Timothy J. Kneafsey

Dewetting of Silica Surfaces upon Reactions with Supercritical CO2 and Brine: Pore-Scale Studies in Micromodels

Wettability of reservoir minerals and rocks is a critical factor controlling CO(2) mobility, residual trapping, and safe-storage in geologic carbon sequestration, and currently is the factor imparting the greatest uncertainty in predicting capillary behavior in porous media. Very little information on wettability in supercritical CO(2) (scCO(2))-mineral-brine systems is available. We studied pore-scale wettability and wettability alteration in scCO(2)-silica-brine systems using engineered micromodels (transparent pore networks), at 8.5 MPa and 45 °C, over a wide range of NaCl concentrations up to 5.0 M. Dewetting of silica surfaces upon reactions with scCO(2) was observed through water film thinning, water droplet formation, and contact angle increases within single pores. The brine contact angles increased from initial values near 0° up to 80° with larger increases under higher ionic strength conditions. Given the abundance of silica surfaces in reservoirs and caprocks, these results indicate that CO(2) induced dewetting may have important consequences on CO(2) sequestration including reducing capillary entry pressure, and altering quantities of CO(2) residual trapping, relative permeability, and caprock integrity.

Challenges, Uncertainties, and Issues Facing Gas Production From Gas-Hydrate Deposits

Challenges, Uncertainties and Issues Facing Gas Production From Gas Hydrate Deposits G.J. Moridis, SPE, Lawrence Berkeley National Laboratory; T.S. Collett, SPE, US Geological Survey; M. Pooladi- Darvish, SPE, University of Calgary and Fekete; S. Hancock, SPE, RPS Group; C. Santamarina, Georgia Institute of Technology; R. Boswell, US Department of Energy; T. Kneafsey, J. Rutqvist and M. B. Kowalsky, Lawrence Berkeley National Laboratory; M.T. Reagan, SPE, Lawrence Berkeley National Laboratory; E.D. Sloan, SPE, Colorado School of Mines; A.K. Sum and C. A. Koh, Colorado School of Mines Abstract The current paper complements the Moridis et al. (2009) review of the status of the effort toward commercial gas production from hydrates. We aim to describe the concept of the gas hydrate petroleum system, to discuss advances, requirement and suggested practices in gas hydrate (GH) prospecting and GH deposit characterization, and to review the associated technical, economic and environmental challenges and uncertainties, including: the accurate assessment of producible fractions of the GH resource, the development of methodologies for identifying suitable production targets, the sampling of hydrate-bearing sediments and sample analysis, the analysis and interpretation of geophysical surveys of GH reservoirs, well testing methods and interpretation of the results, geomechanical and reservoir/well stability concerns, well design, operation and installation, field operations and extending production beyond sand-dominated GH reservoirs, monitoring production and geomechanical stability, laboratory investigations, fundamental knowledge of hydrate behavior, the economics of commercial gas production from hydrates, and the associated environmental concerns. Introduction Background. Gas hydrates (GH) are solid crystalline compounds of water and gaseous substances described by the general chemical formula G•N H H 2 O, in which the molecules of gas G (referred to as guests) occupy voids within the lattices of ice- like crystal structures. Gas hydrate deposits occur in two distinctly different geographic settings where the necessary conditions of low temperature T and high pressure P exist for their formation and stability: in the Arctic (typically in association with permafrost) and in deep ocean sediments (Kvenvolden, 1988). The majority of naturally occurring hydrocarbon gas hydrates contain CH 4 in overwhelming abundance. Simple CH 4 - hydrates concentrate methane volumetrically by a factor of ~164 when compared to standard P and T conditions (STP). Natural CH 4 -hydrates crystallize mostly in the structure I form, which has a hydration number N H ranging from 5.77 to 7.4, with N H = 6 being the average hydration number and N H = 5.75 corresponding to complete hydration (Sloan and Koh, 2008). Natural GH can also contain other hydrocarbons (alkanes C  H 2+2 ,  = 2 to 4), but may also contain trace amounts of other gases (mainly CO 2 , H 2 S or N 2 ). Although there has been no systematic effort to map and evaluate this resource on a global scale, and current estimates of in-place volumes vary widely (ranging between 10 15 to 10 18 m 3 at standard conditions), the consensus is that the worldwide quantity of hydrocarbon within GH is vast (Milkov, 2004; Boswell and Collett, 2010). Given the sheer magnitude of the resource, ever increasing global energy demand, and the finite volume of conventional fossil fuel resources, GH are emerging as a potential energy source for a growing number of nations. The attractiveness of GH is further enhanced by the environmental desirability of natural gas, as it has the lowest carbon intensity of all fossil fuels. Thus, the appeal of GH accumulations as future hydrocarbon gas sources is rapidly increasing and their production potential clearly demands technical and economic evaluation. The past decade has seen a marked acceleration in gas hydrate R&D, including both a proliferation of basic scientific endeavors as well as the strong emergence of focused field studies of GH occurrence and resource potential, primarily within national GH programs (Paul et al., 2010). Together, these efforts have helped to clarify the dominant issues and challenges facing the extraction of methane from gas hydrates. A review paper by Moridis et al. (2009) summarized the status of the effort for production from gas hydrates. The authors discussed the distribution of natural gas hydrate accumulations, the status of the primary international research and development R&D programs (including current policies, focus and priorities), and the remaining science and technological challenges facing commercialization of production. After a brief examination of GH accumulations that are well characterized and appear to be models for future development and gas production, they analyzed the role of numerical simulation in the assessment of the hydrate production potential, identified the data needs for reliable predictions, evaluated the status of knowledge with regard to these needs, discussed knowledge gaps and their impact, and reached the conclusion that the numerical simulation capabilities are quite advanced and that the related gaps are either not significant or are being addressed. Furthermore, Moridis et al. (2009) reviewed the current body of literature relevant to potential productivity from different types of GH deposits, and determined that there are consistent indications of a large production potential at high rates over long periods from a wide variety of GH deposits. Finally, they identified (a) features, conditions, geology and techniques that are desirable in the selection of potential production targets, (b) methods to maximize production, and (c) some of the conditions and characteristics that render certain GH deposits undesirable for production.

