Search

Geological storage of carbon dioxide (CO2) in depleted gas reservoirs represents a cost-effective solution to mitigate global carbon emissions. The surface chemistry of the reservoir rock, pressure, temperature, and moisture content are critical factors that determine the CO2 adsorption capacity and storage mechanisms. Shale-gas reservoirs are good candidates for this application. However, the interactions of CO2 and organic content still need further investigation. The objectives of this paper are to (i) experimentally investigate the effect of pressure and temperature on the CO2 adsorption capacity of activated carbon, (ii) quantify the nanoscale interfacial interactions between CO2 and the activated carbon surface using Monte Carlo molecular modeling, and (iii) quantify the correlation between the adsorption isotherms of activated carbon-CO2 system and the actual carbon dioxide adsorption on shale-gas rock at different temperatures and geochemical conditions. Activated carbon is used as a proxy for kerogen. The objectives aim at obtaining a better understanding of the behavior of CO2 injection and storage into shale-gas formations. We performed experimental measurements and Grand Canonical Monte Carlo (GCMC) simulations of CO2 adsorption onto activated carbon. The experimental work involved measurements of the high-pressure adsorption capacity of activated carbon using pure CO2 gas. Subsequently, we performed a series of GCMC simulations to calculate CO2 adsorption capacity on activated carbon to validate the experimental results. The simulated activated carbon structure consists of graphite sheets with a distance between the sheets equal to the average actual pore size of the activated carbon sample. Adsorption isotherms were calculated and modeled for each temperature value at various pressures. The adsorption of CO2 on activated carbon is favorable from the energy and kinetic point of view. This is due to the presence of a wide micro to meso pore sizes that can accommodate a large amount of CO2 particles. The results of the experimental work show that excess adsorption results for gas mixtures lie in between the results for pure components. The simulation results agree with the experimental measurements. The strength of CO2 adsorption depends on both surface chemistry and pore size of activated carbon. Once strong adsorption sites within nanoscale network are established, gas adsorption even at very low pressure is governed by pore width rather than chemical composition. The outcomes of this paper provides new insights about the parameters affecting CO2 adsorption and storage in shale-gas reservoirs, which is critical for developing standalone representative models for CO2 adsorption on pure organic carbon.

An uncommon facet of Formation Evaluation is the assessment of flow-related in situ properties of rocks. Most of the models used to describe two-phase flow properties of porous rocks assume homogeneous and/or isotropic media, which is hardly the case with actual reservoir rocks, regardless of scale; carbonates and grain-laminated sandstones are but two common examples of this situation. The degree of spatial complexity of rocks and its effect on the mobility of hydrocarbons are of paramount importance for the description of multiphase fluid flow in most contemporary reservoirs. There is thus a need for experimental and numerical methods that integrate all salient details about fluid-fluid and rock-fluid interactions. Such hybrid, laboratory-simulation projects are necessary to develop realistic models of fractional flow, i.e., saturation-dependent capillary pressure and relative permeability. We document a new high-resolution visualization technique that provides experimental insight to quantify fluid saturation patterns in heterogeneous rocks and allows for the evaluation of effective two-phase flow properties. The experimental apparatus consists of an X-ray microfocus scanner and an automated syringe pump. Rather than using traditional cylindrical cores, thin rectangular rock samples are examined, their thickness being one order of magnitude smaller than the remaining two dimensions. During the experiment, the core is scanned quasi-continuously while the fluids are being injected, allowing for time-lapse visualization of the flood front. Numerical simulations are then conducted to match the experimental data and quantify effective saturation-dependent relative permeability and capillary pressure. Experimental results indicate that flow patterns and in situ saturations are highly dependent on the nature of the heterogeneity and bedding-plane orientation during both imbibition and drainage cycles. In homogeneous rocks, fluid displacement is piston-like, as predicted by the Buckley-Leverett theory of fractional flow. Assessment of capillary pressure and relative permeability is performed by examining the time-lapse water saturation profiles. In spatially complex rocks, high-resolution time-lapse images reveal preferential flow paths along high permeability sections and a lowered sweep efficiency. Our experimental procedure emphasizes that capillary pressure and transmissibility differences play an important role in fluid-saturation distribution and sweep efficiency at late times. The method is fast and reliable to assess mixing laws for fluid-transport properties of rocks in spatially complex formations.

