Tracer Experiments in Eastern Devonian Shale

Abstract

The paper was presented at the SPE/DOE Unconventional Gas Recovery Symposium of the Society of Petroleum Engineers held in Pittsburgh, PA, May 16–18, 1982. The material is subject to correction by the author. Permission to copy is restricted to an abstract of not more than 300 words. Write: 6200 N. Central Expwy., Dallas, TX 75206. Abstract For the purpose of characterizing the properties of the Eastern Devonian gas shale, a seven-day tracer experiment was carried out in August of 1981 by the Los Alamos National Laboratory as part of the DOE's Offset-Well Test. Two wells had been drilled in a Columbia Gas Company field in southeastern Ohio, each with a downhole separation of approximately 120 feet from an existing production well. The isoceles triangle formed by the three wells had an apex angle of approximately 110 degrees. About 56,000 SCF of nitrogen were injected into a producing zone located at a depth of 3300 feet in one of the wells. Gas was then produced from the various wells at different rates and pressures for the duration of the test. Both pressure and gas composition in the three wells were monitored throughout the test. Introduction The efficient recovery of methane from many tight formations awaits the implementation of improved stimulation techniques. Determination of the appropriate method can be assisted by a better understanding of the methane storage and transport within the rock. It was to show how this better understanding can be attained that the Los Alamos National Laboratory carried out a seven-day tracer experiment in an eastern gas field as part of the Department of Energy's Offset Well Test. The gas in the field tested is held in Devonian shale. Structurally, these shales consist of a low porosity matrix pierced throughout by a complex network of fractures generally vertical relative to the horizontal bedding planes. Although methane gas has been produced from Devonian shales in the Appalachian Basin for more than fifty years, there is still a wide range of opinion concerning the nature of the underground storage of the gas. Stimulation methods for production wells vary, depending on whether the methane is stored in the pores or in the fractures. One of the purposes of the tracer test was to determine the relative amounts of gas in purposes of the tracer test was to determine the relative amounts of gas in the different storage media. The test was based on a concept originally proposed by Lincoln F. Elkins of Sohio Oil Company. It used an old methane well, which continues to be a useful production well even after eighteen years of flow. In addition, the experiment employed two new wells drilled near the production well as part of the DOE's Offset Well Test. In Elkins's concept, a tracer or tracers would be injected into one of the wells, and then this well back-produced. The concentration-time behavior of the tracers in the well effluent would give information concerning the storage characteristics of the gas in the shale. In our test, we injected 56,000 SCF of nitrogen into one of the new wells and subsequently back-produced this injection well. The old production well and the second new well were allowed to flow at rates production well and the second new well were allowed to flow at rates sufficient for chemical analysis. Rates were sometimes higher than this, due to leaks, but the essential flow rates for all times could be calculated. Species-concentration and pressure data were collected at all three wells throughout the test. BACKGROUND TO EXPERIMENT Geology Broad intercontinental seas deposited the sediments that make up the middle and Upper Devonian shales of Ohio. Descending from the surface, the Devonian shale layers that occur in southeastern Ohio are the Cleveland Member, the Chargin Member, and the Huron Member. The Huron Member is dark-gray to black in color and is the most highly organic of the Ohio Shales. The gas-producing zones are found in this latter member. The Huron shale is composed primarily of silt, clay, and carbonaceous matter. p. 81