Publications

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Measuring the size and orientation of borehole breakouts is one of the primary methods for determining the orientation and magnitudes of the in situ stresses in the subsurface. To better understand the effects of anisotropy on borehole breakouts, experiments were conducted on Mancos Shale, a finely laminated mudrock. A novel testing configuration was developed to conduct borehole breakout experiments in a standard triaxial vessel and load frame. Samples were prepared at three different orientations and deformed under 6.9 to 20.7 MPa confining pressure. The results show a variation of peak strength and breakout geometry depending on the lamination orientation. Samples deformed parallel to laminations failed at a higher maximum compressive stress than samples deformed perpendicular to laminations, which were stronger than inclined samples. These relationships are quantified by a cosine-based failure envelope. Observed breakout shapes in perpendicular samples are V-shaped and symmetric around the borehole, which advance as a series of fractures of increasing size into the sidewalls. In inclined samples, fractures form along weaker laminations planes and grow in an en echelon pattern towards the axial stress direction. In parallel samples, long fractures grow from the wellbore towards the axial stress direction. The observed geometries highlight potential sources of error in calculating in situ stresses from borehole breakouts.

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Pore structure is an important parameter to quantify the reservoir rock adsorption capability and diffusivity, both of which are fundamental reservoir properties to evaluate the gas production and carbon sequestration potential for coalbed methane (CBM) and shale gas reservoirs. In this study, we applied small-angle neutron scattering (SANS) to characterize the total and accessible pore structures for two coal and two shale samples. We carried out in situ SANS measurements to probe the accessible pore structure differences under argon, deuterated methane (CD 4 ), and CO 2 penetrations. The results show that the total porosity ranges between 0.25 and 5.8% for the four samples. Less than 50% of the total pores are accessible to CD 4 for the two coals, while more than 75% of the pores were found to be accessible for the two shales. This result suggests that organic matter pores tend to be disconnected compared to mineral matter pores. Argon pressurization can induce pore contraction because of the mechanical compression of the solid skeleton in both the coal and shale samples. Hydrostatic compression has a higher effect on the nanopores of coal and shale with a higher accessible porosity. Both methane and CO 2 injection can reduce the accessible nanopore volume due to a combination of mechanical compression, sorption-induced matrix swelling, and adsorbed molecule occupation. CO 2 has higher effects on sorption-induced matrix swelling and pore filling compared to methane for both the coal and shale samples. Gas densification and pore filling could occur at higher pressures and smaller pore sizes. In addition, the compression and adsorption could create nanopores in the San Juan coal and Marcellus shale drilled core but could have an opposite effect in the other samples, namely, the processes could damage the nanopores in the Hazleton coal and Marcellus shale outcrop.

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Bedrock fracture is a key element of rock erosion and subsequent surface processes. Here, we test the hypothesis that rock's susceptibility to subcritical cracking, a specific type of fracturing, significantly drives and limits rock erosion. We measured 10Be-derived erosion rates, compressive strength, and crack characteristics on 20 outcrops of different rock units (quartzite, granite, and two metasandstones) in the northern Blue Ridge Mountains of Virginia (USA). We also measured the subcritical cracking index (n), Charles's law velocity constant (A), and fracture toughness (KIC) of samples from four of the same outcrops, representative of each rock type. Erosion rates range from 1.16 ± 0.67 to 32.3 ± 7.8 m/m.y. We find strong correlations- across the four rock units-between average erosion rates and the three subcritical cracking parameters (R2 > 0.85, p < 0.05), but not compressive strength (R2 = 0.6; p > 0.1). We also find a correlative relationship between n and outcrop fracture length (R2 = 0.91; p < 0.05). The latter correlation is consistent with that of published model predictions, further indicating a mechanistic link between subcritical cracking and rock erosion. We infer that subcritical cracking parameters closely tie to erosion rates, because subcritical cracking is the dominant process of mechanical weathering, leading to positive feedbacks relating subcritical cracking rates, crack length, porosity, and water accessibility. These data are the first that directly test and support the hypothesis that subcritical cracking can set the pace of long-term rock erosion.

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The state of stress in the earth is complicated and it is difficult to determine all three components and directions of the stress. However, the state of stress affects all activities which take place in the earth, from causing earthquakes on critically stressed faults, to affecting production from hydraulically fractured shale reservoirs, to determining closure rates around a subterranean nuclear waste repository. Current state of the art methods commonly have errors in magnitude and direction of up to 40%. This is especially true for the intermediate principal stress. This project seeks to better understand the means which are used to determine the state of stress in the earth and improve upon current methods to decrease the uncertainty in the measurement. This is achieved by a multipronged experimental investigation which is closely coupled with advanced constitutive and numeric modeling.

