Geological CO2 Storage Characterization

The Eocene Green River Formation in Wyoming has long served as a standard for lacustrine depositional systems. This lacustrine formation, excluding the culminating phase, was deposited in a closed hydrographic basin. The position of the boundary between lake and mudflat margin was dictated by the inflow/evaporation ratio (inflow greater than evaporation = transgression; inflow less than evaporation = regression). All members of the Green River Formation are characterized by repetitive stratification sequences. In the Tipton and Laney members, the repetitive stratification sequences are laminated, kerogen-rich carbonates with fish fossils overlain by dolostone with numerous desiccation features. In contrast, in the middle member (Wilkins Peak), the typical stratification sequence is trona (evaporate) overlain by dolostone, overlain by kerogen-rich carbonate (oil shale). All these stratification sequences can be explained as products of dynamic climate change and a consequent imbalance between inflow and evaporation which probably resulted from the earth’s processional variations. The evidence for global warming and climate change (prior to anthropogenic green house gas (GHG) emissions) is undeniable. The crucial question is, are anthropogenic GHG emissions accelerating the rate of climate change? The confluence of rising global temperature with substantial increases in GHG emissions since the beginning of the industrial revolution strongly suggests that the answer to this question is yes.

The Wyoming Carbon Underground Storage Project (WY-CUSP) is a statewide effort to identify, inventory, prioritize, and characterize the most outstanding CO₂ storage reservoirs and the premier storage site in Wyoming. The WY-CUSP project is managed by the Carbon Management Institute (CMI) at the University of Wyoming with support from the US Department of Energy, State of Wyoming, and industrial partners. In its search for an optimum carbon dioxide storage reservoir in Wyoming, CMI first inventoried and examined the state’s hydrocarbon reservoirs, for these are reservoirs with proven fluid storage capacity. The inventory and prioritization of storage reservoirs and storage sites was based on the following criteria: (1) thickness, areal extent, and petrophysical properties of the reservoir rocks, (2) presence of a fluid trap and adequate confining layers, (3) suitable temperature, pressure, and rock/fluid chemistry regimes, (4) salinity of the formation fluids in the storage reservoir rocks, and (5) volumetrics of the storage site. It became apparent that the Mississippian Madison Limestone and Pennsylvanian Weber/Tensleep Sandstone were the highest-priority potential CO₂ storage stratigraphic intervals, and that the Rock Springs Uplift (RSU) in southwestern Wyoming was the premier CO₂ storage site in the state. A drill site on the northeastern flank of the RSU was highly prospective in offering high-quality reservoir rock at a depth that provides sufficient temperature and pressure for carbon dioxide storage. A very-large-scale, large-capacity trap on the RSU has several competent sealing rock units, and available data show that the reservoir rocks contain very saline formation water. Abundant sources of carbon dioxide are nearby, notably the Jim Bridger Power Plant.

One of the most important and difficult tasks for any site characterization project is following all regulatory and permitting requirements. Much of the surface access in and around the Rock Springs Uplift #1 well (RSU #1 well) is checkerboarded with federal, state and private surface rights creating a difficult permitting situation. In addition, different phases of the project required permitting from more than one regulatory agency. CMI was required to acquire access permits, regulatory permits, and private letters granting access.

The strategy adapted by the WY-CUSP team to accomplish their primary goal involved the following steps:

The location of a potential carbon capture and sequestration (CCS) project in southwest Wyoming is evaluated with emphasis on the site location, geologic history, location of potential drinking-water aquifers, and proximity to sources of both anthropogenic and natural CO₂. Natural and anthropogenic CO₂ sources were mapped in Wyoming to define their relation to enhanced oil recovery opportunities and prospective storage sites. Of the nearly 60 Mt of anthropogenic CO₂ emissions reported in Wyoming, half were located in the Greater Green River Basin (GGRB) in southwest Wyoming. The Rock Springs Uplift (RSU) CO₂ storage site is located in the GGRB, and is a promising structure for commercial CO₂ storage/surge tank development. Successful economic utilization of natural and anthropogenic CO₂ depends on near-by sources, infrastructure, areas of resource depletion suitable for enhanced recovery, and areas of potential storage.

The RSU is the GGRB’s youngest Laramide uplift (45 m.y.b.p.). Strata on the RSU are largely offset by a blind thrust fault on the western border of the asymmetric anticline. The sedimentary section at the well site consists of more than 4100 m of Paleozoic and Mesozoic rocks and spans nearly 500 m.y. of geologic history. Investigations were conducted to characterize potential Paleozoic reservoirs and their associated seals. The Paleozoic reservoirs are located at depths over 3400 m; approximately 280 m of core was recovered from the Pennsylvanian Weber Sandstone and Mississippian Madison Limestone and associated seals for analysis of reservoir and sealing characteristics. Lithology and geologic history have resulted in a stratigraphic section on the RSU that has deep, thick reservoirs and multiple overlying seals.

