Enhanced Oil Recovery (EOR) is the future for unconventional reservoirs that suffer from rapid depletion and low recovery factor. CO2 EOR method such as huff-n-puff CO2 scheme has been evolving in recent years from lab experiments, and simulation modeling to field-pilots (Hawthorne et al. 2013, Gamadi et al. 2014, Yu et al. 2014, Pu and Li 2015, Todd and Evans 2016, Li and Sheng 2017). However, existing studies use idealized hydraulic fracture geometry and often neglect the effect of fracture complexity in assessing the EOR efficiency in unconventional reservoirs. Our study aims to assess the lithological and geomechanical controls on hydrocarbon recovery in CO2 huff-n-puff and the impact of gas interference to nearby wells via integrated modeling framework.
This study utilizes available well logs in Howard County in the Midland basin. We construct 3D structural models that capture Midland Basin stratigraphy (Spraberry, Dean and Wolfcamp formations). Pressure gradient ranges from 0.46 to 0.52 psi/ft. Matrix porosity ranges from 4% to 12%, while permeability is 0.01 to 10 microDarcy. Assuming isotropic shale material, 3D mechanical earth models are derived based on available logs. We use a fracture simulator to create multi-stage hydraulic fractures that are properly integrated into the geological models. The results of the hydraulic fracture geometry and conductivity profiles are integrated into numerical reservoir simulation models. Several scenarios of CO2 huff-n-puff are evaluated under various operational, geological, and geomechanical constraints to investigate the domain of uncertainty. The study discusses and applies practical completions techniques used in Permian basin into the design of experiment to accurately capture the important parameters and their relative contribution to well performance.
Base case primary depletion is matched with observed production data in Howard County. Adequate matching is acquired by comparing production performance response of various hydraulic fracture realizations. CO2 huff-n-puff results in positive oil increment over the primary depletion in certain huff-n-puff operating conditions and hydraulic fracturing design. Low permeability reservoirs require sufficient soaking period for miscible condition to occur before bringing back on production. Thus, a combination of reasonable injection time and soaking time is recommended. Results suggest 7-year period of huff-n-puff. Simulation results illustrate the interplay between mineralogical content and hydraulic fracture growth which in turn impacts the efficiency of huff-n-puff process.