In the last decade, the widespread application of lateral drilling and hydraulic fracturing technologies has helped promote an unprecedented boom in unconventional oil and gas exploration and production in North America. However, a key challenge remains; accurately predicting propped fracture height growth. Rock mechanical properties calculated from wireline logs generally underestimate the vertical heterogeneity of reservoirs due to log resolution issues. To address this limitation, this study proposes a new method that uses high-resolution Leeb (rebound-hammer) Hardness data to fully capture reservoir heterogeneity. We have developed a new quantitative parameter, Reservoir Geomechanic Heterogeneity Index (RGHI), and used it as an additional input in fracture modeling to more accurately predict hydraulic fracture height growth.
Reservoir geomechanical rock properties (e.g. Young's Modulus and Poisson's Ratio) have long been used as key parameters to design stimulation treatments in unconventional reservoir systems. However, conventional plug or full diameter rock mechanics measurements are commonly widely spaced. In contrast, Leeb Hardness measurements made on core slabs are fast and cost effective, with a flexible sampling frequency. More importantly, Leeb Hardness varies significantly with bulk mineralogy and responds to variation in rock type (e.g. ‘soft’ mudrock vs. ‘hard’ limestone). A mean value of numerous measurements around a small sample area has been shown to be representative of individual lithofacies. For vertically heterolithic reservoirs, a six-inch or higher resolution is recommended to capture the full spectrum of reservoir heterogeneity.
We have established a series of empirical relationships between lab-measured geomechanical attributes (e.g. Unconfined Compressive Strength, Young's Modulus, and Poisson's Ratio) and Leeb Hardness using Core Lab's North America unconventional core database. This has allowed Core Lab to build high-resolution, continuous, geomechanical profiles through many of the most prolific source rock mudstones. The high-resolution geomechanical profiles tend to reveal higher degrees of vertical heterogeneity than those derived from conventional logging data (e.g. sonic logs), especially in heterolithic, thin-bedded reservoirs (e.g. Wolfcamp in the Midland and Delaware basins). The frequency and magnitude of geomechanical changes with depth can be more accurately captured by the high-resolution Leeb Hardness data, allowing more accurate geomechanical modeling.
Reservoir Geomechanic Heterogeneity Index (RGHI) is generated by statistically processing closely spaced Leeb Hardness measurements, and can be used to quantify vertical changes of geomechanical rock properties in the reservoir. RGHI values may vary drastically throughout the same producing interval. A broad range of averaged RGHI values have been acquired based on the studies of various formations in North America, and this quantitative method is consistent with core observations. A continuous RGHI surrogate can be obtained if a core is measured with high-resolution hardness data. This RGHI surrogate has been successfully applied in GOHFER® (Grid Oriented Hydraulic Fracture Extension Replicator) modeling of fracture height growth to better account for ‘layered’ or ‘laminated’ producing intervals.