Physical property changes in hydrate‐bearing sediment due to depressurization and subsequent repressurization

Physical property changes in hydrate-bearing sediment due to depressurization and subsequent repressurization W.F. Waite 1 , T.J. Kneafsey 2 , W.J. Winters 1 , D.H. Mason 1 U.S. Geological Survey, 384 Woods Hole Road, Woods Hole, MA 02543, USA Lawrence Berkeley National Laboratory, 1 Cyclotron Rd., MS 90R1116, Berkeley, CA 94720, USA

Porosity, permeability and fluid flow in the Yellowstone Geothermal System, Wyoming

Cores from two of 13 U.S. Geological Survey research holes at Yellowstone National Park (Y-5 and Y-8) were evaluated to characterize lithology, texture, alteration, and the degree and nature of fracturing and veining. Porosity and matrix permeability measurements and petrographic examination of the cores were used to evaluate the effects of lithology and hydrothermal alteration on porosity and permeability. The intervals studied in these two core holes span the conductive zone and the upper portion of the convective geothermal reservoir. Variations in porosity and matrix permeability observed in the Y-5 and Y-8 cores are primarily controlled by lithology. Y-8 intersects three distinct lithologies: volcaniclastic sandstone, perlitic rhyolitic lava, and non-welded pumiceous ash-flow tuff. The sandstone typically has high permeability and porosity, and the tuff has very high porosity and moderate permeability, while the perlitic lava has very low porosity and is essentially impermeable. Hydrothermal self-sealing appears to have generated localized permeability barriers within the reservoir. Changes in pressure and temperature in Y-8 correspond to a zone of silicification in the volcaniclastic sandstone just above the contact with the perlitic rhyolite; this silicification has significantly reduced porosity and permeability. In rocks with inherently low matrix permeability (such as densely welded ash-flow tuff), fluid flow is controlled by the fracture network. The Y-5 core hole penetrates a thick intracaldera section of the 0.6-Ma Lava Creek ash-flow tuff. In this core, the degree of welding appears to be responsible for most of the variations in porosity, matrix permeability, and the frequency of fractures and veins. Fractures are most abundant within the more densely welded sections of the tuff. However, the most prominent zones of fracturing and mineralization are associated with hydrothermal breccias within densely welded portions of the tuff. These breccia zones represent transient conduits of high fluid flow that formed by the explosive release of overpressure in the underlying geothermal reservoir and that were subsequently sealed by supersaturated geothermal fluids. In addition to this fracture sealing, hydrothermal alteration at Yellowstone appears generally to reduce matrix permeability and focus flow along fractures, where multiple pulses of fluid flow and self-sealing have occurred.

