Abstract

This paper presents the results from a preliminary investigation into the reservoir mechanisms controlling gas production from the Antrim Shale of the Michigan Basin. We used a two-phase, dual porosity, finite-difference reservoir simulator incorporating shale-specific reservoir characteristics to match a well's gas and water production history. We also analyzed the sensitivity of Antrim Shale gas production behavior to drainage area and several reservoir properties.

The results indicate that long-term gas production from the Antrim Shale is primarily controlled by natural fracture properties, desorption properties, and drainage area. Based on development strategies for coalbed methane reservoirs, there is an optimum well spacing for effectively dewatering the Antrim Shale, thus accelerating gas production and improving ultimate recovery. Finally, we found that shale matrix diffusion properties have little effect on short-term gas properties have little effect on short-term gas productivity and preliminary results also suggest long-term productivity and preliminary results also suggest long-term gas productivity does not depend on matrix diffusion.

Introduction

The Antrim Shale of the Michigan Basin is estimated to contain up to 76 Tcf of natural gas. Although commercial quantities of gas have been produced from this Devonian-age formation since the produced from this Devonian-age formation since the 1940s, no comprehensive reservoir engineering studies of the Antrim Shale have been made, and consequently, little is known about the reservoir mechanisms controlling gas production. This lack of knowledge is also due, in part, to the small number of wells completed in the Antrim Shale prior to 1988 and a resulting limited data base for reservoir characterization. However, the drilling pace in the Michigan Basin has recently accelerated considerably, with more than 200 wells completed in the Antrim Shale in 1988 and a projected 1,000 wells in 1989. The increased drilling effort offers an excellent opportunity to develop a basic reservoir engineering understanding of this important new source of gas reserves. As part of its continuing goal of securing reliable sources of domestic natural gas, the Gas Research Institute (GRI) has recently initiated a concerted study program aimed at defining the Production mechanisms and ultimately improving gas Production mechanisms and ultimately improving gas recovery from the Antrim Shale.

Although geologically and lithologically similar to the Devonian-age organic shales in the Appalachian Basin, the Antrim Shale exhibits production characteristics more like coalbed methane reservoirs. Because of high connate water saturations. a typical Antrim well initially produces considerable quantities of water, varying between 0.4 to 1.2 barrels of brine per Mcf of gas, and continues producing water at rates of 10 to 20 BWPD for the well's life. As the water production rates decline, a corresponding increase in gas production is seen, suggesting improvements in relative production is seen, suggesting improvements in relative permeability to gas. permeability to gas. Another production characteristic that is common to both coalbed methane and Devonian shale reservoirs is that Antrim wells often show only a gradual decline in gas production over long periods of time. In one ten-well field drilled In the 1940s in Otsego County. Michigan, four wells are still producing, and two of these wells are actually producing more than when initially completed. Reports of wells starting production at 30 Mcfd and 250 BWPD and improving to 110 Mcfd and 70 BWPD a year later are not uncommon. This gradual decline and sometimes increase in gas production over time suggests gas desorption plays an important role in production from the Antrim Shale. production from the Antrim Shale.

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