Enhanced Recovery in Shales: Molecular Investigation of CO2 Energized Fluid for Re-Fracturing Shale Formations

摘要

The liquid and gas rich shales are low permeability, low porosity but high organic content reservoirs. They need effective sub-surface, in-situ stimulation for economic production. Sometimes due to ineffective initial completion, the shale wells don't produce well. An effective re-stimulation or refracturing can shoot up the production from these mature low producing wells. The fracture fluid plays a key role in determining the fate of such reservoirs. Typically, slickwater or gelled water is used as a fracture-fluid. However, the energized fluids, energized with carbon dioxide (CO2) has shown to increase the performance. They reduce the water volumes, problems associated with water usage and have superior proppant transport capabilities. The Molecular Dynamics (MD) Simulations technique is used in the current work to understand the interaction between carbon dioxide and hydrocarbons rich Type II kerogen in the shale rocks. The Nose-Hoover style non-Hamiltonian equations of motion are used in a molecular simulator to generate positions and velocities of carbon dioxide and kerogen molecules sampled from the canonical (nvt) and isothermal-isobaric (npt) ensembles. In this work, we propose that carbon dioxide energized fluids be used for re-fracturing the low producing shale formations. In shale fractured with carbon dioxide enhanced fracture fluid, some of the CO2 is retained in the formation and doesn't flow back after the fracturing operation. The current work study the fate of the retained portion of CO2 in an energized fracturing operation. MD simulations reveal that carbon dioxide has more affinity than methane and heavier hydrocarbons like octane, to be retained in the organic part known as kerogen (Pathak M., 2015a) found in shales. MD simulations also reveal that the kerogen shrinks as a results of absorption of CO2 which leads to effective decrease in the skin of the formation. This helps in better fluid flow between the formation and fractures. The carbon dioxide dissolved in the kerogen helps to displace hydrocarbons absorbed in the kerogen. The diffusion coefficient of carbon dioxide in kerogen is found to be of an order of magnitude less than methane or octane. MD simulations technique has been used for the first time to explore the interaction between carbon dioxide and organic matter in shales at the molecular scale. The mechanism of the carbon dioxide energized fracture-fluid induced enhanced recovery is understood to understand the key processes that helps in taking decisions of a re-fracture job.