AAPG Middle East Region, Shale Gas Evolution Symposium

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Two-Phase Fluid Flow in Source Rocks: Insights Gained from Nanofluidics


Source rocks exhibit two-phase fluid storage and flow behavior that significantly departs from that of conventional reservoirs because of nanometer-size throat confinements. It is important to quantify two-phase flow in source rocks because of its implications on drainage volume and recovery factors via primary or secondary means. The nanometer range of throat sizes present in source rocks causes two-phase flow to be dominated by throat-wall effects which include electrochemical forces and fluid polarity. This presentation describes how nanofluidics experiments have been used to gain quantitative insight to dominant two-phase flow mechanisms taking place in nano confinements. Emphasis is placed on fluid transport taking place between natural fractures and the rock matrix. The study is carried out with imbibition experiments by inferring equivalent nano-channel mobilities after tracking the time advancement of fluid fronts via differential interference contrast (DIC) microscopy. Dynamic image analysis is necessary to extract the position of liquid within the nanochannels from amidst noise in the DIC image stacks, especially in channels less than 100 nm wide. A signature of DIC microscopy is pairs of light and dark signals produced at feature edges. When fluid invades the nanochannels there is a contrast between the wetted and dry edge signals. We orient an image stack so that the channels are horizontal across the frame and extract 2D slices of selected rows of pixels across the stack in the direction of time. Such a dynamic method is opposed to the typical static image analysis workflow of registering the channel geometry and locating the interface in each frame. We observed that imbibition of various wetting liquids in an array of different-sized, horizontal, two-dimensional silica nanochannels terminated within the channels as a function of hydraulic diameter and liquid type. This front termination is not predicted by the classic Washburn equation for capillary flow, which establishes diffusive dynamics in horizontal channels. The atypical imbibition data are explained by deformed menisci and decreased effective channel diameters. Such occurrences are due to the nanoscale-enhanced influence of surface forces, thin films, and elastocapillary and solid surface deformation due to meniscus-induced negative pressures (suction) and material stresses. Results indicate that nano-scale transport is extremely sensitive to specific surface-liquid-gas combinations. Determination of the net effects of interfaces and surface chemistry on hydrodynamic transport is critical to describing and enhancing the success of liquid transport between induced fracture networks and nano-porous shale. Consequently, hydro-fracturing fluids need to be adapted to specific surface chemistry conditions of source rocks to minimize fluid uptake and maximize matrix production.