Experimental and numerical simulation of dissolution and precipitation: implications for fracture sealing at Yucca Mountain, Nevada

Plugging of flow paths caused by mineral precipitation in fractures above the potential repository at Yucca Mountain, Nevada could reduce the probability of water seeping into the repository. As part of an ongoing effort to evaluate thermal-hydrological-chemical (THC) effects on flow in fractured media, we performed a laboratory experiment and numerical simulations to investigate mineral dissolution and precipitation under anticipated temperature and pressure conditions in the repository. To replicate mineral dissolution by vapor condensate in fractured tuff, water was flowed through crushed Yucca Mountain tuff at 94 degrees C. The resulting steady-state fluid composition had a total dissolved solids content of about 140 mg/l; silica was the dominant dissolved constituent. A portion of the steady-state mineralized water was flowed into a vertically oriented planar fracture in a block of welded Topopah Spring Tuff that was maintained at 80 degrees C at the top and 130 degrees C at the bottom. The fracture began to seal with amorphous silica within 5 days.A 1-D plug-flow numerical model was used to simulate mineral dissolution, and a similar model was developed to simulate the flow of mineralized water through a planar fracture, where boiling conditions led to mineral precipitation. Predicted concentrations of the major dissolved constituents for the tuff dissolution were within a factor of 2 of the measured average steady-state compositions. The mineral precipitation simulations predicted the precipitation of amorphous silica at the base of the boiling front, leading to a greater than 50-fold decrease in fracture permeability in 5 days, consistent with the laboratory experiment.These results help validate the use of a numerical model to simulate THC processes at Yucca Mountain. The experiment and simulations indicated that boiling and concomitant precipitation of amorphous silica could cause significant reductions in fracture porosity and permeability on a local scale. However, differences in fluid flow rates and thermal gradients between the experimental setup and anticipated conditions at Yucca Mountain need to be factored into scaling the results of the dissolution/precipitation experiments and associated simulations to THC models for the potential Yucca Mountain repository.

Laboratory seismic monitoring of supercritical CO2 flooding in sandstone cores using the Split Hopkinson Resonant Bar technique with concurrent x-ray CT imaging

ABSTRACT Accurate estimation of CO2 saturation in a saline aquifer is essential for the monitoring of supercritical CO2 injected for geological sequestration. Because of strong contrasts in density and elastic properties between brine and CO2 at reservoir conditions, seismic methods are among the most commonly employed techniques for this purpose. However the relationship between seismic (P‐wave) velocity and CO2 saturation is not unique because the velocity depends on both wave frequency and the CO2 distribution in rock. In the laboratory, we conducted measurements of seismic properties of sandstones during supercritical CO2 injection. Seismic responses of small sandstone cores were measured at frequencies near 1 kHz, using a modified resonant bar technique (Split Hopkinson Resonant Bar method). Concurrently, saturation and distribution of supercritical CO2 in the rock cores were determined via x‐ray CT scans. Changes in the determined velocities generally agreed with the Gassmann model. However, both the velocity and attenuation of the extension wave (Youngs modulus or ‘bar’ wave) for the same CO2 saturation exhibited differences between the CO2 injection test and the subsequent brine re‐injection test, which was consistent with the differences in the CO2 distribution within the cores. Also, a comparison to ultrasonic velocity measurements on a bedded reservoir rock sample revealed that both compressional and shear velocities (and moduli) were strongly dispersive when the rock was saturated with brine. Further, large decreases in the velocities of saturated samples indicated strong sensitivity of the rocks frame stiffness to pore fluid.

