CO2 Reservoir Research Articles

Controls on Gas Domains and Production Behaviour in A High-Rank CSG Reservoir: Insights from Molecular and Isotopic Chemistry of Co-Produced Waters and Gases from the Bowen Basin, Australia

This paper uses hydrochemical and multi-isotope analysis to investigate geological controls on coal seam gas (CSG) saturation domains and gas well production performance in a high-rank (vitrinite reflectance (Rv) > 1.1) CSG field in the north-western Bowen Basin, Australia. New hydrochemical and stable isotope data were combined with existing geochemical datasets to refine hypotheses on the distribution and origins of CSG in two highly compartmentalized Permian coal seams. Stable isotopic results suggest that geographic variations in gas content, saturation and production reflect the extent of secondary microbial gas generation and retention as a function of hydrodynamics. δ13C and δ2H data support a gas mixing hypothesis with δ13C-CH4 increasing from secondary biogenic values to thermogenic values at depth (δ13C −62.2‰ to −46.3‰), whereas correlated methane and carbon dioxide carbon isotope compositions, Δ13C(CO2–CH4) values and δ13CDIC/alkalinity trends are largely consistent with microbial CO2 reduction. In addition, below 200 m, the majority of δ13C-CO2 values are positive (δ13C: −1.2‰ to 7.1‰) and δ13CDIC shows an erratic increase with depth for both seams that is characteristic of evolution via microbial activity. The progression of carbon isotope values along the CO2 reduction fractionation line suggests progressive depletion of the CO2 reservoir with increasing depth. Faults clearly segment coal seams into areas having significantly different production, with results of geochemical analysis suggesting that pooling of biogenic gas and waters and enhanced methanogenesis occur north of a faulted hinge zone.

Numerical simulation of crystal growth of CO2 hydrate within microscopic Sand pores using phase field model for the estimation of effective permeability

Carbon dioxide capture and storage is an efficient technology to reduce CO2 emission. One of the reservoirs of CO2 is an aquifer in the sub-seabed geological formation under a caprock. However, there is a risk of CO2 leakage even though such probability is extremely low. When the water depth of a storage site is large: say, about 400 m or more, leaked CO2 changes its form to gas hydrate, which may block CO2 rise in the sediment. To estimate the sealing potential of CO2 hydrate, it is necessary to evaluate effective permeability of the sediment after CO2 hydrate forms. In this study, a series of numerical models were used to investigate microscopic hydrate distribution that essentially controls the effective permeability. The method consists of packing sand grains within microscopic computational domains, arranging water and CO2 phases in the pore space of the packed sand grains, placing multiple hydrate nuclei, and growing hydrate in the pores of the sand grains. First, we derived a value of the interface mobility of CO2 hydrate by fitting calculated growth rate of a hydrate spherule to a measurement in the literature. Then, CO2 hydrate formation was simulated within microscopic computational domains consisting of sand grains and water-CO2 two phases. Finally, efficient permeability was estimated using the results of the simulation of water flow through the pore space, regarding the formed hydrate as a solid. The calculation results indicated the differences in hydrate distribution in the pore space and in resulting effective permeability, depending on hydrate saturations, initial water saturations, and contact angles of water on the sand surface.

Sr-isotopic ratios trace mixing and dispersion in CO2 push-pull injection experiments at the CO2CRC Otway Research Facility, Australia

