Even though shale has traditionally been considered a hydrocarbon source rock and/or seal rock, some gas bearing shales have been recognized as major reservoir rocks for unconventional hydrocarbon resources. The exploitation of gas shale across North America has generated much activity in the drilling and completion sector of the oil and gas industry, including drilling horizontal wells and multi-stage fracturing with water-based fracturing fluids. During water fracturing, the shale will interact with water and create consequences (hydration, dehydration, fractures etc.) as that happens in drilling. The consequences from this shale/water interaction will not only alter the shale mechanical properties, but also induce stress changes in the shale. These changes may induce favorable or unfavorable fractures in the shale/water interacted zones.
This paper first outlines lab observations of the shale/water interactions which generating fractures in gas shale. Then, a stress model is developed with poro-elasticity effect and simulation method for studying stress changes due to shale hydration in near-fracture zones. This paper also describes a sample application of the models to investigate stress changes and potential shale failures in shale around major hydraulic fractures in shale gas.
The study presented in this paper that it was mechanically possible to induce multiple secondary fractures around major hydraulic fractures via shale hydration. The modeling in this paper completes the preliminary study and provides a foundation for more complex scenarios in future studies focusing on water fracturing and horizontal well drilling in gas shale. It also provides some new ideas of optimizing hydraulic fracturing in gas shale.