A major concern in long-term CCS is the potential for leakage and the resulting risk of contamination of overlying groundwater aquifers. It is therefore important to characterize the groundwater resources of associated aquifers and active groundwater wells at potential CCS sites. Groundwater use at and near the storage site is sparse: only four groundwater permits for drinking water wells were identified in the area. Two intervals of potential potable water were identified in the Cretaceous Ericson Sandstone during completion of the RSU #1 well. On the basis of an extensive literary search and petrophysical data from the RSU #1 well, sequestering CO₂ in the Weber and Madison aquifers at the study site poses no perceptible threat to current groundwater use or resources.

The Paleozoic strata of the Rocks Springs Uplift (RSU), southwestern Wyoming, have been identified as potential CO₂ storage reservoirs. The lithologic, diagenetic, geochemical, and burial histories of RSU strata were investigated to aid in the geologic characterization of the study site. Log data and core samples were collected from a 12,810 ft (3904 m) stratigraphic test well on the northeast flank of the RSU. The Weber and Madison Formations were found to have porous zones in distinct lithofacies: Weber eolianites have porosity and permeability values that average 6.3 % and 2.7 mD, and Madison dolostones have porosity and permeability values that average 13.1 % and 22.7 mD.

Dolomitization, cementation, pressure solution, dissolution, sulfate reduction, and other diagenetic reactions were largely responsible for the creation or destruction of porosity in these formations. Geologic characterization indicates that both formations are heterogeneous but contain suitable reservoir zones for injection and sequestration. Overall, the Madison Limestone has superior reservoir qualities.

Analysis of primary sealing lithologies at the study site suggests that the lower Triassic section, Amsden Formation, and upper part of the Madison Limestone can confine injected CO₂. Analyses of the sealing formations established that porosity is lower than 4.6 % and micropores are dominant, permeability is below 0.005 mD, and displacement pressures exceed 900 psi (6205 kPa).

Local and regional burial history reconstruction models were completed to augment the reservoir sequestration pressure management plan. A 1-D burial history reconstruction of the study site shows the sedimentary column reaching a maximum depth greater than 8400 m (27,600 ft), maximum temperatures greater than 150 °C (302 °F), and maximum pressures approaching 255 MPa (2.5 kbars). Regional 2-D burial and geochemical history models show that several source rocks have re-entered the hydrocarbon window.

Robust geologic characterization of reservoirs and seals from the study site and interpretation of their diagenetic, geochemical, and burial histories suggest that the Rock Springs Uplift has geologic conditions suitable for the injection and storage of CO₂.

One of the most important steps in characterizing a geologic CO₂ storage site is the construction of 3-D volumes of seismic attributes. Once these seismic attribute volumes are constructed, key attributes can be correlated to core and well-log observations and analytical measurements. These correlations allow standardizations and extrapolations of a variety of determinative rock/fluid characteristics from the well bore of the stratigraphic test well out into the 3-D seismic survey volume, creating a realistic 3-D model of storage reservoirs and seals. This chapter discusses the technique for performing the tasks required to make the seismic attribute volumes; the correlations between key rock/fluid parameters and seismic attributes; and the crucial extrapolations from 1-D core, log, and VSP observations out into 3-D seismic attribute space. Topics covered include surface seismic specifications, seismic data processing, seismic resolution, vertical seismic profiling (VSP) and data acquisition, comparison of VSP and surface seismic data, comparision of VSP and well data (geologic property modeling), horizon mapping and depth conversion, seismic attributes, seismic interpretation, and qualitative permeability determined from seismic attribute analysis.

Formation brine characterization provided the data for analytical permitting requirements, evaluating reservoir confinement, and reaction path modeling. The brines of the Weber Sandstone and Madison Limestone of the Rock Springs Uplift are sodium-chloride type with total dissolved solid concentrations in excess of 75,000 mg/L. Due to the high TDS the Wyoming Department of Environmental Quality has classified these as Class VI groundwater.

Solutes of the brines are enriched above the seawater evaporation trend line, indicating that the brines have been heavily influenced by additional mineral dissolution, specifically halite. The composition of dissolved gases were measured and found to be unique to each formation. Dissimilarities in the brine chemistries and associated dissolved gases indicate that the target formations are isolated from each other.

