Oxidative Hydraulic Fracturing Fluid to Enhance Production from Source Rock Reservoirs

Abstract

Summary The steep production declines generally observed after hydraulic fracturing in unconventional source rock reservoirs has been attributed to several potential causes. Recently a new additive to the stimulation fluid system was proposed to extend economical longer-term production from these formations. Oxidizer-laden fracturing fluid systems are shown to create cracks and deep channels within the organic matter present in the source rock, such as kerogen, thereby increasing the source rock permeability and enhancing the hydraulic conductivity of the exposed fracture faces. To this end, the fluid design and recommendations for its application are illustrated herein. Oxidants composed of ClOn- and BrOn- (n=0-4) are effective for kerogen depolymerization or degradation at depth. This study illustrates the beneficial effects of two specific oxidizers, sodium chlorite NaClO2 and sodium bromate NaBrO3, on kerogen-rich source rock subjected to in-situ reservoir conditions. Source rock samples were cut and polished to test the oxidizer impact on the organic and inorganic regions. Scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS) were performed on the rock surface to identify specific organic matter features. The samples were then chemically treated with varying conditions of NaClO2 or NaBrO3 (concentrations 0.013 M - 0.054 M, temperature 150 °C, and time 3-24 hours). Samples were returned to the SEM for post-treatment analysis. Furthermore, the oxidants were packaged within a slickwater hydraulic fracturing fluid system for field application, and their effects upon viscosity and friction reduction were also studied. SEM images and EDS maps of kerogen-rich rock samples observed before and after treatment with oxidizing fluid showed a series of cracks formed throughout the organic matter domains, where increasing the concentration of oxidizer in the treatment fluid showed a clear increase in the prevalence of cracks throughout the surface. The effect of time was also observed, as short treatment times resulted in porosity/permeability creation in the kerogen, though longer treatment times were associated with more severe degradation. Optimal conditions for NaClO2 and NaBrO3 concentrations in the additive fluid systems, were different and will be herein highlighted. Each oxidizer (10-20 pptg concentration) was added to slickwater with variable friction reducer concentration (1, 2, and 4 gpt), and shear sweeps performed at both 70 °F and 180 °F. Negligible difference is observed between the viscosities of the base fluid and the fluid with either oxidant at low friction reducer concentration. Meanwhile, flow loop tests demonstrated that the oxidizer did not affect the friction reducer except to slightly boost the performance due to the salt effect on the polymer. Two strong oxidants, available as commodity chemicals, are shown to be effective to crack kerogen and any present organic matter thus create permeable channels and enhance the overall permeability of the exposed source rock fracture faces. Meanwhile the proposed fracturing fluid additives display good compatibility with other slickwater fluid components, demonstrating the universal usage in unconventional stimulations. The recommendations for its application as fluid additive in slickwater are herein illustrated.