Impact of Authigenic Surface Roughness on Water Invasion and Flowback in Fractured Media: A Micromodel Study

Invasion distance and flowback of fracture fluids following hydraulic fracturing are directly related to two-phase capillary trapping of immiscible fluids in rocks. Surface roughness occurs in fractured porous media due to geologic processes such as cementation and chemical dissolution. The effects of surface roughness on two-phase flow are currently not well understood. We conducted spontaneous and forced imbibition experiments in porous media micromodels as analogs for fracture fluid invasion. Drainage experiments were run to simulate return-of-the-fracture fluid subsequent to fracturing. Smooth and rough glass micromodels were fabricated with percolating and nonpercolating microfracture networks. Light microscopy and image analysis were utilized to quantify capillary-trapped saturations during two-phase flow experiments. Experiments were conducted until steady state was reached via the injection of hundreds of pore volumes of wetting or nonwetting fluid. At these end point conditions, we determined that surface roughness had a marginal¹ effect on hydrocarbon trapping (less than 10%) during imbibition and drainage when viscous forces were dominant (intermediate to high values, Ca > 10⁻⁵). Such a viscous-dominated and swept environment is found near a wellbore following hydraulic fracturing, although transport beyond this environment is likely capillary dominated (the extent of fluid imbibition will depend on rock quality). Capillary-dominated drainage was not dependent on the presence of surface roughness in our experiments. The effect of microfracture percolation on immiscible fluid trapping was found to be negligible as well. To the authors' knowledge, this is the first systematic quantitative study to couple surface roughness with microfracture connectivity using micromodels. With regard to field applications, results indicate that surface roughness-generating geologic processes such as cementation and dissolution and microfracture propagation - all reservoir quality/lithofacies-dependent variables that impact porosity and permeability- do not likely impact hydrocarbon and fracture fluid trapping during invasion and flowback near wellbores and major fracture conduits. This conclusion indicates that investigations of surface chemistry-driven fluid trapping mechanisms, rather than geometric, may be critical.