Evaluation of the EOR Potential in Shale Oil Reservoirs By CO2 Miscible Displacement Applied in Modified Zipper Fractured Horizontal Wells

Unconventional shale oil/gas reservoirs exist over large quantities in the United States. The success of horizontal wells drilling and multistage hydraulic fracturing technique make it economically viable to produce shale gas reservoirs. However, the natural depletion recovery of shale gas or oil reservoirs is only around 5-10%.  This low recovery rate and the abundance of shale oil or gas plays provide huge potential for enhanced oil recovery by gas injection method in fractured reservoirs.

Modified zipper frac technique is developed in a manner different from zipper frac in which the fractures are stimulated in a staggered pattern.  The benefit of the modified zipper frac is that it will improve the contact area with the reservoir and increase the effective stimulated volume. Studies showed that enhancing fracture complexities in shale gas resources is critical to improving stimulation treatment and well production performance. CO₂ injection enhanced oil recovery (EOR) process under the miscible flooding condition can significantly reduce oil viscosity. Oil viscosity reduction combined with the increased contact area by hydraulic fractures could be the dominant recovery mechanism. Problems associated with gas injection in conventional well patterns such as early breakthrough and channeling through high permeability zones will not likely happen in nano-scale permeable shale oil or gas reservoirs.

 In this paper, we propose miscible CO₂ injection to enhance gas condensate recovery in a horizontal well pair, a gas injection well and a production well, which is stimulated in a modified zipper frac pattern. The approach integrates the advantages of hydraulic zipper fracturing, horizontal wells and miscible gas flooding. Miscible gas flooding has shown the IOR potential in shale oil reservoirs in this simulation and experiment work. We develop a compositional model to simulate complex interactions between the injected gas and reservoir fluids that were not modeled precisely by black-oil simulation. Our simulation results of the Eagle Ford Shale gas condensate indicate that the secondary recovery increased to 86% for 4000 days of secondary production from the 39% of 5 years primary recovery in a 100-ft fracture spacing staggered zipper fracture pattern. The investigation of CO₂ injection in a modified zipper fractured horizontal well pair provides an insight into the EOR performance in nano-pore shale oil reservoirs.