Study on High-Pressure Treatment of Unconventional Reservoirs with Water-Based Fluids

Abstract

ABSTRACT Shales are tight rocks with ultra-low porosity and often contain hydrocarbons (oil and gas). Both the United States and Israel have abundant shale formations containing oil and gas. The majority of shale hydrocarbon production in the United States takes place in Permian basin, Texas, whereas the largest kerogen shale deposits in Israel occur in the Negev area. For the recovery of oil and gas, shale formations are hydraulically fractured by injecting water-based fluids under high pressure. To transition away from fossil fuel use and address atmospheric greenhouse gas levels, sequestration of carbon dioxide (CO2) and hydrogen (H2) storage is now proposed in depleted tight hydrocarbon formations, such as shales. Shales are composed largely of clays and silts, bound by carbonates to form ultra-tight matrices, and containing interconnected nano- to micro- to macro-voids with total porosities of 1-6%. The introduction of fracturing fluids and, potentially, other fluids for storage purposes (e.g., CO2 and H2) may cause changes to the ultra-tight shale matrix, as formations had previously been isolated for millions of years. These changes have been linked to reduced production of oil and gas and may affect possible storage of CO2 and H2. The interaction between artificially introduced fluids and shale formations, leading to softened shale, is attributed to the interlayer expansion of clays and the dissolution of cementing carbonates. This study seeks to investigate of the effects of interaction between various fluids and unconventional reservoir rock at high temperature and pressure for geo-storage implications. Shale-fluid interactions will be quantified through the lens of mineralogical, pore network, and mechanical property changes. of CO2 and H2. INTRODUCTION Shale formations have ultra-low porosity and permeability, and when shales contain hydrocarbons, are considered unconventional reservoirs. Large shale gas reserves are popularly located in China, United States, and Argentina; with 31.6, 17.6, and 22.7 trillion cubic meters of reserve, respectively (U.S. Energy Information Administration, 2015). Horizontal drilling and hydraulic fracturing are major methods of extracting the hydrocarbons trapped in these tight formations (Speight, 2020). Once both a vertical and horizontal well is drilled into the reservoir formation, high-pressure fluid is injected at a designed rate. The fluid, often a water-based brine, also contain proppants chosen to withstand the rock closure pressure as fractures propagate out perpendicular to the horizontal well. These new fractures increase the connectivity of formation pores and allow for the hydrocarbons stored within them to be recovered. Depleted unconventional oil and gas reservoirs therefore offer potential uses for nuclear and wastewater disposal, hydrogen storage, and carbon sequestration, due to their existing wells and fracture networks, and otherwise low porosity.