Permeability of laboratory-formed methane-hydrate-bearing sand: Measurements and observations using x-ray computed tomography

Methane hydrate was formed in two moist sands and a sand/silt mixture under a confining stress in an X-ray-transparent pressure vessel. Three initial water saturations were used to form three different methane-hydrate saturations in each medium. X-ray computed tomography (CT) was used to observe location-specific density changes caused by hydrate formation and flowing water. Gas-permeability measurements in each test for the dry, moist, frozen, and hydrate-bearing states are presented. As expected, the effective permeabilities (intrinsic permeability of the medium multiplied by the relative permeability) of the moist sands decreased with increasing moisture content. In a series of tests on a single sample, the effective permeability typically decreased as the pore space became more filled, in the order of dry, moist, frozen, and hydrate-bearing. In each test, water was flowed through the hydrate-bearing medium and we observed the location-specific changes in water saturation using CT scanning. We compared our data to a number of models, and our relative permeability data compare most favorably with models in which hydrate occupies the pore bodies rather than the pore throats. Inverse modeling (using the data collected from the tests) will be performed to extend the relative permeability measurements.

Correspondence of the Gardner and van Genuchten–Mualem relative permeability function parameters

The Gardner and van Genuchten models of relativepermeability are widely used in analytical and numerical solutions toflow problems. However, the applicab ility of the Gardner model to realproblems is usually limited, because empirical relative permeability datato calibrate the model are not routinely available. In contrast, vanGenuchten parameters can be estimated using more routinely availablematric potential and saturation data. However, the van Genuchten model isnot amenable to analytical solutions. In this paper, we introducegeneralized conversion formulae that reconcile these two models. Ingeneral, we find that the Gardner parameter alpha G is related to the vanGenuchten parameters alpha vG and n by alpha G=alpha vG ~; 1:3 n. Thisconversion rule will allow direct recasting of Gardner-based analyticalsolutions in the van Genuchten parameter space. The validity of theproposed formulae was tested by comparing the predicted relativepermeability of various porous media with measured values.

LABORATORY EXPERIMENTS ON HEAT-DRIVEN TWO-PHASE FLOWS IN NATURAL AND ARTIFICIAL ROCK FRACTURES

Water flow in partially saturated fractures under thermal drive may lead to fast flow along preferential localized pathways and heat pipe conditions. At the potential high-level nuclear waste repository at Yucca Mountain, water flowing in fast pathways may ultimately contact waste packages and transport radionuclides to the accessible environment. Sixteen experiments were conducted to visualize heat-driven liquid flow in fracture models that included (1) assemblies of roughened glass plates, (2) epoxy replicas of rock fractures, and (3) a fractured specimen of Topopah Spring tuff. Continuous rivulet flow was observed for high liquid flow rates, intermittent rivulet flow and drop flow for intermediate flow rates, and film flow for lower flow rates and wide apertures. Heat pipe conditions (vapor-liquid counterflow with phase change) were identified in five of the seven experiments in which spatially resolved thermal monitoring was performed, but not when liquid-vapor counterflow was hindered by very narrow apertures, and when inadequate working fluid volume was used.