Analysis of 87Sr/86Sr ratios and modelling of formation water, injection water and produced water compositions from the CO2CRC Otway Research Facility in Victoria, Australia are used to test tracer behaviour and response in push-pull experiments. Such experiments are an essential pre-requisite to understanding the controls imposed by reservoir heterogeneities on CO2 dissolution rates which may be an important stabilising mechanism for geological carbon storage. The experiments (Otway stage 2B extension in 2014) comprised two sequential tests in which ~100 t of CO2-saturated water was injected with combinations of Sr and Br or Li and Fluorescein tracers, each injection being followed by two staged extractions of ~10 t and a final extraction of ~50 t all spaced at ~10 day intervals. Analysis of the 87Sr/86Sr ratios of the produced fluids from the first injection, spiked with SrCl2 and NaBr, is consistent with Sr behaving conservatively. This contrasts with previous interpretations in which Br was argued to have behaved conservatively while Sr, which dilutes ~three times as fast as Br, was thought to be lost to a mineral phase. Such Sr-loss cannot explain the evolution of 87Sr/86Sr ratios. The analysis of 87Sr/86Sr ratios in the waters produced after the second injection episode, spiked with LiCl and Fluorescein tracers, allows calculation of the fractions of the formation waters and the injection waters from both tests 1 and 2. The Sr, Li and SO4 tracers (the later formed by oxidation of formation sulphide) all indicate similar rates of dilution that is consistent with conservative behaviour. The results of the two injection episodes with spaced extractions are compared with two subsequent push-pull injections in which the produced waters, spiked with methanol, were extracted continuously. These continuous extraction experiments exhibited significantly less dilution over the same range of produced to injected water volumes (up to only ~0.6) than the earlier experiments with spaced extractions. This implies that some process related to the pauses in extraction enhances mixing of injected and formation waters. Achieving the objective of using push-pull experiments to assess reservoir heterogeneities and CO2 dissolution rates will require better assessment of the various tracers to establish which behave conservatively followed a proper understanding of the causes of the variations in mixing as fluids are extracted from the formations.

Rapid Loss of CO2 From the South Pacific Ocean During the Last Glacial Termination

AbstractDuring the termination of the last ice age, atmospheric CO2 rose ~80 ppm, but the origin of this carbon has not been fully resolved. Here we present novel constraints on the patterns and processes of deglacial CO2 release using three marine sediment cores from the southwest Pacific. Carbon isotopes (δ13C) and boron to calcium ratios (B/Ca) of benthic foraminiferal calcite provide records of the δ13C of total dissolved inorganic carbon (DIC) and carbonate ion concentrations ([CO32−]) in seawater, respectively. Together these properties indicate enhanced storage of respired CO2 between 1.2‐ and 2.5‐km water depth during the Last Glacial Maximum (19–23 thousand years ago, ka). The first major rise in atmospheric CO2 during the last deglaciation, at the time of Heinrich Stadial 1, was accompanied by increases in δ13C and [CO32−] at all core depths. The initial increases could be attributed to southward shifted westerly winds driving increased upwelling in the Southern Ocean, sending a signal of enhanced ventilation northward into the Pacific. Our results confirm that southern Pacific interior water masses served as an important reservoir for CO2 during the last glacial period, likely extracted from the atmosphere via the biologic pump. Some abrupt changes in Pacific carbon storage coincide with changes in Southern Ocean pH (Rae et al., 2018, https://doi.org/10.1038/s41586‐018‐0614‐0), upwelling indicators (Anderson et al., 2009, https://doi.org/10.1126/science.1167441), and pCO2 (Monnin et al., 2001, https://doi.org/10.1126/science.291.5501.112), indicating that portions of the deep Pacific carbon pool can be ventilated rapidly to the atmosphere via the Southern Ocean.

Multi-physics modeling of injected nanoparticles effect on remediation of CO2 leakage through cracks

One potential risk in CO2 sequestration is the leakage of carbon dioxide, which can result in contamination of underground water, creating potential threats to existing ecosystems. The common leakage pathway is through the pre-existing fractures or discontinuities within cement in the wellbore, incurred by the environmental conditions imposed on the cement. Injecting nanoparticles into pre-existing cracks is one of the most recently proposed ideas for mitigating fracture propagation in cement CO2 sequestration. To demonstrate the feasibility of this new technology, a numerical approach was taken in this work, as it is challenging to investigate it in a laboratory setting. We proposed a coupled ALE (Arbitrary Lagrangian–Eulerian)–DEM (Discrete Element Method)–peridynamic modeling strategy within the LS-Dyna package to investigate the intertwined interaction among the CO2 fluid flow, native fluid (e.g., brine), particle clusters, and cracks within the cement. The numerical results demonstrate that injected nanoparticles can effectively reduce the pressure exerted on the crack surface. Accordingly, the potential fracture propagation at the crack tip would be reduced compared to corresponding cases without nanoparticels as pressurized by fluid flow. This result verifies the effectiveness of proposed nanoparticle injection technology. Finally, using this established modeling strategy, the effect of filling particles on the fracture mitigation for different crack geometries (e.g. particle cluster patterns, aspect ratio of crack aperture and length) and CO2 reservoir pressure are examined. The result shows that injected particles successfully reduce the fracture propagation in these scenarios.

