Low-permeability Rocks Research Articles

Quantitative validation of pore structure characterization using gas slippage measurements by comparison with predictions from bundle of capillaries models

The use of gas slippage measurements as a tool for quantitative pore structure geometry characterization was investigated by comparing pore sizes estimated from gas slippage measurements to pore sizes predicted by bundle of capillaries models. A large data set of gas slippage measurements was generated for the study by combining measurements made in our laboratory with measurements from seven previously published studies. Pore size was estimated from the gas slippage measurements using two models, one assuming a circular cross sectional pore geometry and the other slot shaped. When using a porosity function generated for the data set using experimental data and a theoretically derived tortuosity function, pore sizes estimated from bundle of capillaries models yielded a good match to the pore sizes independently estimated from gas slippage data. Better agreement between the models exists when pores in high permeability rocks are modelled as having slot shaped cross sectional geometry and pores in low permeability rocks circular cross sectional geometry, suggesting a systematic shift in pore morphology with changing permeability.

The Effective Transmissivity of a Plane‐Walled Fracture With Circular Cylindrical Obstacles

AbstractStimulated and propped fractures provide the conductive pathways in low matrix permeability rocks. We study the impact of fracture aperture and proppant size and fraction on the effective transmissivity of a stimulated fracture filled with a partial monolayer of proppant grains. The proppant grains are treated as circular cylindrical obstacles, and the fracture walls are planar. The key geometric parameters are the obstacle fraction f and the ratio between the fracture aperture and the obstacle diameter α. We use three‐dimensional Stokes flow numerical simulations to demonstrate that the fracture flow model given by the Reynolds equations may largely overestimate the flow rate. To circumvent the inability of the Reynolds model to fulfill the no‐slip boundary condition at the rims of the obstacles, we use the Brinkman flow model adapted to the case of fracture flow. The relative difference between the effective transmissivities computed with the Stokes and Brinkman models for systems with multiple obstacles is below 10% for fractions up to 0.5. We use the Brinkman model to study the effective transmissivity of a plane‐walled fracture with circular cylindrical obstacles. Systematic numerical simulations show that the normalized effective transmissivity is predominantly dependent on f and α, and the effects of obstacle ordering are minor. The presented numerical results can be used by petroleum engineers for estimating transmissivities of propped fractures under in situ conditions. The model can also be applied to microfluidic systems and for deriving first‐order estimates of the effective transmissivity of rough‐walled, natural fractures with load‐bearing contact areas.

Frost heaving and frost cracking of elliptical cavities (fractures) in low-permeability rock

A 9% expansion of freezing water in the elliptical cavities (fractures) of rock leads to frost heaving pressure and thermal stress, which are the main cause of the freeze-thaw damage of a fractured rock mass. Considering the influence of thermal stress and water flow in elliptical cavities (fractures), a mathematical model for predicting frost heaving pressure in the elliptical cavities (fractures) of rock is presented, assuming that the rock matrix is impermeable. The maximum tensile stress and frost cracking angle on the elliptical cavity are deduced and the critical condition for simplifying the elliptical cavity to planar fracture is suggested based on the method of complex variable function and elastic mechanics theory. Moreover, the effects of the main parameters on frost heaving pressure and frost-cracking conditions are determined to investigate the frost cracking of water-filled elliptical cavities (fractures). The flow flux of water, the aspect ratios of cavities (fractures) and the elastic moduli of rock have a great direct influence on the frost heaving pressure and cracking angle, whereas the dip angle of the cavity (fracture) and the thermal expansion coefficient of rock indirectly affect them by changing the thermal stress. It is suggested that the stress intensity factor is adopted to predict the frost propagation of planar elliptical cavities only if the aspect ratio exceeds fifteen.

