Reaction of the Marcellus Shale with hydraulic fracturing fluid: An experimental study

Abstract

Technological advancements in the oil and gas industry in the past few decades have led the Marcellus Shale to be one of the most productive unconventional reservoirs in the United States. Increased production has resulted in more fundamental questions regarding how the fluids being injected interact with the reservoir. Previous research at the National Energy Technology Laboratory experimentally characterized the geochemical and physical changes to the Marcellus Shale as a function of rock reaction with hydrofracturing fluids of various compositions, based on average compositions used in the Appalachian Basin. In these studies, synthetic fracking additives were mixed up to 3 weeks prior to experimental operations and then injected into Marcellus Shale cores. This is in contrast to field operations where mixing of chemicals typically happens at the wellhead as fracking operations commence. Observed variations in dissolution and precipitation in these experiments is potentially due to time differences between mixing and injection; it is possible that the additives are degrading over time. In the current study, synthetic fracturing fluids of the same composition, as the previous studies (excluding ammonium persulfate) were prepared and then immediately injected into artificially fractured Marcellus Shale cores. Marcellus Shale cores were held at representative conditions of 150°F, pore pressures of 2800 psi, and confining pressures of 3000 psi. Mineral dissolution and precipitation inferred from computed tomography (CT) images in conjunction with mineral saturation indices calculated from ion chromatography (IC), inductively coupled plasma mass spectrometry (ICP-MS), and inductively coupled plasma optical emission spectroscopy (ICP-OES) of the reacted fluids were used to characterize the chemical and physical changes in the rock as a function of time exposed to fracturing fluid. The reactivity of the fracturing fluid with ammonium persulfate with the Marcellus Shale can be related to changes in time between preparing the fracturing fluid and injection. Without the ammonium persulfate additive, temporal variations in preparing and using the fluid did not influence barite precipitation. Instead the composition of the Marcellus Shale proved to be the dominant control on mineral dissolution. Ammonium persulfate appears to drive important chemical and physical changes though. Given hydraulic fracturing fluid prepared within an hour of use without ammonium persulfate, the guar gel additive, used to increase viscosity for proppant placement, did not degrade, potentially preventing chemical reactions and removing proppant. Results from this study provided insight on fluid stability relative to temporal changes in preparing synthetic fracturing fluid in laboratory studies on fluid-rock interactions.