Environmental conditions of deposition of the Lower Cretaceous lacustrine carbonates of the Barra Velha Formation, Santos Basin (Brazil), based on stable carbon and oxygen isotopes: A continental record of pCO2 during the onset of the Oceanic Anoxic Event 1a (OAE 1a) interval?

This study presents new stable carbon and oxygen isotope data from Lower Cretaceous lacustrine carbonate rock samples recovered from a well drilled in the Santos Basin, offshore southeast Brazil. These samples represent a record of a continental environment just prior to the opening of the South Atlantic Ocean and the ultimate break-up of Gondwanaland. The geochemical data, along with carbonate mineralogy, indicate repeated cycles of lake level variation that could be attributed to climatic oscillations. Despite the absence of correlations between δ13C and δ18O values, facies analysis and the isotopic and mineralogical data suggest that lake hydrology was essentially closed for most of the depositional interval studied here. The existence of persisting trends of nearly constant δ13C values with a spread in δ18O values though, suggests long water residence times in the palaeolake, equilibrium between atmosphere and lake water CO2, as well as significant evaporation of water. The overall geological model that emerges unveils a more comprehensive picture of the depositional conditions that favoured the continuity of a significant carbonate factory in the middle of the Gondwanan continent, corroborating previous studies that suggested the lasting existence of a large and somewhat shallow endorheic lake in the area during the Early Cretaceous. As a result of this recorded trend strongly suggesting equilibrium between lake waters DIC (dissolved inorganic carbon) reservoir and atmospheric CO2, the data are most consistent with lacustrine deposition rather than precipitation of travertine, contrasting with some suggestions for the genesis of the carbonates of the Barra Velha Formation. Finally, this apparent equilibrium with the atmosphere likely left a preserved record in the continental carbonates of the final stages that preceded a major global environmental disturbance associated with an increase in atmospheric CO2, known for this time as the Oceanic Anoxic Event (OAE) 1a. If this is correct, it also helps to put further time constraints on this studied interval, which should not be younger than Barremian age, and to provide a regional continental perspective on a global event.

Quantifying pore scale and matrix interactions of SCCO2 with the Marcellus shale

Marcellus Shale was investigated as a storage reservoir for CO2, applying CO2 as an enhanced hydrocarbon recovery agent, and examining the CO2 alteration of the shale as a sealing unit. The investigation was comprised of observing the geochemical interaction of CO2 and water with two Marcellus Shale samples (MS-1 [silicate-rich] and MS-4 [carbonate-rich]) using various analytical techniques. In-situ Fourier Transform infrared (FT-IR) spectroscopy at geological storage conditions (high temperature and pressure) was used to examine the geochemical interaction of CO2 (with and without water) and Marcellus Shale. FT-IR results showed that CO2 interacted physically with the clay and kerogen components of dehydrated Marcellus Shale samples while dissolving in the water layer in the hydrated Marcellus Shale samples to promote carbonate dissolution. Feature relocation scanning electron microscopy was utilized to document the physical changes and the pore alterations based on exposure to CO2 with and without water. SEM indicated significant carbonate dissolution to the MS-4 sample after exposure to CO2 and water. Density function theory (DFT) and sorption isotherms were used for pore size analysis and to quantify CO2 sorption. The results were very different for the two Marcellus Shale samples studied. The MS-4 showed dissolution of carbonate to be a major reaction under hydrated conditions with a loss of mesopore volume from 68% to 40%, while MS-1 often showed an opposite trend. Both the MS-1 and MS-4 samples showed that significant volume of macropores larger than 200 nm were present. Isotherm results suggested uptake of CO2 of the MS-4 sample is four times greater than the MS-1 sample.

Wellbore cement alteration during decades of abandonment and following CO2 attack – A geochemical modelling study in the area of potential CO2 reservoirs in the Pannonian Basin

In East-Central Europe, the sedimentary rocks and saline reservoirs of the Pannonian Basin provide the greatest potential for geological sequestration of CO2. However, there is no knowledge about the integrity of cement casing and plugs in abandoned wellbores drilled in the last century, which surround the potential CO2 geological storage sites. Thermodynamic and kinetic batch, and 1D kinetic reactive transport models have been built up in PHREEQC to estimate the present composition of hydrated cement in different depths (106, 1478, 2136 and 2718 m) of these abandoned wellbores and, to access cement reactivity for the effect of potentially injected CO2. The wellbore cements are all predicted to presently consist of mainly calcium silicate hydrate (CSH) and portlandite but differences occur due to the stability of ettringite connected to the temperature of the geological environment. With the depth, the amount of potentially dissolved CO2 increases which induces the breakdown of portlandite and CSH in the cement, and the major precipitation of calcite and amorphous silica. The transformation affects the whole width of the cement casing after about 2–3 years in most of the depths. However, the calculated mass transfer among minerals indicate a 2–6% porosity drop, which raises the attention to potential pore clogging. This process significantly reduces the risks of abandoned wellbores when implementing CO2 geological storage.