Computed-Tomography Measurements of Water Block in Low-Permeability Rocks: Scaling and Remedying Production Impairment

Summary Fracturing-fluid invasion into the rock matrix can generate water block that potentially reduces hydrocarbon production, especially in low-permeability reservoirs. Here, we experimentally investigate the dynamics of water block under different flow scenarios (i.e., without shut-ins) and rock permeabilities through multiple coreflood experiments. A computed-tomography (CT) scanner is used to obtain the saturation profile within the core throughout the experiment, while the overall hydrocarbon productivity is measured from the overall pressure drop across the core. On the basis of the saturation and pressure measurements, we interpret the potential physical mechanism regarding the productivity reduction from water block and its self-mitigation facilitated by the capillary imbibition. Our interpretation also matches the observed scaling with rock permeability and the optimal shut-in time.

A 2D Model of Hydraulic Fracturing, Damage and Microseismicity

We present a model for hydraulic fracturing and damage of low-permeable rock. It computes the intermittent propagation of rock damage, microseismic event locations, microseismic frequency–magnitude distributions, stimulated rock volume and the injection pressure. The model uses a regular 2D grid and is based on ideas from invasion percolation. All damaged and connected cells during a time step constitute a microseismic event, where the size of the event is the number of cells in the cluster. The magnitude of the event is the $$\log _{10}$$ of the event size. The model produces events with a magnitude–frequency distribution having a b value that is approximately 0.8. The model is studied with respect to the physical parameters: permeability of damaged rock and the rock strength. “High” permeabilities of the damaged rock give the same b value $$\approx 0.8$$ , but “moderate” permeabilities give higher b values. Another difference is that “high” permeabilities produce a percolation-like fracture network, while “moderate” permeabilities result in damage zones that expand circularly away from the injection point. In the latter case of “moderate” permeabilities, the injection pressure increases substantially beyond the fracturing level. The rock strength and the time step do not change the observed b value of the model for moderate changes.

Thermoporoelastic effects during heat extraction from low-permeability reservoirs

Thermoporoelastic effects during heat extraction from low permeability geothermal reservoirs are investigated numerically, based on the model of a horizontal penny-shaped fracture intersected by an injection well and a production well. A coupled formulation for thermo-hydraulic (TH) processes is presented that implicitly accounts for the mechanical deformation of the poroelastic matrix. The TH model is coupled to a separate mechanical contact model (M) that solves for the fracture contact stresses due to thermoporoelastic compression. Fractures are modelled as surface discontinuities within a three-dimensional matrix. A robust contact model is utilised to resolve the contact tractions between opposing fracture surfaces. Results show that due to the very low thermal diffusivity of the rock matrix, the thermally-induced pore pressure partially dissipates even in the very low-permeability rocks that are found in EGS projects. Therefore, using the undrained thermal expansion coefficient for the matrix may overestimate the volumetric strain of the rock in low-permeability enhanced geothermal systems, whereas using a drained thermal expansion coefficient for the matrix may underestimate the volumetric strain of the rock. An “effective” thermal expansion coefficient can be computed from the drained and undrained values to improve the prediction for the partially-drained matrix.

Water Saturation Relations and Their Diffusion‐Limited Equilibration in Gas Shale: Implications for Gas Flow in Unconventional Reservoirs

AbstractLarge volumes of water are used for hydraulic fracturing of low permeability shale reservoirs to stimulate gas production, with most of the water remaining unrecovered and distributed in a poorly understood manner within stimulated regions. Because water partitioning into shale pores controls gas release, we measured the water saturation dependence on relative humidity (rh) and capillary pressure (Pc) for imbibition (adsorption) as well as drainage (desorption) on samples of Woodford Shale. Experiments and modeling of water vapor adsorption into shale laminae at rh = 0.31 demonstrated that long times are needed to characterize equilibrium in larger (5 mm thick) pieces of shales, and yielded effective diffusion coefficients from 9 × 10−9 to 3 × 10−8 m2 s−1, similar in magnitude to the literature values for typical low porosity and low permeability rocks. Most of the experiments, conducted at 50°C on crushed shale grains in order to facilitate rapid equilibration, showed significant saturation hysteresis, and that very large Pc (∼1 MPa) are required to drain the shales. These results quantify the severity of the water blocking problem, and suggest that gas production from unconventional reservoirs is largely associated with stimulated regions that have had little or no exposure to injected water. Gravity drainage of water from fractures residing above horizontal wells reconciles gas production in the presence of largely unrecovered injected water, and is discussed in the broader context of unsaturated flow in fractures.

