Impact of CO₂ treatment on fracture development and mineralogical composition in organic-rich and organic-lean unconventional reservoirs

Abstract

Fracturing associated with carbon dioxide (CO₂) treatment in unconventional hydrocarbon reservoirs presents a huge potential for CO₂ geological sequestration in carbonate mudrock and shale reservoirs and the use of CO₂ as a working fluid for oil recovery. This study aims to examine the impact of CO₂ treatment on fracture growth and propagation in unconventional carbonate reservoirs. Experimental studies were conducted on both organic-rich (Jordanian) and organic-lean (Wolf Camp) mudrock samples and a careful examination of fracture initiation and propagation induced by CO₂ treatment was examined. After an extensive 30-day treatment with carbonate brine (i.e., a mixture of brine and CO₂) under 75 °C temperature and 4158 psi pressure, mineralogical and geochemical results revealed an increase in the carbonate mineral content (calcite) in the organic-rich sample, along with a decline in total organic carbon content from after CO₂/carbonate brine treatment. The organic-lean sample, inversely, showed negligible change in the calcite content and almost no change in the total inorganic carbon. Microscopic investigations showed that the organic-rich sample exhibited a remarkable development of a new set of horizontal (bedding-parallel) microfractures, while the organic-lean sample did not display any significant fracture development. In addition, the surface roughness of the organic-rich sample increased significantly (from 20 to 37 uµ on average), after carbonate brine treatment, while the organic-lean sample showed a slight increase from 18 to 19 uµ. The outcomes of this study will be used to develop a comprehensive understanding of the mechanisms driving the fracture growth and propagation in organic-rich unconventional reservoirs induced by CO₂ treatment, which can inform strategies to enhance oil/gas recovery. They will also aid in defining the potential benefits of CO₂-induced fracturing for underground CO₂ storage, such as improving the permeability and porosity of unconventional rock formations, and for oil recovery, such as enhancing the recovery factor.