100'sof microdarcies. Data obtained from the observation well during the interference test suggests that the pressure distribution from the production well was not radially symmetric and thus the reservoir was modeled as an anisotropic naturally fractured reservoir. Numerical modeling techniques including a dual porosity reservoir model were used to simulate this naturally fractured tight reservoir. Included in our analysis was a parametric sensitivity study for a number of salient well and reservoir parameters, such as wellbore damage and the effects of boundaries. Several months of sustained winter pipeline production that followed well testing attests to the fact that the natural fracture system within the Cozzette extends over a large area. This paper details some of the studies carried out in the Cozzette sandstone and includes core analysis, and the results of the MWX well testing.