Characterizing unsaturated diffusion in porous tuff gravel

latest developments in instrumentation and techniques. Improved understanding of unsaturated diffusion in Evaluation of solute diffusion in unsaturated porous gravel is very gravel will help in the characterization and remediation important for investigations of contaminant transport and remediation, risk assessment, and waste disposal (e.g., the potential high-level effort in gravel deposits at the Hanford Reservation nuclear waste repository at Yucca Mountain, Nevada). For a porous (Washington). It will also help in the invert diffusion aggregate medium such as granular tuff, the total water content is barrier concept for the potential underground high-level comprised of surface water and interior water. The surface water radioactive waste repository at Yucca Mountain, Necomponent (water film around grains and pendular water between vada, where tuff gravel has been considered as an invert the grain contacts) could serve as a predominant diffusion pathway. material (material filling the bottom of a tunnel having a To investigate the extent to which surface water films and contact circular cross-section) to contain radionuclide transport. points affect solute diffusion in unsaturated gravel, we examined the The invert placed between the waste package or drip configuration of water using X-ray computed tomography (CT) in shield and the tuff host rock at Yucca Mountain is an partially saturated gravel and made quantitative measurements of difintegral component of the repository’s performance. If fusion at multiple water contents using two different techniques. In the first, diffusion coefficients of KCl in 2to 4-mm granular tuff at effective, an invert diffusion barrier (caused by slow multiple water contents were calculated from electrical conductivity radionuclide diffusion through the invert) can greatly (EC) measurements using the Nernst–Einstein equation. In the secenhance waste-isolation capacity. Conca and Wright ond, we used laser ablation with inductively coupled plasma–mass (1992) measured effective diffusion coefficients (De) in spectrometry (LA/ICP-MS) to perform microscale mapping, allowing unsaturated soil, gravel, bentonite, and whole rock for the measurement of diffusion coefficients for a mixture of chemical a wide range of volumetric water contents (this free tracers for tuff cubes and tetrahedrons having two contact geometries water content does not include interlayer water in clays (cube–cube and cube–tetrahedron). The X-ray computed tomography or other structural water; Conca and Wright, 2000). images show limited contact between grains, and this could hinder They found that De values in all media were primarily the pathways for diffusive transport. Experimental results show the a function of volumetric water content and not material critical role of surface water in controlling transport pathways and hence the magnitude of diffusion. Even with a bulk volumetric water characteristics. CRWMS M&O (2000b) reported that content of 1.5%, the measured solute diffusion coefficient is as low this diffusion data set was well correlated in terms of a as 1.5 10 14 m2 s 1 for tuff gravel. Currently used diffusion models power-dependence (Archie’s Law type) on the volumetrelating diffusion coefficients to total volumetric water content inaderic water content, and in this study a resultant “univerquately describe unsaturated diffusion behavior in porous gravel at sal” power function was used to represent diffusive very low water contents. transport of radionuclides through the invert. However, in waste emplacement drifts, characterized by a humid environment with or without the presence of liquid S of flow and transport in gravels have rewater, crushed porous rock may provide unique characcently received attention because of the importance teristics that vary greatly from this generic power funcof gravel aquifers, the need to understand contamination (Wang et al., 2001; Hu and Wang, 2003). For examtion characterization and remediation of gravel deposits ple, Conca (1990) placed four different size fractions of in the vadose zone, and the use of gravel as capillary bartuff gravel samples (2–4, 4–6.3, 6.3–9.5, and 15–25.4 mm) riers for waste isolation. As stated in Tokunaga et al. for equilibrium inside a chamber with a nearly 100% (2003), relatively little information is available on the humidity atmosphere. After equilibrating about 70 d, unsaturated hydraulic properties of gravels; this is also all samples of individual grains were observed to be dry, true for transport processes in unsaturated gravel sysdespite the 2.7% intragranular water content. No EC tems. Conca and coworkers published pioneering work could be measured on these samples, resulting in an examining chemical diffusion behavior in porous gravinferred diffusion coefficient below 10 15 m2 s 1, which els, but this work was conducted more than 10 yr ago is the detection limit reported by Conca (1990) using (Conca, 1990; Conca and Wright, 1990, 1992). With the EC for estimating the diffusion coefficient. The inferred improved understanding of water distribution in gravel, low diffusion value of 10 15 m2 s 1 at this water content there is a strong need to investigate the diffusion prodeviates significantly from the “universal” power funccesses in unsaturated porous gravel by employing the tion with a diffusion coefficient (≈2.8 10 12 m2 s 1), which is obtained with continuous fluid introduction. In other words, at the same water content, diffusion in samQ. Hu and J.J. Roberts, 7000 East Ave., MS L-231, Lawrence Livermore National Laboratory, Livermore, CA 94550; T.J. Kneafsey, L. ples prepared using high humidity (without fluid source Tomutsa, and J.S.Y. Wang, 1 Cyclotron Road, MS 90-1116, Lawrence Berkeley National Laboratory, Berkeley, CA 94720. Received 12 Nov. Abbreviations: CT, computed tomography; EC, electrical conductiv2003. Original Research Paper. *Corresponding author (hu7@llnl.gov). ity; ICP-MS, inductively coupled plasma-mass spectrometry; LA/ICPMS, laser ablation with inductively coupled plasma-mass spectromePublished in Vadose Zone Journal 3:1425–1438 (2004).