Stress dependent permeability and porosity of low-permeability rock

The seepage property of low-permeability rock is of significant importance for the design and safety analysis of underground cavities. By using a self-developed test system, both permeability and porosity of granite from an underground oil storage depot were measured. In order to study the influence of rock types on permeability, a tight sandstone was selected as a contrast. The experimental results suggested that the porosity of this granite is less than 5% and permeability is low to 10–20 m2 within the range of effective stress. During the loading process, both exponential relationship and power law can be utilized to describe the relationship between effective stress and permeability. However, power law matches the experimental data better during the unloading condition. The stress dependent porosity of granite during loading process can be described via an exponential relationship while the match between the model and experimental data can be improved by a power law in unloading paths. The correlation of permeability and porosity can be described in a power law form. Besides, granite shows great different evolution rules in permeability and porosity from sandstone. It is inferred that this difference can be attributed to the preparing of samples and different movements of microstructures subjected to effective stress.

Imbibition Oil Recovery From Tight Rocks With Dual-Wettability Networks in the Montney

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 185076, “Imbibition Oil Recovery From Tight Rocks With Dual-Wettability Pore Networks: A Montney Case Study,” by Ali Javaheri and Hassan Dehghanpour, SPE, University of Alberta, and James Wood, SPE, Encana, prepared for the 2017 SPE Canada Unconventional Resources Conference, Calgary, 15–16 February. The paper has not been peer reviewed. Previous studies demonstrate that Montney rock samples present a dual-wettability pore network. Recovery of the oil retained in the small hydrophobic pores is uniquely challenging. In this study, the authors applied dual-core-imbibition (DCI) methods on several Montney core plugs and introduced the imbibition-recovery (IR) trio to investigate the recovery mechanisms in rocks with dual-wettability pore networks. Introduction Spontaneous imbibition of aqueous phases (water, brine, or surfactant solutions) in fractured sandstone has been studied as a possible mechanism for enhanced oil recovery. Extensive experimental and mathematical investigations have been conducted for relating the imbibition rate and total oil recovery to the capillary and gravity forces and geometrical parameters. However, rock/fluid interactions in tight and shale reservoirs are more complicated than those seen in conventional reservoirs. In addition to capillary forces, organic materials and reactive clay minerals can influence the fluid flow and storage in the small pores of low-permeability rocks. The affinity of reservoir rock to a particular fluid in such formations depends especially on rock mineralogy and properties of the organic matter that coats and fills the pores. Previous comparative imbibition tests show that the affinity of the Montney samples to oil is significantly higher than their affinity to water. This behavior was explained by the presence of water-repellent pores within or coated by solid bitumen or pyrobitumen. In the complete paper, the authors focus on imbibition oil recovery of samples cored from the Montney Formation and investigate the role of rock-fabric complexities, such as dual-wettability characteristics, in oil recovery by water imbibition. A detailed discussion of materials used in the spontaneous-imbibition and oil-recovery tests, including rock and fluid properties, is included in the complete paper. Methodology The authors conducted three sets of comparative tests on five twin core plugs, which were dry cut from Montney cores. The samples are characterized by measuring mineral concentration, total-organic-carbon (TOC) content, porosity, and permeability. The methodology is fully described in the complete paper.

