Proper Evaluation of Shale Gas Reservoirs Leads to a More Effective Hydraulic-Fracture Stimulation

Abstract

Abstract Shale-gas formations are currently being explored in the Rocky Mountain region from Montana to New Mexico. These shale- gas formations include the Cody, Hilliard, Baxter, Mancos, Gothic, Pierre, Lewis, and others. Whether the well is drilled vertically or horizontally, shale-gas wells need to be hydraulically fracture-stimulated to produce commercial amounts of natural gas. Because each shale play has unique attributes, a systematic approach to well construction, data collection, and prefrac diagnostics is an essential component in the quest for the most effective hydraulic-fracture stimulation and the best chance to achieve commercial gas production. The first step in the process is a thorough understanding of the shale's petrophysical attributes. Coupling openhole wireline-log information with laboratory measurements of core or cutting samples provides a basis to calibrate the petrophysical model that describes essential geomechanical and geochemical characteristics of the shale. With a calibrated petrophysical log-analysis model, a basic openhole, wireline-log suite consisting of a gamma ray, porosity, and resistivity is a useful evaluation tool. The inclusion of additional wireline measurements, like the spectral gamma ray, microlog, dipole sonic, and electrical borehole-image logs, will further enhance the description of the shale. Core testing can determine Young's Modulus, Poisson's Ratio, Brinell hardness, total organic carbon, kerogen type, gas content, mineral composition, fluid sensitivity, and acid solubility. The end result of this mating of logs and core data is a model that provides an understanding of the mineralogy, mechanical rock properties, britteleness, organic content, and natural fractures of the shale. The next component of the process is to use all the petrophysical analysis and tribal knowledge (current known information) to design the hydraulic-fracture treatment and select the completion intervals. Completion intervals are first selected on the basis of the brittle zones and the zones that will most likely serve as frac barriers. The selection and volumes of the appropriate fracturing fluid and proppant is based on the shale brittleness, geomechanical, and geochemical properties. The final step is to close the loop by evaluating the overall effectiveness of the stimulation treatment. This is accomplished by doing a detailed postjob treatment-pressure analysis. Microseismic mapping during the hydraulic frac treatment is also a valuable technique to evaluate the effectiveness of the frac job. The goal of this systematic process is to shorten the "ideal" frac learning cycle and provide a framework for moving into frontier areas and new shale plays. Introduction In recent years, natural-gas production from shale has become of increasing interest in the quest for future energy supplies. Throughout the last 40 years, the petroleum industry has progressed from conventional gas reservoirs, to tight gas reservoirs, to ultra-low matrix permeability unconventional shale-gas reservoirs. Each type of reservoir has presented its own unique challenges, but it is especially difficult to coax commercial quantities of hydrocarbons from shale reservoirs. The matrix permeability of the reservoir of interest has gone from millidarcies (conventional), to microdarcies (tight gas), to nanodarcies (shale). It then becomes essential for the prospective shale-gas reservoir to have enhanced permeability beyond the matrix permeability, usually in the form of existing natural fractures. Even with enhanced permeability from natural fractures, a well-designed hydraulic-fracturing program is needed to provide successful production results.