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 provide additional insight into the processes required to develop low permeability lenticular gas sandstone reservoirs. 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 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's of microdarcies. Data obtained permeability on the order of 100's of 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 a 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.

INTRODUCTION AND BACKGROUND

At the MWX site the Mesaverde Formation is found to lie at a depth of between 4,000 and 8,300 ft. Outcrop studies indicate that most of the Upper Mesaverde Formation was deposited by meandering stream systems and includes a meandering fluvial system, that grades into more carbonaceous but gas-bearing paludal deposits. Three continuous marine blanket sandstones are paludal deposits. Three continuous marine blanket sandstones are at the base of the column, the Rollins which is water-saturated, Cozzette and Corcoran sandstone which are gas-bearing.

lsopachous contours in the Cozzette sandstone show that the interval thickens markedly eastward, and there is an east-northeast zone of thinning that must reflect an underlying anticline or sandstone buildup. The Cozzette lies between two thick deposits of marine Mancos shale and is separated into two zones by about 50 feet of mudstone and siltstone. The two zones were appropriately designated the Upper and Lower Cozzette. The well testing described herein was carried out in the more productive Upper Cozzette. The logs shown in Figure 2 detail the location of the Cozzette within a portion of the marine interval. The surface and sub-surface location of the wells, MWX-1 and MWX-2 are shown in Figure 3 along with the distance between wells at the depth of the Cozzette. (123 ft). During test periods, bottomhole pressure, bottomhole temperature, surface tubing pressure, casing pressure, fluid flow rates and other pertinent information were pressure, fluid flow rates and other pertinent information were acquired, manipulated and stored using a PDP-11/34 computer system. The field computer, interfacing hardware, accompanying calibration instrumentation and repair facility are all housed within the DOE/ Sandia Well Testing Facility located at the MWX site. Most of the subsequent analysis was performed on a Cyber 76 computer, DOE Nevada Operations and a VAX 730, CER Corp.

MWX COZZETTE CORE ANALYSIS

A total of 3,700 feet of 4 in. diameter core, including some oriented and pressurized were obtained from MWX-1 and MWX-2. The MWX cores were sealed at the site and shipped to several different laboratories for analysis. The following procedures were described by Randolph of IGT in performing their analysis.

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