BACKGROUND
Over the past decade, the focus of the U.S. oil and gas industry has shifted to exploration and production of unconventional oil and gas reservoirs. In unconventional or self-sourced reservoir plays, the natural system matrix permeability is very low and a combination of long, horizontal wells and aggressive hydraulic fracturing (“fracking”) is necessary for economic fluid recovery. Despite the successes, overall recovery from the largest unconventional plays (e.g., Permian Basin, Eagle Ford, Bakken) is low (typically <30%) and high initial production rates are followed by rapid declines (often >70% in the first year). A key aspect of these reservoirs is the mechanically layered nature of the rocks that constitute these tight reservoirs, which is a critical input to planning and executing a successful fracking program. Presently, detailed mechanical stratigraphic characterization is restricted to scenarios where companies have collected subsurface core—a time-consuming and expensive endeavor—and performed limited rock mechanics testing on a small number of samples from the core. The objectives of this research project were to
Determine correlations between rebound data and rock mechanical properties in diverse rock types.
Evaluate the influence of rock composition and porosity on rebound.
Develop, test, and demonstrate a practical approach for estimating rock failure envelopes from rebound measurements in order to provide better mechanical stratigraphic characterization services for oil and gas industry clients.
Assemble these tools and associated database into a marketable package suitable for assisting clients to optimize hydraulic fracturing in their quest to improve oil and gas recovery.
APPROACH
This project leveraged SwRI data collected through past IR&D and consortium projects and released proprietary data to establish relationships between rock mechanical properties and rebound measured in outcrop and core. Additional data were gleaned from literature, and supplemental measurements and testing were performed to complete or expand datasets. After determining rock mechanical correlations and controls on rebound, we developed a practical approach for leveraging the rebound measurements to construct failure envelopes for rocks, which allow predictions of failure mode under natural or imposed deformation. Our rapid and cost-effective approach provides unprecedented bed-by-bed mechanical stratigraphic characterization of oil industry core and definition of corresponding rock failure envelopes that, along with stress history, can be used to predict natural failure modes, fracture orientations, and deformation during hydraulic fracturing.
ACCOMPLISHMENTS
Project results include
Augmentation of the rebound and mechanical property database for a range of samples including traditional experimental rocks (e.g., Berea sandstone, Carrara marble, Westerly granite, Indiana limestone) as well as oil and gas reservoir rocks (e.g., Eagle Ford Formation, Austin Chalk, Wolfcamp Formation, Bone Spring Formation, Buda Limestone).
Assessment of uncertainty introduced by rebound measurement protocol (i.e., testing substrate, sample thickness).
Analyses of controls on rebound (including rock type, density, porosity) using ordinary regression and multivariate analyses.
Collection of rebound and fault geometry data to better understand the relationship between rebound, slip and dilation tendency, and failure envelope prediction, and to support a peer-reviewed publication.