Both the Weber and Madison fluids are supersaturated with respect to dolomite and calcite. With respect to anhydrite, the Weber is saturated or slightly undersaturated and the Madison is undersaturated. Simulations of CO₂ injection into the formation brines suggest a decrease in pH and an increase in the partial pressure of CO₂, calcite dissolution and anhydrite precipitation. These reactions will likely cause an increase in porosity of 1–3 %.

Hydrogen sulfide concentrations in the reservoir increased between the first and second sampling set (0.04 to 127 mg/L in the Weber and from 29 to 87 mg/L in the Madison). Although the cause of this increase is unknown, it is recommended that careful consideration be given to reservoir management during CO₂ injection. Additionally, H₂S monitoring should be an element of water production scenarios for CCUS.

Estimates of permeability in carbonate rocks from porosity alone are highly uncertain but can be improved when pore geometry information is incorporated. We developed a permeability model for a 400-ft-thick carbonate reservoir on the Rock Springs Uplift, Wyoming, with the objective of increasing the accuracy of flow simulation during CO₂ sequestration. Core data was used to identify hydraulic flow units within the reservoir and to further distinguish them through lithofacies analysis of thin sections. We used both the Flow Zone Indicator (FZI) and Winland’s R ₃₅ method to identify the flow units. FZI and pore throat radius values were obtained from the log-of-permeability-versus-porosity crossplot of the core sample measurements. For the rock types composing the Madison Limestone on the Rock Springs Uplift, both the FZI and R₃₅ methods proved to be effective techniques for rock-type classification. We found that acoustically derived porosity estimates within the Madison Limestone stratigraphic interval correlate well with those derived from the FZI. Sonic velocity in carbonates is a function not only of total porosity but also of the predominant pore type that determines the permeability of the rock. Hence our permeability estimation used both the density porosity and calibrated sonic porosity from conventional wireline logs. In the Madison Limestone, vug development within dolomitized sparitic carbonates has resulted in layered structures of super-permeable zones sandwiched between non-vuggy, less permeable micritic dolostones. Among the various vuggy zones of the Madison stratigraphic interval, permeability was found to vary by two to three orders of magnitude.

We also estimated the permeability in a 670-ft-thick sandstone unit within the Weber Sandstone on the Rock Springs Uplift with the objective of increasing the accuracy of our CO₂ flow simulation program. We used core data to identify the porosity-permeability relationship for the cored depth interval. On the basis of this relationship and well log data, we constructed a continuous vertical permeability profile. Seismically derived porosity values along the Weber horizon were used to model spatial permeability variations away from the RSU #1 well. The resulting statistical estimators of the permeability distribution led us to classify the Weber Sandstone as highly heterogeneous and variably permeable strata.

In order to implement CO₂ storage in deep saline aquifers a diverse set of geologic, geophysical, and geochemical parameters must be characterized in both the targeted reservoir intervals and at the storage site.

All observational, experimental and theoretical information and laboratory measurements are integrated into a comprehensive geologic model in order to obtain an accurate characterization of a specific set of potential storage reservoirs and a targeted storage site. The integration is achieved through a series of performance assessments for a diverse set of storage scenarios utilizing numerical simulation techniques. Two of the important fluid-flow parameters that are investigated with the numerical simulations are CO₂ storage capacity and CO₂ injectivity. Reliable estimates of these two parameters are essential to both governments making energy policies and environmental regulations, and to industry trying to make quality business decisions.

Three analytical techniques are utilized to evaluate CO₂ storage capacity in both the Madison Limestone and Weber Sandstone on the Rock Springs Uplift: (1) a static volumetric approach, (2) a dynamic fluid-flow simulation approach using a homogenous reservoir model, and (3) a dynamic fluid-flow simulation approach using a more realistic 3-D heterogeneous reservoir model. The results from these three approaches demonstrate how as the descriptive characterization of the spatial distribution of the determinative reservoir parameters become more realistic, the uncertainties of the CO₂ storage performance assessments are substantially reduced.

Using comprehensive regional geologic, structural, geochemical, log suite, core, and seismic data, we present field-scale heterogeneous reservoir models for the Madison Limestone and Weber Sandstone on the Rock Springs Uplift (RSU). These models were used to evaluate uncertainty in critical geologic carbon storage (GCS) performance metrics: storage capacity and well injectivity. The geologic setting of the RSU is presented first and is followed by a description of the techniques used to determine porosity and permeability heterogeneity based on analytic results from log suites, cores, and 3-D seismic data. Random realizations of permeability and porosity (i.e., extrapolations of reservoir properties away from the stratigraphic well into the 3-D seismic volume) are then generated for each of the geologic units including the storage targets (the Weber Sandstone and the Madison Limestone) and sealing formations (the Amsden, Dinwoody, and Chugwater, among others). These heterogeneous property fields then are used to simulate non-isothermal CO₂ injection over a 50-year period.

