Alteration of Mancos Shale by synthetic hydrofracturing fluid

Abstract:

Shale gas produced through hydrofracturing has changed the energy perspective in the United States. Shale gas extraction is complicated by the fast decline in wellbore production, to mitigate which re-stimulation and drilling new wells are required. Our goal is to conduct laboratory experiments that examine methane transport from low-permeability matrices to fracture networks induced by hydraulic fracturing. In particular, we address whether mineralogical alteration of shale by hydrofracturing fluids has an effect on matrix-to-fracture methane transport. We performed a set of laboratory experiments addressing the alteration of Mancos shale by synthetic hydrofracturing fluid at hydrothermal conditions (90 °C). Both powdered shale and shale chips were investigated. Solid characterization was done using bulk and micro-X-ray diffraction (XRD, μXRD), and micro-X-ray Fluorescence mapping (µXRF). Analysis of the aqueous samples was done using ion chromatography (IC) for major anions and cations and inductively coupled plasma mass spectrometry (ICP-MS) - for trace metals. Our results indicate that calcium, barium, strontium , magnesium, manganese, silica, sodium, chloride and sulfate were released from Mancos shale after reaction with hydrofracturing fluid. Altered zones on shale surface after 2 months of reaction are thin – likely, within a few microns. The XRD patterns normalized to the 100% peak for quartz indicate that the dolomite, calcite, biotite, and kaolinite peaks decrease in intensity relative to the quartz peaks with increased alteration time, indicative of the partial dissolution of these minerals. Understanding mineralogical composition of an altered layer of Mancos shale will provide insight whether methane transport through these zones will be affected compared to the unaltered material. Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-AC04-94AL85000.