Acid fracturing can be potentially used in calcareous shale/mudstone reservoirs to stimulate the unpropped fractures. The unpropped fractures are created in large amount during hydraulic fracturing and provide considerable productivity, which, however, fails to sustain in the production stage. To understand how acid fracturing affects the fracture performance in laboratory is a critical step before field trial and can provide valuable guidance for fracture design.
In this paper, we benchmarked the performance of acid-treated to brine-treated unpropped fracture using a "half-core" approach. The half cores to compare split from one shale core had very close mineralogy, they were treated with brine or acid independently and then form a composite core with a stainless steel half for conductivity measurement. This approach minimized the effect of mineralogy anisotropy in shale and cyclic stress that involved in traditional methods and enabled more meaningful comparison. Preserved Eagle Ford shale categorized into low, medium and high carbonate content were used; uniform and non-uniform flow patterns were compared in the treatment. Fracture conductivities and their response after cyclic stress, etched surface profiles and mechanical properties were systematically investigated.
With uniform flow, acid resulted in lower fracture conductivity and restored conductivity after cyclic stress than brine. Both conductivities further decreased with carbonate content. On the contrary, non-uniform flow led to a higher acid-treated than brine-treated conductivity. The difference reached two orders of magnitude at reservoir pressure in sample with high carbonate content. The restored conductivities after acid treatment were higher than or close to that after brine treatment. Non-uniform etching was shown critical in a successful stimulation of unpropped fractures, and it could be achieved by viscous fingering or presence of carbonate veins. Most samples after acid etching developed isolated pits with depth within 50 µm on surface, channels were only observed for the sample with carbonate content etched with non-uniform flow and the one with calcite vein. Acid treatment reduced hardness in all samples, and the reduction tended to increase with the original carbonate content in samples. Mineralogy impacted the unpropped fracture conductivity, even in the cases of brine treatment, where no chemical reaction happened.
We experimentally demonstrate the feasibility of acid fracturing in shale and provide insight to shale candidate selection for acid fracturing.