The main objectives of this project are to quantify the changes in fracture porosity and multiphase transport properties as a function of confining stress. These changes will be integrated into conceptual and numerical models that will improve our ability to predict and optimize fluid transport in fractured system. This report details our progress on a. developing the direct experimental measurements of fracture aperture and topology using high-resolution x-ray microtomography, b. modeling of fracture permeability in the presence of asperities and confining stress, and c. simulation of two-phase fluid flow in a fracture and a layered matrix.The three-dimensional surface that describes the large-scale structure of the fracture in the porous medium can be determined using x-ray micro-tomography with significant accuracy. The distribution of fracture aperture is a difficult issue that we are studying and developing methods of quantification. The difficulties are both numerical and conceptual. Numerically, the threedimensional data sets include millions, and sometimes, billions of points, and pose a computational challenge. The conceptual difficulties derive from the rough nature of the fracture surfaces, and the heterogeneous nature of the rock matrix. However, the high-resolution obtained by the imaging system provides us a much needed measuring environment on rock samples that are subjected to simultaneous fluid flow and confining stress. Pilot multi-phase experiments have been performed, proving the ability to detect two phases in certain large fractures.The absolute permeability of a fracture depends on the behavior of the asperities that keep it open. A model is being developed that predicts the permeability and average aperture of a fracture as a function of time under steady flow of water including the pressure solution at the asperity contact points.
The main objectives of this project are to quantify the changes in fracture porosity and multi-phase transport properties as a function of confining stress. These changes will be integrated into conceptual and numerical models that will improve our ability to predict and optimize fluid transport in fractured system. This report details our progress on: a. developing the direct experimental measurements of fracture aperture and topology and fluid occupancy using high-resolution x-ray micro-tomography, b. The three-dimensional surface that describes the large-scale structure of the fracture in the porous medium can be determined using x-ray micro-tomography with significant accuracy. Several fractures have been scanned and the fracture aperture maps have been extracted. The success of the mapping of fracture aperture was followed by measuring the occupancy of the fracture by two immiscible phases, water and decane, and water and kerosene.The distribution of fracture aperture depends on the effective confining stress, on the nature of the rock, and the type and distribution of the asperities that keep the fracture open. Fracture apertures at different confining stresses were obtained by micro-tomography covering a range of about two thousand psig.Initial analysis of the data shows a significant aperture closure with increase in effective confining stress.Visual and detailed descriptions of the process are shown in the report. Both extensional and shear fractures have been considered.A series of water imbibition tests were conducted in which water was injected into a fracture and its migration into the matrix was monitored with CT and DR x-ray techniques. The objective was to iv understand the impact of the fracture, its topology and occupancy on the nature of mass transfer between the matrix and the fracture. Counter-current imbibition next to the fracture was observed and quantified, including the influence of formation layering.A group of Shear fractures were studied, with layers perpendicular and parallel to t he main axis of the sample. The structures of the fractures as well as their impact on absolute permeability and on oil displacement by water were evaluated. Shear fractures perpendicular to the layers lead to a wide distribution of pores and to an overall increase in absolute permeability. Shear fractures parallel to the layers lead to an overall increase in absolute permeability, but a decrease in displacement efficiency. This DoE project funded or partially funded three Ph.D. and four M.Sc. students at the Pennsylvania State University. The results from the research have yielded several abstracts, presentations and papers. Much of the work is still in the process of being published.
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