Numerical study on the formations of gas channels and subsequent bubbles in unconsolidated sandy seabed sediment using a coupled LBM-DEM method

Carbon dioxide capture and storage is an efficient technology to reduce CO2. A candidate CO2 reservoir is a sub-seabed aquifer under the cap rock. However, there is a risk of CO2 leakage even though such probability is low. Once CO2 seeps at the seafloor, it forms bubbles/droplets and dissolves during their rise. The rising speed and the dissolution rate significantly depend on the bubble/droplet size. Interestingly, past observations of natural seepage of CO2 indicated that the bubble/droplet sizes were not very different, regardless of the seepage depths. In this study, a numerical method to simulate 3D solid-liquid-gas three-phase flow in the unconsolidated particle layers was developed and applied to elucidate the influences of gas flux, porosity, and particle size on the formations of gas channels and subsequent bubbles. The results suggested that the bubble size did not depend on the flux, but on the gas channel size at the level of gas flux expected in possible seepage events.

A novel multi-technique approach used in the petrophysical characterization of the Maquoketa Group (Ordovician) in the southeastern portion of the Illinois Basin: Implications for seal efficiency for the geologic sequestration of CO2

Quantifying the petrophysical properties of low-permeability sedimentary units helps to determine the possibility of upward flow of supercritical carbon dioxide when evaluating a site for safe confinement of geologic carbon storage. This research examines fine-scale pore characteristics that affect the sealing capacity of the Upper Ordovician Maquoketa interval, a thick and heterogeneous sequence of carbonates, siltstones, and clay-rich rock units in the Illinois Basin. This unit has been previously identified as a regional caprock that would likely isolate and effectively store any CO2 injected into underlying reservoirs. We applied a multi-technique approach to quantify pore-size distribution, pore surface area, porosity, permeability, and capillary entry pressure. These laboratory-based techniques include mercury porosimetry, gas adsorption, portable X-ray fluorescence, X-ray diffraction, total organic carbon analyses, and, to a lesser extent, scanning electron microscopy and petrography. In addition, we developed a lithofacies model that interpreted the combined wireline responses from multiple well locations. This model confirms that the Maquoketa Group is dominated by muddy limestone, dolomitic/calcitic shale, and silty shale. The results of these evaluations indicate that these sequences have low porosity (0.4–3.1 %) and low permeability (0.04–7.1 mD) values and capillary entry pressures adequate to inhibit invasion of supercritical CO2 driven by buoyancy forces. Laboratory results also indicate that portions of the Maquoketa Group may also function as a low-volume reservoir for CO2. That is, should supercritical CO2 migrate upward and percolate into this unit, most of the CO2 will likely be securely trapped by means of capillary mechanisms.

CO2 storage in the Paluxy formation at the Kemper County CO2 storage complex: Pore network properties and simulated reactive permeability evolution

The Paluxy formation is being considered as a prospective CO2 reservoir at the Kemper County CO2 Storage Complex. Here, the pore and pore-throat size distributions and connectivity of the Paluxy formation is evaluated through analysis of 3D X-ray Computed Tomography images. In spite of resolution limitations that constrain the pore-throat sizes detectable by imaging, the permeability contributing pore-throats are successfully characterized through 3D imaging analysis. Image-obtained pore and pore-throat size distributions and pore connectivity are then utilized to construct pore network models and simulate permeability. After CO2 is injected, it will dissolve into formation brine and create conditions favorable for dissolution of primary minerals and precipitation of secondary minerals. These reactions will alter the porosity and permeability of the system to varying degrees depending on the spatial location of reactions. Here, the possible porosity-permeability evolution is simulated using pore network models considering mineral reactions occurring uniformly and non-uniformly throughout the network. For a given change in porosity, there is a large range of possible permeability outcomes. Depending on the extent and spatial location of mineral reactions, permeability may decrease by more than one order of magnitude as minerals precipitate. During dissolution, simulated permeability increases as much as 500%.

