Occurrence Characteristics of Methane and Water in the Matrix Near-Wellbore Area during the Development of Marine Shale

During drilling and fracturing of shale gas wells, huge amounts of drilling fluid and fracturing fluid are kept in marine shale reservoirs, forming a water invasion layer. Gas-water displacement and gas production experiments, water absorption, and gas driving water experiments were used in this work to replicate the hydraulic fracturing-soaking-gas production process in shale gas development. Methane and water in marine shale are detected and quantitatively investigated using nuclear magnetic resonance technologies, and their occurrence characteristics are obtained. The results show that fluids, such as drilling fluid and fracturing fluid, are first adsorbed on the surface of pores and fractures to form bound water. With the increase of the water content, capillary pores are filled to form capillary water and then aggregated into free water on large-scale pores. Bound water film promotes the adsorbed gas to desorb, namely gas-water replacement, which improves the development effect of gas wells. Easily produced free gas makes the gas well have the characteristics of an early high yield, and the continuous desorption of adsorbed gas ensures the long-term stable production of gas wells. The flow of gas is accompanied by the discharge of water. Airflow initially removes the capillary water column and some free water. The thickness of the bound water film steadily decreases with the adsorbed gas released and the airflow carried. Capillary water and free water make up most reflowing fluids. Bound water, sensitive to microforces, is challenging to transport out of the reservoir, which results in the low flowback ratio of shale gas wells. This study offers a fresh perspective on the characteristics of competitive adsorption, water absorption, and flowback law in shale reservoirs.