Coreflood Effluent and Shale Surface Chemistries in Predicting Interaction between Shale, Brine, and Reactive Fluid

Abstract

Summary Field and laboratory observations to date indicate that the efficiency of hydraulic fracturing, as it relates to hydrocarbon recovery, depends significantly on geochemical alterations to rock surfaces that diminish accessibility by partial or total plugging of the pore and fracture networks. This is caused by mineral scale deposition, such as coating of fracture surfaces with precipitates, particle migration, and/or crack closure, because of dissolution under stress. In reactive flow-through experiments, mineral reactions in response to acidic fluid injection significantly reduced system porosity and core permeability. The present study focuses on changes to fluid chemistry and shale surfaces (inlet and fracture walls) resulting from shale-fluid interactions and integrates these findings for an improved estimate of transport-related consequences. The pre- and post-reaction shale surfaces were examined by spatially resolved scanning electron microscopy-energy dispersive spectroscopy (SEM-EDS) analysis. Importantly, inductively coupled plasma-mass spectrometry/optical emission spectroscopy (ICP-MS/OES) was utilized to probe the chemical evolution of the coreflood effluents. The three study cores selected from the Marcellus formation represent different mineralogies and structural features. In flow-through experiments, laboratory-generated brine and HCl-based fracture fluid (pH = 2) were injected sequentially under effective stress (up to 500 psi) at reservoir temperature (80°C). SEM-EDS results confirmed by the ICP concentration trends showed significant Fe hydroxide precipitates in the clay- and pyrite-rich outcrop sample because of partial oxidation of Fe-bearing phases in the case of intrusion of low salinity water-based fluids. Porosity reduction in the Marcellus Shale Energy and Environmental Laboratory (MSEEL) carbonate-rich sample is related to compaction of cores under stress because of matrix softening with substantial dissolution, and pore filling by hydroxides, as well as secondary barite and salts. Despite the same fluid compositions and experimental conditions used for both MSEEL samples, barite precipitation was much more intense in the MSEEL clay-rich sample because of its greater sorption capacity and additional sulfate source as well as its fissile nature with multiple lengthwise cracks. ICP tests revealed time-resolved concentration behavior in produced brine and reactive fluids that in turn complemented the pre/post-reaction SEM-EDS observations. The greatest release of metal ions into brine was in clay-rich systems indicating the importance of chemical compatibility between in-situ shale and nonequilibrated injection solutions. A thorough examination of surface and effluent data pointed to the substantial influence of formation brine in the shales, mixing of brine with fracture fluid during flow, and shale mineralogy on mineral dissolution and scale formation that significantly affect flow efficiency.