Alterations of Fractures in Carbonate Rocks by CO2-Acidified Brines.

Fractures in geological formations may enable migration of environmentally relevant fluids, as in leakage of CO2 through caprocks in geologic carbon sequestration. We investigated geochemically induced alterations of fracture geometry in Indiana Limestone specimens. Experiments were the first of their kind, with periodic high-resolution imaging using X-ray computed tomography (xCT) scanning while maintaining high pore pressure (100 bar). We studied two CO2-acidified brines having the same pH (3.3) and comparable thermodynamic disequilibrium but different equilibrated pressures of CO2 (PCO2 values of 12 and 77 bar). High-PCO2 brine has a faster calcite dissolution kinetic rate because of the accelerating effect of carbonic acid. Contrary to expectations, dissolution extents were comparable in the two experiments. However, progressive xCT images revealed extensive channelization for high PCO2, explained by strong positive feedback between ongoing flow and reaction. The pronounced channel increasingly directed flow to a small region of the fracture, which explains why the overall dissolution was lower than expected. Despite this, flow simulations revealed large increases in permeability in the high-PCO2 experiment. This study shows that the permeability evolution of dissolving fractures will be larger for faster-reacting fluids. The overall mechanism is not because more rock dissolves, as would be commonly assumed, but because of accelerated fracture channelization.

Fracture-matrix fluid exchange in oil-bearing unconventional mudstones.

The poromechanical properties of unconventional reservoir materials are in large part dictated by their mineralogy. Since these properties govern the response to stress experienced during hydraulic fracturing, fluid production, and fluid injection, they play a central role in the formation of microcracks or bedding delaminations which ultimately dominate mass transport. In this work we study access to the porosity of end member unconventional reservoir materials, where the end members are predominantly dictated by carbonate content. Access to the porosity is quantified using state of the art 3D x-ray computed tomography coupled with physics informed data analytics. Xenon gas, which attenuates x-rays, provides a spatiotemporal map of access to the porosity. The accessible porosity is quantified over a range of net confining stress relevant to the manmade disturbances listed above. These experiments demonstrate that heavily carbonated mudstones are nearly impermeable at the core (~ cm) scale, while carbonate free analogues afford better access to the microstructure. Consistent with previous qualitative 2D radiographs, access to the interior of the clastic mudstones is first observed along planar microcracks, followed by slow penetration into the surrounding matrix. Physics informed data analytics of the 3D tomography measurements presented here show that these microcracks do not permit uniform access to the adjacent rock matrix. In addition, variation of the effective pressure elucidates the mechanisms that govern fracture/matrix fluid exchange. Under conditions consistent with hydrocarbon production fluid accumulates in the immediate vicinity of the nearest microcrack. While there is clear evidence that, as intended, part of this accumulation is from the more distant matrix, fluid is also squeezed out of the microcrack. The fluid build-up at the microcrack indicates that migration out of the rock is hindered by the coupled poroelastic response of the microcrack and adjacent rock matrix. We show that these mechanisms ultimately account for the meager oil recovery factors realized in practice. These insights have implications for making reservoir scale predictions based on core scale observations, and provide a basis for devising new asset development techniques to access more porosity, and enhance fluid extraction. Finally, these findings shed light on key features and mechanisms that govern shale storage capacity, with relevance to other important industrial processes, such as geologic CO2 storage.

Dataset documenting reaction-induced changes to five fractured foamed wellbore cement cores during CO2 fluid flow.

The integrity of wellbore cement is vital for the long-term success of applications such as enhanced oil recovery and carbon storage. Intact cemented well casings are crucial to preventing leakage and fluid migration, as well as maintaining safety of operations. To investigate the changes to fractures in foamed wellbore cement in a carbon storage scenario, four cores were fractured lengthwise and injected with deionized water at equilibrium with CO2. The experiment duration was five days for the first core and was increased for each successive test, with the final test lasting 20 days. The fractured cores were periodically imaged with a NorthStar M5000 Industrial Computed Tomography (CT) scanner, documenting the changes to the fracture during dissolution, as well as the reaction zone in the surrounding cement matrix. For two cores with the most robust reactions, the fracture and two reaction zones (proximal and distal to the fracture) were segmented from the raw CT data. They were quantified volumetrically and in the form of fracture aperture maps. A Local Cubic Law (LCL) modeling suite was used to map out localization of flow within the open portions of the fractures.