Carbon dioxide supersaturation in high‐elevation oligotrophic lakes and reservoirs in the Sierra Nevada, California

AbstractTo better understand the contribution of alpine lakes to global CO2 emissions, carbon dioxide concentrations and fluxes to the atmosphere were measured in five high‐elevation lakes and five reservoirs in the Sierra Nevada, California. Median summer surface concentrations of dissolved CO2 (reservoirs: 21.1 μM, lakes: 23.7 μM) were supersaturated for most of the ice‐free season. Median diffusive flux of CO2 was low as compared to other inland waters (lakes: 260 mg CO2 m−2 d−1, reservoirs: 192 mg CO2 m−2 d−1). Linear mixed modeling demonstrated that the length of ice cover, persisting for 5–9 months and allowing for accumulation of under‐ice CO2, was a strong predictor of summer surface CO2. During the ice‐free period, surface evasion of CO2 was highest for the first 40 d after ice‐off when carbon dioxide that had accumulated during winter was released, although supersaturation and evasion continued until fall at most sites despite low rates of ecosystem metabolism. This study suggests that the contribution of high‐elevation, oligotrophic lakes and reservoirs in the Sierra to global CO2 emissions are small despite persistent supersaturation, and are primarily driven by the duration of ice cover.

Transport properties of saline CO2 storage reservoirs with unconnected fractures from brine-CO2 flow-through tests

CO2 storage in fractured reservoirs may lead to fast CO2 flow through interconnected fracture networks; but the role of isolated fractures on brine-CO2 multiphase flow systems remains unclear. We present the results of a brine-CO2 flow-through experiment in which we assess the change in transport properties of a synthetic sandstone plug (a surrogate of a saline siliciclastic CO2 reservoir) containing non-connected fractures aligned 45° from its axis. The test was performed at 40 MPa of constant hydrostatic confining pressure and ~11 MPa of pore pressure, at room temperature (~19.5 °C), using pure liquid-CO2 and 35 g L−1 NaCl salt solution. The injected CO2-brine volume fraction was increased from 0 to 1 in 0.2 units-steps (drainage). Upon achievement of the maximum CO2 saturation (SCO2 ~0.6), the plug was flushed-back with the original brine (imbibition). During the test, we monitored simultaneously pore pressure, temperature, axial and radial strains, and bulk electrical resistivity. The fractured sample showed lower values of cross- and end-points in the relative permeability curves to CO2 compared to non-fractured samples, from comparable experiments performed at similar pressure and brine salinity conditions, but different temperature. Our results suggest that a non-connected fracture network affects the mobility of the individual phases, favouring the trapping of CO2 in the porous medium and improving the storage efficiency of the reservoir. These evidences show the need of a better understanding of fracture connectivity prior to discard fractured reservoirs as unsuitable geological formations for CO2 storage.

Kerogen deformation upon CO2/CH4 competitive sorption: Implications for CO2 sequestration and enhanced CH4 recovery

The low permeability of kerogen governs the storage and production of shale gas. The flexible kerogen constantly experiences mechanical deformation induced by reservoir environment and complex interplay with geofluids. However, the kerogen deformation associated with CH4/CO2 competitive sorption remains poorly understood. In this work, the effect of preloaded moisture on the deformation of kerogen with different organic types was investigated with molecular dynamics simulation. The kerogen deformation upon CH4/CO2 competitive sorption was quantified with the combination of grand canonical Monte Carlo simulations and poromechanics theory. The effects of various factors and their corresponding contributions were discussed in detail. The effects of kerogen deformation on CH4/CO2 diffusion were studied. Some implications for CO2 sequestration and enhanced gas recovery (CS-EGR) were proposed. Our results verify the theoretical feasibility of CS-EGR in shale gas reservoir. CO2 is observed to have a higher affinity with kerogen and a lower diffusion coefficient compared with CH4, facilitating it to replace CH4 and retain in the kerogen matrix. The rising CO2 composition can induce larger kerogen swelling, thus opening fluid flow pathways and increasing shale gas production. There are optimum moisture content and reservoir pressure corresponding to the maximum effective pore size in kerogen. It could be feasible to enhance the efficiency of CS-EGR by manipulating the reservoir moisture and CO2 injection timing. Thermal stimulation in deep shale reservoir may not be efficient for CS-EGR. CH4/CO2 competitive sorption can induce significant swelling of kerogen. The flexible nature of kerogen should be considered to improve the evaluation on both gas-in-place and CO2 storage capacity.