Digital rock models of Precambrian and Paleozoic tight formations from Poland

Properties of selected Precambrian and Paleozoic sedimentary clastic rocks were analysed with respect to their reservoir potential. Multidimensional analysis of laboratory results and borehole logging data was used to construct digital models of pre-Mesozoic, deeply buried formations, present as tight, low-porosity and low-permeability rocks. This modern statistical and deterministic approach as applied to laboratory and borehole logging results worked to integrate data at different scales. The results obtained are useful not only in further scientific research but also found a use in industrial application. As a first step, statistical methods, including clustering and separation of homogeneous groups, enabled digital rock model creation on the basis of the results of such laboratory measurements as pycnometry, mercury porosimetry, nuclear magnetic resonance spectroscopy or computed X-ray tomography. Next, the models constructed were applied in borehole logging interpretation to find intervals with similar petrophysical properties within the group and different properties between the groups. This approach allowed implementation of upscaling procedures of laboratory experiments at micro- and nano-scale to borehole logging scale. High correlations were established between the log petrophysical parameters within the digital models. This approach can be used to divide the succession cored into intervals with different petrophysical parameters.

Lithological control on fracture cementation in the Keuper Marl (Triassic), north Somerset, UK

AbstractThe spatial arrangement of gypsum veins as preserved natural hydraulic fractures have been characterized in the Triassic Keuper Marl Formation (UK), a caprock for hydrocarbon reservoirs and CO2sequestration. The marls cropping out are subdivided into five discrete fracture units based on the presence and abundance of gypsum veins. The nodular gypsum in evaporite horizons provides excess gypsum for nodule-rooted horizontal gypsum veins. Our petrographic observations demonstrate that the development of gypsum veins in beds lacking macroscopic evaporites is closely associated with disseminated gypsum cement in the marls. We interpret that the gypsum veins in marl are sourced from disseminated gypsum cements in the host rocks, based on stratigraphic correlations, and much lower Sr concentrations than gypsum nodules. Gypsum was transported to adjacent veins mainly through diffusion in the low-permeability marls. The localization of gypsum veins and varied Sr concentrations of veins and nodules indicate that the diagenetic fluids are a mix of connate water with meteoric water rather than brines transported from evaporite beds along faults to non-evaporite beds. This results in the absence of gypsum fillings in fractures in rocks without primary gypsum cements. The study implies that the cementation of natural fractures in low-permeability rocks can highly depend on the presence of cement minerals in the host rock.

Prediction of fracture intensity of carbonates based on the results of paleostress estimation by fault kinematic analysis based on the 3D seismic data (Kuyumba Oil Field)

The approach of prediction of fracture intensity of low permeability rocks is presented based on slip and dilation tendency analyses. The input data are faults kinematics data and the stress components that regulate their formation. As an example, data obtained from the Riphean complex of the Kuyumba oilfield and the concentrators of fractured oil reservoirs in the investigated region, is examined. Refs 19. Figs 5. Table 1.

Evaluation of wettability alteration and IFT reduction on mitigating water blocking for low-permeability oil-wet rocks after hydraulic fracturing

A significant amount of water is lost into shale reservoirs after hydraulic fracturing, and this can cause the permeability reduction that hinders hydrocarbon production. A coreflood workflow was designed in this work to explore the mechanisms behind the formation and mitigation of water blocking in rock matrix. Since shale is typically considered as mixed-wet, one core sample was altered from natively water-wet into oil-wet and tested through an identical experimental procedure; this provided a direct comparison of water blocking in water-wet and oil-wet states. Recently, surfactants have been widely proposed to enhance the oil production from shales, through either wettability alteration or interfacial tension (i.e., IFT) reduction. A microemulsion-forming surfactant was then tested in the core sample at the oil-wet state, also through the identical experimental procedure; this provided another direct comparison on rock permeability enhancement from either one of these two techniques on the same porous media.Our results showed the increase of rock permeability to hydrocarbon is a naturally-triggered process under the water-wet condition, where higher pressure-drawdown delays the increase; however, it is a pressure-drawdown-driven process under the oil-wet condition, where higher pressure-drawdown promotes the permeability increase. Moreover, when oil-wet rock is altered into water-wet, the late-time rock permeability is enhanced but the amount of flowback water is significantly reduced. However, permeability reduction due to matrix-fracture interaction may take place at the early time, the degree of which depends on the level of IFT reduction. For low-permeability rocks that the steady state requires long time to achieve, it might be a better option to use surfactants that give ultralow IFT reductions.

