Insight Gained From Experimental Gas-Cycling Eor in the Unconventional Montney and Duvernay Formations
Authors: Thomas F. Brent , Piwowar Michael , Noroozi Mehdi , Gibb William , Marin Juan , Zhang Hongmei 
Published: August 2020

A radial-flow, matrix-frac experimental procedure was developed to incorporate the mechanisms of Gas-Cycling Enhanced Oil Recovery (GCEOR) phase behavior, IFT change, swelling, viscosity reduction and residual-liquid shrinkage in the presence of porous media possessing propped hydraulic fractures and matrix. Relatively large hydrocarbon pore volumes are possible using this technique whereby effluent compositions, densities and volumes are measured. The importance of rock and fluid properties is investigated along with operating pressure, injection gas composition and levels of primary depletion. More than fifty primary depletions followed by GCEOR Huff and Puff operations have been conducted and some of the more interesting results have been assembled for discussion.

Two dominant flow regimes are incorporated: matrix mass transfer into and from the fracture(s) and flow within the fracture(s). Reservoirs tested exhibited pressures from 3000 to over 5000 psi and temperatures from 140 to 220 F. Design parameters were changed from run to run allowing for insight into GCEOR operation and design. Of particular note is the ability to run different fluid systems in the same porous media in order to breathe insight into the relative importance of geology and phase behavior. Simulation of some experimental results with subsequent scale-up for field forecasting was performed, although not all systems were simulated as yet.

Measured results indicate that recovery of OOIP can be more than doubled compared to primary production, in some cases, by implementing GCEOR. The role of injection gas composition, operating pressure, soak/ Huff time is commented on and appears to change from system to system. Analyzing measured oil flux in the experiments allowed the calculation of experimental Peclet numbers, indicating the relative importance of convection compared to diffusion. From the accumulated data base, the following have been observed:

Cycling pressure should be optimized (highest pressure does not necessarily perform the best). Gas quality can, in some cases, play a major role but should be considered and quantified in GCEOR applications. Soak time/ Huff time may be optimized to maximize production cycles and minimize injection cycles. Gas utilization values, for well-designed GCEOR systems, are low compared to conventional continuous gas injection projects causing Huff and Puff GCEOR to approach gas storage performance. Gas utilization appears to be sensitive to the mechanisms at work in GCEOR. Less depletion before GCEOR initiation may accelerate recovery and may, in some cases, access residual oil that was not produced at higher levels of primary depletion. However, significant increases in recovery factor have not been observed with decreased degree of depletion on primary production. Contrary to expectation, the rock character may dominate GCEOR performance. In a subset of this testing, the rock heterogeneity had a more dominant role than fluid properties including miscibility. It appears from this ongoing testing that design of GCEOR projects may be dominated by different parameters from field to field, and possibly well to well. Simulation would provide a good approach in order to bridge the experimental measurements to field design. Experimental GCEOR measurements provide an objective means in order to calibrate the mathematical models in order to forecast field GCEOR upside.