Reservoir CO2 evasion flux and controlling factors of carbon species traced by δ13CDIC at different regulating phases of a hydro-power dam

As the world largest hydropower reservoir, the Three Gorges Reservoir (TGR) significantly impacted on the carbon cycle since reservoirs are sources of carbon sink. This study was carried out to investigate the effects of damming on the carbon cycle. δ13CDIC and δ13CDOC were used to trace the origin of dissolved organic (DOC) and inorganic carbon (DIC). The estimated CO2 evasion flux in two regulating phases (discharge and recharge) with averages of 111 mg/m2 h and 264 mg/m2 h, respectively. At the basin scale, average CO2 flux was about 188 mg/m2 h and varies from −158 mg/m2 h to 1092 mg/m2 h. The highest average pCO2 (1294 ppmv) was observed during the discharge period, which was oversaturated than atmospheric equilibrium value; hence, the TGR act as a considerable sink of atmospheric carbon. The δ13CDIC varies from −8.95‰ to 0.00‰ with mean −1.87‰; these enrich isotope values indicated that metabolic process (photosynthesis and respiration) and the rapid kinetics of carbonate weathering by soil CO2 control the pCO2. The low pCO2 of reservoir water caused the rapid dissolution of CO2 from the atmosphere during the recharge period. The δ13CDOC varies between −30.64‰ to −23.05‰, which is similar to the values of C3 vegetation; thus, the source of DOC would be the degradation of soil organic matter. Overall, this study revealed the δ13CDIC signature coupled with soil CO2 dissolution and admixture of atmospherically equilibrated waters resulting in the sink of atmospheric CO2 of the reservoir and impoundment of the dam alters the carbon cycle and aquatic carbon budget in TGR. The findings of this study provide a global image on the contribution of reservoirs to the carbon cycle and aquatic carbon budget. Coupling with isotope signatures and elemental concentrations, investigation of the biogeochemical cycle of the carbon can be effectively traced.

Carbon dioxide emission from drawdown areas of a Brazilian reservoir is linked to surrounding land cover

Reservoir sediments exposed to air due to water level fluctuations are strong sources of atmospheric carbon dioxide (CO2). The spatial variability of CO2 fluxes from these drawdown areas are still poorly understood. In a reservoir in southeastern Brazil, we investigated whether CO2 emissions from drawdown areas vary as a function of neighboring land cover types and assessed the magnitude of CO2 fluxes from drawdown areas in relation to nearby water surface. Exposed sediments near forestland (average = 2733 mg C m−2 day−1) emitted more CO2 than exposed sediments near grassland (average = 1261 mg C m−2 day−1), congruent with a difference in organic matter content between areas adjacent to forestland (average = 12.2%) and grassland (average = 10.9%). Moisture also had a significant effect on CO2 emission, with dry exposed sediments (average water content: 13.7%) emitting on average 2.5 times more CO2 than wet exposed sediments (average water content: 23.5%). We carried out a systematic comparison with data from the literature, which indicates that CO2 efflux from drawdown areas globally is about an order of magnitude higher than CO2 efflux from adjacent water surfaces, and within the range of CO2 efflux from terrestrial soils. Our findings suggest that emissions from exposed sediments may vary substantially in space, possibly related to organic matter supply from uphill vegetation, and that drawdown areas play a disproportionately important role in total reservoir CO2 emissions with respect to the area they cover.

Validation of a reactive transport model for predicting changes in porosity and permeability in carbonate core samples