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In situ measurements of geological materials under compression and with hydrostatic fluid pressure are important in understanding their behavior under field conditions, which in turn provides critical information for application-driven research. In particular, understanding the role of nano- to micro-scale porosity in the subsurface liquid and gas flow is critical for the high-fidelity characterization of the transport and more efficient extraction of the associated energy resources. In other applications, where parts are produced by the consolidation of powders by compression, the resulting porosity and crystallite orientation (texture) may affect its in-use characteristics. Small-angle neutron scattering (SANS) and ultra SANS are ideal probes for characterization of these porous structures over the nano to micro length scales. Here we show the design, realization, and performance of a novel neutron scattering sample environment, a specially designed compression cell, which provides compressive stress and hydrostatic pressures with effective stress up to 60 MPa, using the neutron beam to probe the effects of stress vectors parallel to the neutron beam. We demonstrate that the neutron optics is suitable for the experimental objectives and that the system is highly stable to the stress and pressure conditions of the measurements.

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CO₂ geological storage in saline aquifers results in acidification of resident brine. Chemical reactions between acidified brine and rock minerals lead to dissolution and precipitation of minerals at various time scales. Mineral dissolution and precipitation are often neglected in assessing the mechanical integrity of target storage formations, yet, changes in rock strength and deformational behavior can impact trapping mechanisms. This paper shows the impact of exposure to CO₂-charged brine on shear strength and stiffness of various outcrop rocks evaluated through triaxial testing. The tested rocks were exposed to CO₂-charged brine over geological time at a naturally occurring near-surface seepage along the Little Grand Wash Fault and Salt Wash Grabens, which include the Crystal Geyser site near the town of Green River, Utah. Prior work suggests that this site provides a near-surface structural analog for possible fault-controlled CO₂ leakage over time scales that exceed expected injection time scales (10–100 years). Results show mechanical alteration in various aspects: (1) CO₂-charged brine alteration at near-surface conditions results in mineral dissolution/precipitation and reduction of shear strength and brittleness of Entrada sandstone and Summerville siltstone samples, and (2) carbonate precipitation in fractured Mancos shale leads to matrix stiffening and fracture mineralization resulting in overall stiffer and likely tighter shale. Additional discrete element simulations coupled with a bonded-particle-model confirm the role of cement bond size alteration as one of the main controls for rock chemo-mechanical alteration in sandstones. The chemo-mechanical alteration path that mimics cement dissolution (under stressed subsurface conditions) results in vertical compaction and lateral stress relaxation. Overall, results show that rock exposure to CO₂-charged brine can impart distinct petrophysical and geomechanical changes according to rock lithology and location with respect to major CO₂ conduits. Finally, while mineral dissolution in the storage rock may result in undesired reservoir strains and changes of stresses, mineral precipitation downstream from a leakage path can help seal potentially induced fractures.

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Desirable outcomes for geologic carbon storage include maximizing storage efficiency, preserving injectivity, and avoiding unwanted consequences such as caprock or wellbore leakage or induced seismicity during and post injection. To achieve these outcomes, three control measures are evident including pore pressure, injectate chemistry, and knowledge and prudent use of geologic heterogeneity. Field, experimental, and modeling examples are presented that demonstrate controllable GCS via these three measures. Observed changes in reservoir response accompanying CO2 injection at the Cranfield (Mississippi, USA) site, along with lab testing, show potential for use of injectate chemistry as a means to alter fracture permeability (with concomitant improvements for sweep and storage efficiency). Further control of reservoir sweep attends brine extraction from reservoirs, with benefit for pressure control, mitigation of reservoir and wellbore damage, and water use. State-of-the-art validated models predict the extent of damage and deformation associated with pore pressure hazards in reservoirs, timing and location of networks of fractures, and development of localized leakage pathways. Experimentally validated geomechanics models show where wellbore failure is likely to occur during injection, and efficiency of repair methods. Use of heterogeneity as a control measure includes where best to inject, and where to avoid attempts at storage. An example is use of waste zones or leaky seals to both reduce pore pressure hazards and enhance residual CO2 trapping.