A new stereolithography experimental porous flow device.

A new method for constructing laboratory-scale porous media with increased pore-level variabilities for two-phase flow experiments is presented here. These devices have been created with stereolithography directly on glass, thus improving the stability of the model created with this precision rapid construction technique. The method of construction and improved parameters are discussed in detail, followed by a brief comparison of two-phase drainage results for air invasion into the water-saturated porous medium. Flow through the model porous medium is shown to substantiate theoretical fractal predictions.

Design and implementation of a shearing apparatus for the experimental study of shear displacement in rocks.

Fluid flow in the subsurface is not well understood in the context of "impermeable" geologic media. This is especially true of formations that have undergone significant stress fluctuations due to injection or withdrawal of fluids that alters the localized pressure regime. When the pressure regime is altered, these formations, which are often already fractured, move via shear to reduce the imbalance in the stress state. While this process is known to happen, the evolution of these fractures and their effects on fluid transport are still relatively unknown. Numerous simulation and several experimental studies have been performed that characterize the relationship between shearing and permeability in fractures; while many of these studies utilize measurements of fluid flow or the starting and ending geometries of the fracture to characterize shear, they do not characterize the intermediate stages during shear. We present an experimental apparatus based on slight modifications to a commonly available Hassler core holder that allows for shearing of rocks, while measuring the hydraulic and mechanical changes to geomaterials during intermediate steps. The core holder modification employs the use of semi-circular end caps and structural supports for the confining membrane that allow for free movement of the sheared material while preventing membrane collapse. By integrating this modified core holder with a computed tomography scanner, we show a new methodology for understanding the interdependent behavior between fracture structure and flow properties during intermediate steps in shearing. We include a case study of this device function which is shown here through shearing of a fractured shale core and simultaneous observation of the mechanical changes and evolution of the hydraulic properties during shearing.

Two-phase flow in a rough fracture: experiment and modeling.

To develop and test theory-based procedures for modeling two-phase flow through fractures, it is important to be able to compare computational results for a fracture with experiments performed on the exact same fracture. Unfortunately for real fractures, any attempt to image the fracture and to produce a numerical model of the fracture accessible to computer modeling unavoidably results in a coarsening of the resolution, with the very small-scale features of the imaged fracture averaged to produce the numerical representation used in modeling. Contrary to the hope that these high-resolution features would be unimportant, several modeling efforts have shown that such changes in resolution do affect the flow. Therefore, the numerical representation is different from the real fracture because of this unavoidable coarsening of the resolution. To remove the problems caused by the use of different fractures in the experiment and in the model, the fracture used in our experiments was stereographically constructed from the same numerical representation used in the modeling so that the only difference between the experimental "fracture" and the modeling "fracture" is a manufacturing error of approximately 3% or less in the aperture sizes of the manufactured experimental model. Using several models not unlike others in the literature, we modeled injection of air into the water-saturated fracture. The modeling results are compared to experimental results for injection of air into the water-saturated stereolithographically constructed fracture. A comparison between modeling and experimental results for the essentially identical fractures shows a much better detailed agreement than obtained in other studies, which compared experimental flows on the real fracture with modeling results for a lower resolution representation of the real fracture. This suggests that many of the differences between experiment and modeling in previous work resulted from the differences between the experimental and modeling fractures. For our low capillary-number cases, the best agreement with experiment is for a modification of invasion percolation with trapping (IPwt) that included approximations to viscosity ratio effects and to the interfacial tension effects in reducing very short-range curvature.