The CO2-EOR and geologic CO2 storage (GCS) communities working in carbonate reservoirs would like better estimates for long-term changes to CO2 reservoir permeability and porosity that account for changes caused by the dissolution of carbonate minerals. One such model derived key parameters by matching pressure, single-phase solution chemistry, and dissolution features from fifteen core-flood experiments from two different carbonate reservoirs (Weyburn-Midale unit, Hao et al., 2013; Smith et al., 2013; Arbuckle unit, Smith et al., 2017). This model’s development is followed here with a validation study using samples from a third distinct reservoir. Forward simulations are compared with experimental results from five new carbonate samples from the Duperow Formation (Montana, USA). Simulations sample the parameter ranges for calcite and dolomite rate constants (log kcal,25C = −6.5, −5.5, −4.5; and log kdol,25C = −8, −7, −6)), and exponent n (1.5, 3, and 8) used to estimate permeability response from evolving porosity change, and model domains honor the initial mineral and void space distributions determined from non-destructive characterization as well as the initial bulk permeability of each sample prior to CO2 exposure. The forecast envelopes generated by the reactive transport model capture the pressure declines and pre-breakthrough porosity generation trends observed in these new experiments. The magnitude of model permeability forecasts spans orders of magnitude and is most sensitive to the value of the porosity-permeability exponent and sample heterogeneity.

Effective stress constraints on vertical flow in fault zones: Learnings from natural CO2 reservoirs

Some fault zones leak vertically to the ground surface or seafloor, whereas most others remain naturally sealed. Understanding the factors that cause this leakage is essential for predicting and preventing such leakage for both conventional reservoir development and subsurface CO2 storage. This study, a comparison of leaking and nonleaking natural CO2 gas accumulations, provides such constraints. We compare and contrast trap configurations, fluid pressures, and stress states for several natural CO2 accumulations from the Colorado Plateau. Extensive surface geologic data are integrated with subsurface data from a large suite of groundwater and hydrocarbon wells. Leakage of CO2 is documented by geochemical surveys and the occurrence of extensive travertine deposits. The leakage occurs exclusively in fault fracture damage zones where the total fluid pressure reduces the minimum horizontal effective stress to approximately zero. These results are consistent with natural and accidentally induced fault seeps from some deep-water hydrocarbon reservoirs. These criteria can be used to evaluate the potential for fault zones to provide vertical leakage pathways and loss of fluid containment.

Current travertines precipitation related to artificial CO2 leakages from a natural reservoir (Gañuelas-Mazarrón Tertiary Basin, SE Spain)

In the framework of a natural CO2 reservoir with CO2 leakages as an analogue of a failed CO2 deep geological storage, the current precipitation of travertines and the associated upwelling of CO2-rich saline groundwater were analysed. This natural analogue is located in the Gañuelas-Mazarrón Tertiary Basin (SE Spain). The study comprises of the chemistry of both groundwater and travertines, including stable isotopes, mineralogy and petrography of the travertines, all this performed after a review of the geology of the basin. In this sense, the basin gathers the main features of a safe natural CO2 reservoir in a deep saline aquifer sealed by a thick marl formation. The aquifer was artificially perturbed by the drilling of wells, inducing the travertines precipitation at these water discharge points. Groundwater is saline, slightly acid, oversaturated in aragonite and calcite and with significant concentrations of heavy elements, some of them toxic. From an isotopic viewpoint, the relative constant δ13C-DIC values suggest that carbon is mainly inorganic in origin with minor organic and mantle contributions. Travertines are basically composed of aragonite or calcite, their precipitation being controlled by a sudden CO2 degassing and minor biological activity. Their δ13C signatures indicate that carbon mainly has an inorganic origin, although some contribution of organic carbon must be considered as well. Furthermore, these carbonate deposits did not precipitate in isotopic equilibrium, as determined by δ18O values. Finally, it is suggested that the appearance of travertines along with their carbon isotopic signatures represent efficient tools for detecting CO2 leakages from any CO2 storage site.

Prediction of CO2 storage site integrity with rough set-based machine learning

CO2 capture and storage (CCS) and negative emissions technologies (NETs) are considered to be essential carbon management strategies to safely stabilize climate. CCS entails capture of CO2 from combustion products from industrial plants and subsequent storage of this CO2 in geological formations or reservoirs. Some NETs, such as bioenergy with CCS and direct air capture, also require such CO2 sinks. For these technologies to work, it is essential to identify and use only secure geological reservoirs with minimal risk of leakage over a timescale of multiple centuries. Prediction of storage integrity is thus a difficult but critical task. Natural analogues or naturally occurring deposits of CO2, can provide some information on which geological features (e.g., depth, temperature, and pressure) are predictive of secure or insecure storage. In this work, a rough set-based machine learning (RSML) technique is used to analyze data from more than 70 secure and insecure natural CO2 reservoirs. RSML is then used to generate empirical rule-based predictive models for selection of suitable CO2 storage sites. These models are compared with previously published site selection rules that were based on expert knowledge.