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Results of axisymmetric compression tests on weak, porous Castlegate Sandstone (Cretaceous, Utah, USA), covering a range of dilational and compactional behaviors, are examined for localization behavior. Assuming isotropy, bulk and shear moduli evolve as increasing functions of mean stress and Mises equivalent shear stress respectively, and as decreasing functions of work-conjugate plastic strains. Acoustic emissions events located during testing show onset of localization and permit calculation of observed shear and low-angle compaction localization zones, or bands, as localization commences. Total strain measured experimentally partitions into: A) elastic strain with constant moduli, B) elastic strain due to stress dependence of moduli, C) elastic strain due to moduli degradation with increasing plastic strain, and D) plastic strain. The third term is the elastic-plastic coupling strain, and though often ignored, contributes significantly to pre-failure total strain for brittle and transitional tests. Constitutive parameters and localization predictions derived from experiments are compared to theoretical predictions. In the brittle regime, predictions of band angles (angle between band normal and maximum compression) demonstrate good agreement with observed shear band angles. Compaction localization was observed in the transitional regime in between shear localization and spatially pervasive compaction, over a small range of mean stresses. In contrast with predictions, detailed acoustic emissions analyses in this regime show low angle, compaction-dominated but shear-enhanced, localization.

Here, the injection of carbon dioxide (CO₂) into geological formations results in a chemical re-equilibration between the mineral assemblage and the pore fluid, with ensuing mineral dissolution and re-precipitation. Hence, target rock formations may exhibit changes of mechanical and petrophysical properties due to CO₂ exposure. We conducted batch reaction experiments with Entrada Sandstone and Summerville Siltstone exposed to de-ionized water and synthetic brine under reservoir pressure (9–10 MPa) and temperature (80°C) for up to four weeks. Samples originate from the Crystal Geyser field site, where a naturally occurring CO₂ seepage alters portions of these geologic formations. We conducted micro-scratch tests on rock samples without alteration, altered under laboratory conditions, and naturally altered over geologic time. Scratch toughness and hardness decrease as a function of exposure time and water salinity up to 52% in the case of Entrada and 87% in the case of Summerville after CO₂-induced alteration in the laboratory. Imaging of altered cores with SEM-EDS and X-ray microCT methods show dissolution of carbonate and silica cements and matrix accompanied by minor dissolution of Fe-oxides, clays, and other silicates. Parallel experiments using powdered samples confirm that dissolution of carbonate and silica are the primary reactions. The batch reaction experiments in the autoclave utilize a high fluid to rock volume ratio and represent an end member of possible alteration associated with CO₂ storage systems. These types of tests serve as a pre-screening tool to identify the susceptibility of rock facies to CO₂-related chemical-mechanical alteration during long-term CO₂ storage.

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Imaging techniques for the analysis of porous structures have revolutionized our ability to quantitatively characterize geomaterials. Digital representations of rock from CT images and physics modeling based on these pore structures provide the opportunity to further advance our quantitative understanding of fluid flow, geomechanics, and geochemistry, and the emergence of coupled behaviors. Additive manufacturing, commonly known as 3D printing, has revolutionized production of custom parts with complex internal geometries. For the geosciences, recent advances in 3D printing technology may be co-opted to print reproducible porous structures derived from CT-imaging of actual rocks for experimental testing. The use of 3D printed microstructure allows us to surmount typical problems associated with sample-to-sample heterogeneity that plague rock physics testing and to test material response independent from pore-structure variability. Together, imaging, digital rocks and 3D printing potentially enables a new workflow for understanding coupled geophysical processes in a real, but well-defined setting circumventing typical issues associated with reproducibility, enabling full characterization and thus connection of physical phenomena to structure. Here we report on our research exploring the possibilities that these technologies can bring to geosciences for coupled multiscale experimental and numerical analysis using 3D printed fractured rock specimens.

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A series of constant mean stress (CMS) and constant shear stress (CSS) tests were performed to investigate the evolution of permeability and Biot coefficient at high mean stresses in a high porosity reservoir analog (Castlegate sandstone). Permeability decreases as expected with increasing mean stress, from about 20 Darcy at the beginning of the tests to between 1.5 and 0.3 Darcy at the end of the tests (mean stresses up to 275 MPa). The application of shear stress causes permeability to drop below that of a hydrostatic test at the same mean stress. Results show a nearly constant rate decrease in the Biot coefficient as the mean stress increases during hydrostatic loading, and as the shear stress increases during CMS loading. CSS tests show a stabilization of the Biot coefficient after the application of shear stress.

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Greater Aneth oil field, Utah’s largest oil producer, was discovered in 1956 and has produced over 483 million barrels of oil. Located in the Paradox Basin of southeastern Utah, Greater Aneth is a stratigraphic trap producing from the Pennsylvanian (Desmoinesian) Paradox Formation. Because Greater Aneth is a mature, major oil field in the western U.S., and has a large carbonate reservoir, it was selected to demonstrate combined enhanced oil recovery and carbon dioxide storage. The Aneth Unit in the northwestern part of the field has produced over 160 million barrels of the estimated 386 million barrels of original oil in place—a 42% recovery rate. The large amount of remaining oil made the Aneth Unit ideal to enhance oil recovery by carbon dioxide flooding and demonstrate carbon dioxide storage capacity.

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