Relationship of groundwater geochemistry and flow to volcanic stratigraphy in basaltic aquifers affected by magmatic CO2, Jeju Island, Korea

Groundwater from pristine volcanic rock aquifers on dormant Jeju Island, Korea, was investigated to identify sources and geochemical processes of dissolved inorganic carbon (DIC) and related solutes along with characteristics of groundwater flow using a multi-isotope approach. δ18O, δ2H, and 3H of groundwater indicated the water is of meteoric origin, and is mainly derived from recent recharge, except Na-rich mineral water which has a significant contribution of 3H-free old water. 87Sr/86Sr of dissolved Sr suggests that solutes in groundwater were mainly derived from old alkali basalt in the lower part of lava flows. δ13CCO2(g) calculated using δ13CDIC and measured hydrogeochemical parameters indicated that (1) biogenic soil CO2 is a dominant source of DIC in low mineralized water, (2) CO2-rich water had mostly mantle-derived magmatic CO2, and (3) mineral water contained both biogenic and magmatic CO2. δ13CCO2(g) in mineral water was affected by precipitation of calcite, which complicated the estimation of the contribution of CO2 sources to DIC. Concentrations of mobile elements were mainly controlled by the amount of magmatic CO2 and groundwater residence time. δ13CCO2(g) and 3H content were used to distinguish four types of groundwater with different spatial patterns of occurrence in relation to the volcanic stratigraphy of Jeju Island. Groundwater types I and II are low mineralized water with lower levels of DIC from biogenic CO2, composed of either recent recharge (<40years, type I) or older water with negligible 3H (>60years, type II). Type III is Mg-rich mineral water with significant recent recharge while type IV is Na-rich mineral water with older residence time. Types III and IV had somewhat elevated levels of DIC (up to 7.6mM) with mixed signatures of magmatic and biogenic CO2, while CO2-rich water (a subset of type III) had DIC up to 100mM with signatures of mantle-derived magmatic CO2. Groundwater in the eastern area of Jeju Island is mostly type I, which can be attributed to a lack of both low-permeability rocks and recent volcanic rocks in that area. Types III and IV were found in the western area where the latest volcanic rocks are distributed. In the southwestern area, type III is associated with permeable basaltic aquifers with a contribution of magmatic CO2 and type IV is from the younger lava flows dissected by older massive trachytic rocks and shallower hydrovolcanic tuffs, which led to the higher degree of mineralization. A conceptual model is proposed to integrate the chemical and isotopic constraints of groundwater with volcanic stratigraphy and suggest the processes that generated groundwater types with different residence times and chemical compositions. This study may contribute to understanding the processes of groundwater flow and mineralization in volcanic aquifers affected by deep-source CO2 and variable residence time of groundwater in both dormant and active volcanic areas.

On the Theory of Solitons of Fluid Pressure and Solute Density in Geologic Porous Media, with Applications to Shale, Clay and Sandstone

We here analyze a new model of transients of pore pressure p and solute density ρ in geologic porous media. This model is rooted in the nonlinear wave theory, its focus is on advection and effect of large pressure jumps on strain. It takes into account nonlinear and also time-dependent versions of the Hooke law about stress, rate and strain. The model solutions strictly relate p and ρ evolving under the effect of a strong external stress. As a result, the presence of quick and sharp transients in low permeability rocks is unveiled, i.e., the nonlinear “Burgers solitons”. We, therefore, show that the actual transport process in porous rocks for large signals is not only the linear diffusion, but also a solitons presence could control the process. A test of a presence of solitons is applied to Pierre shale, Bearpaw shale, Boom clay and Oznam-Mugu silt and clay. An application about the presence of solitons for nuclear waste disposal and salt water intrusions is also discussed. Finally, in a kind of “theoretical experiment” we show that solitons could also be present in higher permeability rocks (Jordan and St. Peter sandstones), thus supporting the idea of a possible occurrence of osmosis also in sandstones.

