In-Situ Stress Tests and Acoustic Logs Determine Mechanical Propertries and Stress Profiles in the Devonian Shales

Abstract

Summary. The in-situ stress contrast between a reservoir rock and the surrounding formations is important to the design and analysis of hydraulic fracture treatments. The stress contrast between layers controls the fracture's vertical (height) growth, which in turn affects the fracture length and width. As part of the Gas Research Inst.'s (GRI's) Comprehensive Study Well (CSW) program in the Devonian shales of the Appalachian basin, we measured in-situ stresses directly and used these values to calibrate coustic log measurements to develop stress profiles across the Devonian shales. This discusses the measurement and interpretation of in-situ stresses, the use of acoustic logs to determine mechanical properties and a more complete stress profile, and the practical use of the stress profile in fracture-treatment design and analysis. In-situ stresses were measured directly throughout the Devonian shale interval with both openhole and cased-hole stress tests, conducted with nitrogen, a specially designed downhole shut-in tool, and a downhole quartz pressure gauge with surface readout. The paper describes the procedure used and presents interpreted stress-test results from two CSW'S. Full-waveform acoustic tools were run to define elastic properties of the Devonian shales. These properties also were determined under both static and dynamic conditions from whole-core plugs taken in the shales. Logs were calibrated to core-measured values, and then elastic properties determined from the logs were used to compute an in-situ-stress profile, which was calibrated against field measurements of in-situ stresses. Finally, we discuss how these data are used to design and analyze hydraulic fracture treatments in CSW'S. Introduction Successful development of low-permeability gas reservoirs generally requires an effective hydraulic fracturing program. Designing fracture treatments to achieve optimal results requires the measurement or estimation of several reservoir properties. One of the most important rock properties required for fracture design is insitu stress, both in and Surrounding the prospective productive zone. The in-situ-stress profile will strongly influence the created fracture height and width during a treatment. Accurate estimation of the fracture-height growth expected during a fracture treatment is critical for the fracture-design engineer to achieve the effective propped-fracture half-length required to optimize stimulation results. Several methods are available for measuring or estimating insitu stress, including core measurements, openhole and cased-hole stress tests, and stress calculations from acoustic log measurements. Although the best method for determining in-situ stress is direct measurement (core measurements and stress tests), it is prohibitively expensive for most routine applications. Acoustic logs, however, can be run as part of the openhole or cased-hole logging suite at a reasonable cost. If calculations from these acoustic measurements can be made to yield accurate estimates of in-situ stress, this approach could provide a practical, cost-effective method to estimate in-situ stress. This method also provides a continuous stress profile as a function of depth, as opposed to the discrete point values of stress obtained by other methods, The only drawback to making acoustic log measurements in open hole is that the borehole must be loaded with a liquid to provide an acoustic coupling between the wireline tool and the formation. Special core analysis has determined that the predominant clay in the Devonian shale is illite, with little to no swelling clays present. Therefore, adding liquid to the wellbore should not significantly affect production. Other investigators have found that acoustic-log-calculation procedures can be calibrated empirically with measured in-situ-stress information to yield an accurate stress profile. Whitehead et al. presented an approach for the Travis Peak tight gas sands in east Texas, part of the GRI's Tight Gas Sands Research Program. GRI also conducted a stimulation research project in the Devonian shales of the eastern U.S. This project required a method for estimating in-situ stress to design fracture treatments in the Devonian shales. Our paper presents a method for using measured stress data from cores and stress tests to develop an accurate log-based method for estimating stress profiles in the Devonian shales. The Devonian shales of the eastern U.S. are a significant source of natural gas because they act as both a source rock and a natural gas reservoir. Unfortunately, the Devonian shales usually have low permeability, unless natural fractures or other localized permeable features exist. Although nearly always gas-productive, most Devonian shale intervals must be stimulated to achieve economic flow rates. Most stimulation treatments in the Devonian shales, although providing some stimulation, do not provide the desired effective fracture geometry. Effective propped-fracture half-lengths >30 to 50 ft are rarely calculated from poststimulation transient-test analyses. Abnormally low reservoir pressure, low proppant concentrations, liquid cleanup problems, and uncontrolled vertical fracture-height growth have been identified as possible reasons for the lack of overall stimulation effectiveness. To improve stimulation results in the Devonian shales, we are trying to develop a practical method for estimating in-situ stress to predict vertical fracture-height growth better. Whole-core data have been collected and analyzed to estimate rock mechanical properties. Openhole and cased-hole stress tests and cased-hole breakdown tests have been conducted to measure in-situ stress. Acoustic logs have been run to estimate in-situ stress. The objective of these measurements is to provide an empirical method for accurately calculating in-situ stress from acoustic log data. With an accurate, log-determined stress profile, 3D fracture-design simulators can be used to predict created fracture geometry more accurately, which will lead to improved fracture designs and stimulation effectiveness. Core, stress test, and log data were collected on CSW's as part of GRI's Devonian shale research program. We will present data from two wells: Sterling Drilling and Production Inc's Jarvis No. 1143 in Calhoun County, WV (CSW 2) and Ashland Exploration Inc.'s E.J. Evans No. 91 in Breathitt County, KY (CSW 4A). Fig. 1 shows the wells approximate locations. Results of our analyses of data collected on these wells are presented. Estimation of In-Situ Stress From Wireline Logs New wireline logging tools have been introduced that allow an accurate estimation of shear-wave velocity, v, in a wellbore by complex analysis of the digitally recorded full-waveform data. When vs is combined with the more conventional log measurements of compressional-wave velocity, Vc, and bulk density, Pb, various elastic moduli from the basic elasticity theory for homogeneous, isotropic solids are easily calculated. SPEFE P. 248⁁