Dustin Crandall

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.

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.

Foamed Cement Analysis With Computed Tomography

Foamed cements are widely used for cementing oil or gas wells that require lightweight slurries, gas migration prevention, or wells in high-stress environments. When this manufactured slurry solidifies in the sub-surface environment the distribution of gas voids can affect the resultant strength, permeability, and stability of the wellbore casing. Researchers at the National Energy Technology Laboratory have produced the first high-resolution X-ray computed tomography (CT) three-dimensional images of atmospheric and field generated foamed cement across a range of foam qualities. CT imaging enabled the assessment and quantification of the foamed cement structure, quality, and bubble size distribution in order to provide a better understanding of this cement. Ultimately, this research will provide industry the knowledge to ensure long-term well integrity and safe operation of wells in which foamed cements are used. Initial results show that a systematic technique for isolating air voids can give consistent results from the image data, laboratory generated foamed cements tend to be uniform, and that high-gas fraction foamed cements have large interconnected void spaces.Copyright

Modeling Two-Phase Flow in Porous Media Including Fluid-Fluid Interfacial Area

We present a new numerical model for macro-scale two-phase flow in porous media which is based on a physically consistent theory of multi-phase flow. The standard approach for modeling the flow of two fluid phases in a porous medium consists of a continuity equation for each phase, an extended form of Darcy’s law as well as constitutive relationships for relative permeability and capillary pressure. This approach is known to have a number of important shortcomings and, in particular, it does not account for the presence and role of fluid–fluid interfaces. An alternative is to use an extended model which is founded on thermodynamic principles and is physically consistent. In addition to the standard equations, the model uses a balance equation for specific interfacial area. The constitutive relationship for capillary pressure involves not only saturation, but also specific interfacial area. We show how parameters can be obtained for the alternative model using experimental data from a new kind of flow cell and present results of a numerical modeling study.© 2008 ASME

Effects of Atmospheric Dynamics on CO2 Seepage at Mammoth Mountain, California USA

In the past few decades, atmospheric effects on the variation of seepage from soil have been studied in disciplines such as volcanology, environmental protection, safety and health hazard avoidance. Recently, monitoring of potential leakage from the geologic sequestration of carbon has been added to this list. Throughout these diverse fields, barometric pumping and presence of steady winds are the two most commonly investigated atmospheric factors. These two factors have the effect of pumping gas into and out of the unsaturated zone, and sweeping the gas in the porous medium. This study focuses on two new factors related to atmosphere in order to explain the CO2 seepage anomalies observed at the Horseshoe Lake tree kill near Mammoth Mountain, CA, where the temporal variation of seepage due to a storm event could not be explained by the two commonly studied effects. First, the interaction of the lower atmospheric dynamics and the ground topography is considered for its effect on the seepage variation over ...

Modeling of Immiscible, Two-Phase Flows in a Natural Rock Fracture

One potential method of geologically sequestering carbon dioxide (CO2 ) is to inject the gas into brine-filled, subsurface formations. Within these low-permeability rocks, fractures exist that can act as natural fluid conduits. Understanding how a less viscous fluid moves when injected into an initially saturated rock fracture is important for the prediction of CO2 transport within fractured rocks. Our study examined experimentally and numerically the motion of immiscible fluids as they were transported through models of a fracture in Berea sandstone. The natural fracture geometry was initially scanned using micro-computerized tomography (CT) at a fine volume-pixel (voxel) resolution by Karpyn et al. [1]. This CT scanned fracture was converted into a numerical mesh for two-phase flow calculations using the finite-volume solver FLUENT® and the volume-of-fluid method. Additionally, a translucent experimental model was constructed using stereolithography. The numerical model was shown to agree well with experiments for the case of a constant rate injection of air into the initially water-saturated fracture. The invading air moved intermittently, quickly invading large-aperture regions of the fracture. Relative permeability curves were developed to describe the fluid motion. These permeability curves can be used in reservoir-scale discrete fracture models for predictions of fluid motion within fractured geological formations. The numerical model was then changed to better mimic the subsurface conditions at which CO2 will move into brine saturated fractures. The different fluid properties of the modeled subsurface fluids were shown to increase the amount of volume the less-viscous invading gas would occupy while traversing the fracture.Copyright