Contribution of osmotic transport on oil recovery from rock matrix in unconventional reservoirs

The fluid transport mechanisms in fracture and rock matrix play a critical role in economic viability of hydrocarbon recovery from unconventional reservoirs. Several transport mechanisms, including advection, diffusion, and convection, have varying degrees of contribution to hydrocarbon recovery depending on the transport properties of the formation. The contribution of the concentration gradient driven diffusion, or osmotic transport on hydrocarbon recovery has been numerically investigated in this study. The main objective of this study is to determine when the osmotic transport plays an important role on the mass exchange between the fractures and the rock matrix.In this study, a model for the mass transport between the rock matrix and the fractures has been formulated and validated with the experimental data. The numerical results showed that osmosis is an important force imbibing water into low permeability rock matrix and enhancing the effectiveness of low salinity waterflooding on oil recovery. The imbibition of water into oil-wetted shale matrix is mainly driven by the osmotic transport and wettability alteration. The contribution of the osmotic transport on oil recovery continues a long period of time, typically in a few years. This contribution increases if the membrane efficiency is high and the matrix block size is small. However, shale membrane efficiency is typically less than 10% considerably reducing the oil recovery factor by osmosis to less than 5%. Higher membrane efficiency and lower diffusion coefficient of dissolved ions increase the contribution of osmosis on the oil recovery from shale matrix.

Estimation of the differential stress from the stress rotation angle in low permeable rock

AbstractRotations of the principal stress axes are observed as a result of fluid injection into reservoirs. We use a generic, fully coupled 3‐D thermo‐hydro‐mechanical model to investigate systematically the dependence of this stress rotation on different reservoir properties and injection scenarios. We find that permeability, injection rate, and initial differential stress are the key factors, while other reservoir properties only play a negligible role. In particular, we find that thermal effects do not significantly contribute to stress rotations. For reservoir types with usual differential stress and reservoir treatment the occurrence of significant stress rotations is limited to reservoirs with a permeability of less than approximately 10−12 m2. Higher permeability effectively prevents stress rotations to occur. Thus, according to these general findings, the observed principal stress axes rotation can be used as a proxy of the initial differential stress provided that rock permeability and fluid injection rate are known a priori.

Genesis of natural hydraulic fractures as an indicator of basin inversion

Satin spar (fibrous gypsum) veins, which occur in evaporite basins worldwide, provide significant insights into host rock deformation and fluid flow, although the genetic mechanism remains obscure. Satin spar veins in the red marls of the Triassic Mercia Mudstone of the Bristol Channel Basin were characterized in the context of regional and local setting. The vein network in the Keuper Marl (lower Mercia) exhibits a lack of systematic cross-cutting between three distinct vein sets. Two sets of veins are observed in the overlying Tea Green Marl, with one set clearly crossing the other. The gypsum veins commonly contain a blocky median zone of multiple thin bands of host-rock inclusions and alabastrine gypsum crystals, exhibiting crack-seal patterns. Fibrous zones on either side of the median zone consist of pure parallel-aligned gypsum fibres that are oblique to vein walls, indicating a hybrid shear-extensional mode of vein widening. Veins developed within reverse-reactivated faults contain fibre lineations in the median zones and also on vein surfaces, suggesting a minimum of two phases of fault slip. The veins are interpreted to have formed as a result of overpressure in the low-permeability mudstones by tectonic compression during basin inversion, giving rise to the median zone. Growth of gypsum fibres commenced when the initial fractures were completely sealed. The timing of vein formation is suggested to be Miocene, based on cross-cutting relationships with folds formed during the latest phase of basin inversion. Because the fractures initiated in response to fluid overpressures, they provide a useful analog to subsurface hydraulic fracture systems in low-permeability rocks subjected to tectonic compression. The fibrous widening of the veins post-dates their initiation and led to continued propagation and increased connectivity.

