(270b) The Impact of Acid-Base Stimulation Sequence on Mineral Stability for Tight/Impermeable Unconventional Rocks: Delaware Basin Case Study

Technological innovations in fracture stimulation of unconventional reservoirs have spurred an energy boom, dramatically lowering CO₂ emissions in the US due to the large-scale substitution of natural gas for coal for electricity generation. However, mineral precipitation due to reactions with injected fluids during unconventional stimulation is a well-recognized problem. Laboratory experiments using a typical hydraulic fracture fluid (HFF) formulation for the Delaware Basin shows that the injection of this fluid initiates two major chemical processes: (i) mineral dissolution, creating beneficial secondary porosity; and (ii) precipitation of mineral scale (e.g., barite, Fe(III)-(hydr)oxides, etc.) on fracture surfaces and in the shale matrix that can reduce porosity and permeability. These two processes have strong competing effects on the rock that can significantly impact production. Additionally, shale reservoirs are fractured using injections of strong acid (15% HCl) followed with sand slurries containing chemical amendments (pH 7-10) resulting in large pH variations. Consequently, chemical conditions in stimulated reservoirs can depart significantly from simplified model fluids containing only a few targeted chemicals (such as scale inhibitors, biocide, etc.) that have been studied. The goal of this study was to evaluate secondary mineral precipitation and permeability attenuation under “real-world” chemical injection scenarios (formulations, injection sequences, T/P, etc.) specific to the Delaware basin.

Whole cores (1” diameter x 1” length) and ground shale (150-250 μm) from carbonate-rich Bone Spring Formation, Delaware Basin TX, were reacted at 80°C and 85 bar using a HFF recipe and sequential injection typical of Delaware Basin. One subset of ground samples was reacted in glass serum bottles (80°C and ~1 bar) with solution sampling every 72 hours for 3 weeks to evaluate solute evolution with time. We then analyzed these reacted shales and solutions using a variety of laboratory-based and synchrotron-based techniques to characterize both the chemical and spatial distribution of secondary mineral precipitation and identify changes in permeability and mineralogy.

Despite being relatively impermeable (< 5 nD), the HFF was able to penetrate the shale and react with the Fe-bearing phases. The initial pH of the solution is ~0 due to the HCl spearhead. Subsequent additions of borate crosslinked slickwater (pH of 9.2) caused a dramatic increase in solution pH due to a combination of dilution/neutralization by the slickwater and reaction of HCl with the calcite and dolomite the rock. Synchrotron-based X-ray fluorescence mapping coupled with X-ray absorption spectroscopy (both bulk and micro) showed that almost all the iron had been oxidized to Fe(III). These results demonstrate the importance of the acid spearhead and subsequent formulation in order to move the HFF into the rock. These results also improve our understanding of the geochemical reactions occurring in shale reservoirs during fracture stimulation.