Insights from the Marcellus Shale Energy and Environment Laboratory (MSEEL)

Timothy R Carr, Thomas H. Wilson, Payam Kavousi, Shohreh Amini, Shikha Sharma, West Virginia University: Jay Hewitt, Ian Costello, B. J. Carney, Emily Jordon, Northeast Natural Energy LLC: Malcolm Yates, Keith MacPhail, Natalie Uschner, Mandy Thomas, Si Akin, Oluwaseun Magbagbeola, Adrian Morales, Asbjoern Johansen, Leah Hogarth, Olatunbosun Anifowoshe, Kashif Naseem, Schlumberger: Richard Hammack, Abhash Kumar, Erich Zorn, Robert Vagnetti, and Dustin Crandall, National Energy Technology Laboratory, US Department of Energy.

In Situ Contact Angle Measurements of Supercritical CO2, Brine, and Sandstone Cores Using Micro-CT Imaging

Accurate descriptions of contact angles and fluid wettability are crucial to the understanding of multiphase behavior and storage efficiency factors for potential Geologic Carbon Storage (GCS) reservoirs. The contact angle is the angle between the fluid-fluid interfaces with a solid. Contact angle measurements on non-flat surfaces are challenging and not traditionally obtained. Tests are underway at the U.S. Department of Energy National Energy Technology Laboratory (NETL) building upon previous experiments that seek to utilize micro-CT scans to visually measure contact angles and determine how samples behave under various pressure, temperature, and geochemical settings. Before and after threedimensional (3D) images of a sample at residual CO2 saturation reveal the space occupied by supercritical CO2 (scCO2). These volumes of CO2 are isolated and the contact angle of the CO2 is shown to be variable within this space due to additional forces acting upon the trapped bubbles. Careful selection of a region of interest for measuring contact angles is of utmost importance to extrapolate a representative population of contact angles from a single volume [1]. This research incorporates conditions more representative of those found in a GCS reservoir to determine accurate contact angle measurements.

Dynamic Imaging of Multiphase Flows in Rock Using Computed Tomography

Understanding the mechanisms for multiphase flow within the subsurface is critical to the planning and execution of a multitude of energy related projects including, enhanced oil recovery, geologic carbon sequestration, geothermal energy extraction, and gas production from tight shale reservoirs. This paper provides a brief review of the use of X-ray computed tomography (CT) scanning to visualize multiphase flows within geologically relevant rock cores, and then provides recent examples of multiphase flow at the National Energy Technology Laboratory. With modern CT scanning techniques we show how it is possible to visualize dynamic flooding of cores with fluids, as well as calculate changes in the saturation within the porous matrix from image analysis of the scans.Copyright

Measurement of Interfacial Area Production and Permeability Within Porous Media

An understanding of the pore-level interactions that affect multi-phase flow in porous media is important in many subsurface engineering applications, including enhanced oil recovery, remediation of dense non-aqueous liquid contaminated sites, and geologic CO2 sequestration. Standard models of two-phase flow in porous media have been shown to have several shortcomings, which might partially be overcome using a recently developed model based on thermodynamic principles that includes interfacial area as an additional parameter. A few static experimental studies have been previously performed, which allowed the determination of static parameters of the model, but no information exists concerning the interfacial area dynamic parameters. A new experimental porous flow cell that was constructed using stereolithography for two-phase gas-liquid flow studies was used in conjunction with an in-house analysis code to provide information on dynamic evolution of both fluid phases and gas-liquid interfaces. In this paper, we give a brief introduction to the new generalized model of two-phase flow model and describe how the stereolithography flow cell experimental setup was used to obtain the dynamic parameters for the interfacial area numerical model. In particular, the methods used to determine the interfacial area permeability and production terms are shown.