Abstract
We examine how a mechanically-induced fracture network may influence geochemical reaction between shale and stimulation fluid used in hydraulic fracturing. Two different types of bedding-parallel fractures, a simple fracture between bedding planes and a damage zone with multiple fractures, were induced in the Whitby Mudstone (Early Jurassic) from the UK. Both fractured shales were subsequently reacted with stimulation fluid at 10 MPa and 100°C for about 2000 hours. pH increased from 2.1 to about 6 after 1000 hours of reaction in both shales, but pH increased slightly more rapidly by reaction in the shale with the damage zone. Total dissolved inorganic carbon evolved in similar fashion in both experiments and did not readily distinguish between the two styles of fracturing induced in the Whitby Mudstone.
Highlights
Interest in shale has grown with the advent of new techniques to extract hydrocarbons from these rocks
Multiple fractures formed in the shale that was subjected to direct shear parallel to bedding (Figure 2)
After 136 hours of reaction between shale and stimulation fluid, pH increased from the initial value of 2.1 to 4.4 in the experiment with the simple fracture and to 4.7 in the experiment with shale that was fractured by direct shear (Figure 3)
Summary
Interest in shale has grown with the advent of new techniques to extract hydrocarbons from these rocks. Development of unconventional shale oil and gas reservoirs by ‘hydraulic fracking’ combines chemical and mechanical stimulation with new technologies for drilling horizontal wells that extend for hundreds to thousands of meters within organicrich beds. Stimulation fluids, a mixture of water and chemical additives, are injected into these horizontal wells at pressures that propagate fractures into adjoining beds. Chemical imbalance between the shale and injected fluids potentially drives geochemical reactions that may alter mechanical properties of the rock. At the same time, fracturing may drive physical changes that alter geochemical reaction paths of water-rock interactions. We examine one small aspect of the problem, how a mechanically-induced fracture network may influence geochemical reaction between stimulation fluid and shale.
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