The most critical problem with commercial scale geological CO₂ sequestration is management of displaced fluids. All of the high quality numerical simulations of carbon capture, utilization, and storage (CCUS) on the Rock Springs Uplift (RSU), utilizing realistic 3-D reservoir models, demonstrate that commercial-scale geological CO₂ storage will require the removal of formation brines in approximately 1:1 ratio of injected CO₂ to displaced fluid. Without the production of formation brines the simulations suggest that very quickly injected CO₂ will cause pressures in the storage domain to exceed fracture pressures. To solve this problem, Carbon Management Institute (CMI) proposed a strategy that includes integration of fluid production/treatment with injection of CO₂. The treatment of the brines involved three important steps: (1) use of the temperature of the produced brines (~100 °C) to produce electricity via a heat exchanger to power the treatment facility, (2) to separate fresh water from the brines via nanofiltration and reverse osmosis, and (3) to recover metals, notably lithium, from the residual brines after partial evaporation. The impact of this approach; production of electricity, fresh water, and metals such as lithium from produced brines transform an anticipated carbon storage penalty into a revenue center.

The Powder River Basin (PRB) offers an opportunity to illustrate the advantages to Wyoming of deploying an innovative, multiple-resource development strategy designed to foster the sustainability of the state’s energy and environmental resources. Such a multiple resource development plan is based on viewing the PRB’s particular assemblage of energy/environmental resources as a synergistic system rather than a collection of disparate parts. This approach relies on synergistic relationships among resource elements in order to increase the efficiency of development, minimize environmental degradation, sustain long-term resource use, and maximize revenue to the state.

The key resource elements of an integrated development strategy for the PRB are:

By developing this suite of resource elements as a system, it would be possible to optimize the benefits to the energy industry while maximizing the sustainability of energy resource development, and maximizing state revenues for future generations. In addition, the strategy described here would reverse the regional trend of coal- and energy-related job loss. Most importantly, all of this resource development can be accomplished within the existing regulatory framework and without significantly increasing the industrial footprint. It is vital to our future that Wyoming seek new, more effective, efficient, and sustainable approaches to energy development.

Rich in energy resources, China’s Ordos Basin shares many similarities with Wyoming’s Greater Green River Basin and Powder River Basin. As a result, the energy development strategy employed in Wyoming basins should be applicable to the Ordos Basin. The Ordos Basin’s coal, coalbed methane, and natural gas reserves are ranked first in China, and its oil reserves are ranked fourth. The coal deposits in the Ordos Basin account for 39 % (3.98 Tt) of Chinese coal resources, and six of the thirteen largest coal mines in China are in the basin. China’s large energy base and the facilities essential to its fast-growing coal-to-chemicals industry are located in the Ordos Basin. The concurrent development of relatively new coal conversion industries with existing oil and gas industries in the Ordos Basin (Northern Shaanxi Province) provides the opportunity to apply the systematic approach to energy production developed in Wyoming: the integration of geologic CO₂ storage and enhanced oil recovery (EOR) using CO₂ flooding (CO₂-EOR). The coal conversion industry (coal-to-methanol, coal-to-olefins, etc.) provides affordable, capture-ready CO₂ sources for developing large-scale CO₂-EOR and carbon storage projects in the Ordos Basin. Compared with other CCUS projects, the ability to use CO₂ from the coal-conversion industry for CO₂-EOR and geologic CO₂ storage will make these projects in the Ordos Basin more cost-effective and technologically efficient while reducing CO₂ emissions to the atmosphere.

The goals of the WY-CUSP program were to improve estimates of geological CO₂ reservoir storage capacity, to evaluate the long-term integrity and permanence of confining layers, and to manage injection pressure and produced brine at the Rock Springs Uplift CO₂ storage site. In the process of achieving these goals, a new and substantially more effective strategy and technology has been developed to achieve the most accurate performance assessments and resultant risk reductions for detailed geologic CO₂ storage site characterization. The strategies and technologies used to perform the tasks resulting in achieving the project goals are the subject of this work—optimizing CO₂ storage efficiency at the Rock Springs Uplift study site and elsewhere. The ultimate mission of the WY-CUSP program, managed by the University of Wyoming Carbon Management Institute—delivery of a certified commercial CO₂ storage site in Wyoming that could be used as a surge tank for CO₂ utilization—has been accomplished.