Relative Permeability of CO2 in a Low-permeability Rock: Implications for CO2 Flow Behavior in Reservoirs with Tight Interlayers

In many of the underground CO2 storage sites, the sedimentary reservoirs are not petrologically uniform. Reservoir formations usually comprise high-permeability layers and low-permeability interbeds. Such sedimentary structure has been observed in CO2 storage reservoirs; for example, the Utsira formation, Sleipner, Norway, and also in the Haizume formation, Nagaoka, Japan. During the upward migration of CO2 plume from lower to upper layers, CO2 flow is inevitably affected by these tight interlayers. Therefore, it is important to know the CO2 relative permeability in these layers to predict CO2 flow behaviour. However, measurements of CO2 relative permeability in low-permeability rocks are rare in literature. The reason is perhaps due to the difficulty in controlling saturation and specific conditions for measuring relative permeability in low-permeability rocks using conventional techniques. In this study, we designed a new quasi-steady state approach for the measurement of CO2 relative permeability in a low-permeability sandstone cored from an underground formation of a CO2 storage site. The rock sample is characterized by a low permeability (approximately 0.16 mD) and relatively strong heterogeneity in subcore scale. The low-permeability characteristics make the conventional steady-state way impractical, because a very long time is necessary for building each steady-state saturation profile. Furthermore, the spatial heterogeneity also leads to that it is unsuitable for using unsteady state methods, e.g. JBN method, which is limited to the application in the displacement with uniform fluid distribution. Rock heterogeneity in the sample may lead to an incorrect estimation of relative permeability. In our new method, we use X-ray CT imaging technique to monitor the CO2 saturation changes and formation of fluid flow path during pressure adjustments. Considering the formation of CO2 flow path, we initially injected CO2 at a considerable high pressure drop until the saturation reaches a targeted value, to create the flow path. Then we lowered the pressure drop (so as to flow rate) and let the saturation profile reach a quasi-steady state and stop to increase. The saturation and fluid flow path during this operation have almost no change in the sample. Then we repeated the operation of lowering pressure drop several times and measured pressures and flow rates. If the relative permeability of CO2 has no change, the plot of pressure drop with respect to flow rate should give a linear relationship with a constant slope. Using this method, the saturation at quasi-steady states in relative permeability measurement can be controlled and the operation time can be significantly reduced. The results show a linear relationship between the pressure drop and flow rate at several quasi-steady states as expected. The results indicate the relative permeability of CO2 in low-permeability rocks can be still high as in high-permeability rocks (e.g. Berea sandstone). The changes of CO2 relative permeability are greatly dependent on the formation of CO2 flow path and the effective area of CO2 flow path in pore space as shown by the X-ray CT images. Our results are essential for understanding the behaviour of CO2 migration and trapping in CO2 storage reservoirs with low-permeability tight interbeds and the transition part of caprocks. The measurement results give important constraint in reservoir simulations for CO2 storage.

Movement of CO2 Charged Fluids in Low Permeability Rocks During Deformation: Migration Patterns in the Carmel Formation, Utah

Understanding geodynamic reservoir and seal performance is important in order to ensure safe storage of CO2. In Utah leakage from natural gas reservoirs through sandstones and along faults may be observed as diagenetic alteration traces. Aeolian deposits, stained red by iron oxides, have commonly been bleached by reducing fluids of varying density. At the study site, bleaching is observed within and along the base of a low-permeability, regionally sealing unit; the lowermost Carmel Formation. Compactional stress during progressive deformation in the San Rafael monocline induced pressure gradients driving flow through the seal and towards venting fractures. Chemical transport processes are interpreted to be a combination of diffusion and advection. Capillary barriers may be overcome as transient in situ pressures and temperatures affecting fluid properties. The same geological unit may behave as a conduit during some parts of burial/uplift-history and as a sealing unit at other times.