Comparison of Diagnostic Tools for Selecting Completion Intervals in Devonian Shale Wells
Authors: McBane R.A. , Campbell R.L. , Truman R.B. 
Published: December 2007

Summary.
Commonly available tools used by producers for selecting intervals for stimulation provide different, and sometimes conflicting, information on which zones have the best potential to produce gas. Data on the presence of hydrocarbons and permeable features required for migration of hydrocarbons to a well can be acquired from mud logs, noise and temperature logs. borehole television logs, and geophysical well logs. The Gas Research Inst. (GRI) has funded the collection of these data in more than 30 Devonian shale wells as part of its Eastern Devonian Gas Shales project area, permitting an evaluation of the utility of these diagnostic tools for selecting treatment intervals. Special interpretive methods were developed for analyses of data from geophysical logs, borehole television logs, and experimental mud-logging techniques. Data from each technique were analyzed to determine the geologic and reservoir characteristics of the zones with gas production potential. Production logs were collected in the study wells after stimulation and cleanup to determine the relative contribution of each completed zone to the total production of the well. The comparison of actual productivity of each completed zone to the anticipated gas potential as indicated by each diagnostic technique can provide producers with a better understanding of how to interpret and use diagnostic data collected during drilling and completion of Devonian shale wells.

Introduction
One premise of the GRI's Devonian shale research program is that, to improve productivity of Devonian shale wells, it is necessary to develop a better understanding of the permeable pathways that allow gas to migrate from the matrix to the wellbore. Studies under the U.S. DOE's Eastern Gas Shales Program have shown that gas is contained in the rock matrix throughout the Appalachian basin. Yet productivity of development wells can vary significantly, even when offsetting other development wells. Understanding how to recognize the combination of geologic features necessary for good production can improve the producer's ability to complete wells in the best zones.
The objective of the Geologic Production Controls project is to establish a fuller understanding of the relationships between productive gas flows and various geologic features that control the distribution and magnitude of production in Devonian shale gas wells. To accomplish this objective, the GRI is investigating 30 newly drilled wells in West Virginia, Kentucky, and Ohio. The purpose of this study is to determine in individual wells which zones are producing gas and then to characterize the geologic and reservoir properties of both the productive and nonproductive zones. The relative contribution to production of each perforated zone is determined by running production logs in each well after stimulation. The geologic and reservoir properties are then characterized by standard diagnostic methods: mud logs, temperature logs, noise logs, borehole television logs, geophysical logs, sidewall cores, and surface well tests. Through this approach, the GRI plans to determine how to identify productive zones in Devonian shale wells better.
One outcome of this study has been an evaluation of the productivity of completion zones selected on the basis of various diagnostic techniques. Through the course of the project, the producers have selected zones for perforation and stimulation. Often, data from several diagnostics supported the selection of a particular zone. Frequently, however, one indicator, such as a temperature anomaly or fractures visible on the borehole television, was used as the criterion for completion. The comparison of the relative contribution to production of the selected zones to the interpretation of the diagnostic methods is discussed below.

Production Logs
While nearly the last set of data collected in a well, production logs are the most important data in determining where gas is flowing into a well. Production logs were run after stimulation and cleanup of each of the study wells immediately before drawdown and buildup well tests were conducted. These logs were interpreted to determine the relative contribution to total production of each perforation in the well. A production profile was constructed for each well that showed an estimate of the actual productivity of each zone. This production profile was used to compare actual productivity of each zone to the potential productivity indicated by each diagnostic method. While the contribution of each perforation can be determined by careful interpretation of production logs, it is not possible to determine from the available data whether the gas is produced directly from the perforated zone or whether the gas has migrated vertically through an induced fracture to that perforation.
Recording useful poststimulation production logs in typical low-gas-flow-rate, low-pressure Devonian shale wells required well-maintained, high-quality equipment and special logging procedures and interpretation techniques. The production log suite run in Jackson Well 10 in Jackson County, WV, included the full-bore flow-meter (spinner), gradiomanometer (fluid density), manometer (pressure), and thermometer (temperature survey). This logging suite permits the detection of gas entries into the wellbore through the perforations, but not necessarily where gas was stored in the formation, especially when the well has been hydraulically stimulated. In addition, the gas flow rate at standard conditions can be determined at any depth in the well. This permits evaluation of the contribution of any perforation to the total flow rate from the well. For example, the perforation in Jackson Well 10 at 4,276 ft [ 1303 m] (Fig. 1) contributes 24.6 Mcf/D [697 meters cubed per d] to the total production.

Mud Logging
Natural gas in the drilling fluid is detected at the flowline at the surface and recorded on the mud log. A hypothetical mud-log total-gas curve recorded in a well where air was used as the drilling fluid is shown in Fig. 2. (Interpretation of this figure assumes constant drilling fluid flow rate and constant drilling rate of penetration of the formation.)
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