Laboratory evaluation of tight oil production processes requires a unique coreflood apparatus that portrays both the tight rock matrix and fractures—a dual-permeability system. However, with some tight formations such as the Bakken having no available outcrop, full-sized actual cores are very difficult and sometimes impossible to obtain. In this study, a large-volume coreflood apparatus was developed to incorporate the detailed physics of ultra-tight, dual-porosity, dual-permeability flow in porous media. Also, large synthetic cores were made to represent the permeability, porosity, and mineralogy of actual tight reservoir rocks. To generate the desired dual-permeability system, fractures are simulated by creating sand-filled branched pathways in the large synthetic core. During a coreflood test, fluids are injected through the peripheral high-permeability belt into the tight matrix and recovered through the central fracture. This simulates the flow of matrix to fracture, or vice versa, seen in an ultra-tight reservoir. As well, either continuous gas/water injection or cyclic gas injection modes can be tested with the model. Two large-volume dual-permeability corefloods were conducted to study the effectiveness of immiscible field-produced gas flooding using recombined reservoir oil and synthetic tight cores. The corefloods simulated a practical field production sequence that included primary recovery (pressure depletion), injection of field-produced gas, pressurizing and soaking with this gas, and pressure depletion. The higher-recovery run produced 28.3% original oil in place (OOIP) including primary and tertiary (enhanced) processes. The dual-permeability corefloods demonstrated more representative performance of actual field operations than do traditional one-dimensional corefloods.

The new matrix–fracture coreflood system better reflects the extreme permeability contrast between matrix and fractures that characterizes tight oil reservoirs. Therefore, history matching of its results using a numerical simulator is expected to provide much better representation in scaling up and predicting realistic field operations.

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