Reservoir simulators often predict poor injectivity in polymer enhanced oil recovery (EOR) projects because of the high polymer viscosity, which is a deterrent for the project. However, field studies have shown much higher injectivity than predicted by the models. The objective of this work is to perform grain-scale, coupled fluid dynamics and geomechanics modeling to predict the injectivity of viscous, non-Newtonian polymers in wellbores. Fluid-rock interactions are modeled by coupling computational fluid dynamics (CFD) and the discrete element method (DEM). Fluid flow is determined using an open-source CFD software that solves the volume-averaged Navier-Stokes equation using the finite volume method on Eulerian grids. Grain-scale geomechanics (DEM) is used to explicitly solve the particle trajectories in a Lagrangian reference system. The simulation results confirm the hypothesis of fracture initiation and sand failure near the injector. The results show that the polymer-driven fracture initiation is associated with sand shear failure, while the fracture geometry is the result of the localization of sand shear failure and fluidization of unconsolidated sand at the fracture tip. The injection of a viscous fluid can create fractures in the direction perpendicular to the applied minimum principal stress. The presence of fractures increases the injectivity. The peak injection pressure is more than 3 times greater than the applied minimum principal stress. The viscosity increase of polymers promotes the initiation of fractures and results in a greater fracture aperture. The injection of polymer can promote the initiation of fractures, and therefore, increase the polymer injectivity. This work, for the first time, uses a grain-scale model to predict polymer-driven fracture initiation and demonstrates the improved injectivity observed in the field.

Long-term integrity and practical storage of CO2 is contingent upon its seal performance and the dynamic sealing capacity of faults for the CO2 storage site. Faults are prone to reactivation with reservoir pressurization caused by CO2 injection. The goal of this study is to create and verify a reservoir elasto-plastic model capable of capturing short-term evolution of fault reactivation and the resulting change of permeability. This model is then used to explore the effects of coupling geomechanics with reservoir fluid flow on the reactivation of faults. In this paper, we introduce a workflow for modeling of fault reactivation with fault elements as gridblocks instead of surfaces. Reservoir simulation, with coupled fluid flow and geomechanics, was used for this purpose. The simulation models utilize a geomechanical module to capture elasto-plasticity and a compositional numerical scheme based on an equation of state (EOS) to calculate CO2-brine interaction. The geomechanical module used in this study is based on Hierarchical Single Surface (HISS) model that captures strain softening and hardening, and therefore post-yield plastic deformations related to fault reactivation. The compositional numerical scheme based on EOS calculates the amount of CO2 solubilization in brine as well as the density and viscosity of the CO2- and aqueous-rich phase. In this approach, the flow properties, i.e. permeability and porosity, dynamically change in response to geomechanical effects. The dynamic change was captured through a volumetric strain-permeability law. Our simulation results show that the model is capable of capturing short-term evolution of fault reactivation and the resulting change of permeability along the fault. The dynamic changes of fault properties control the extent of fault reactivation, the pressure relief during injection, and the fault sealing capacity.

The reduction of pore pressure caused by depletion can induce significant reservoir compaction, especially in unconsolidated reservoirs. Experiments using unconsolidated core samples are often sparse and costly. We develop a numerical approach based on computer-based simulations of rock samples and mechanical tests. The numerical sample consists of crushable grains simulated with the discrete element method (DEM) and the bonded-particle model (BPM). Model parameters are calibrated through numerical single-grain-crushing tests which reproduce the experimentally-measured sand strength. Grain crushing induced by the uniaxial strain stress path results in a pronounced reduction of porosity and permeability, which manifests more readily for samples with large grain size. The change of particle size distribution indicates that the high effective stress causes grain crushing and produces a significant amount of fines. We perform numerical uniaxial strain tests on numerical samples comprising stiff and soft mineral grains. Simulation results indicate that the presence of soft grains and inclusions (e.g. shale fragments) facilitates the grain crushing. Reservoir simulations, incorporating the change of porosity and permeability as a compaction table, show that the upscaled compaction can enhance production due to compaction drive but also reduces production rate by impairing the reservoir permeability. This multiscale numerical workflow bridges particle-scale compaction behavior and field-scale reservoir production. In this paper, (a) DEM simulations provide a useful tool to investigate compaction effects and complement laboratory experiments; (b) the multi-scale numerical approach can predict the depletion-induced evolution of reservoir production.

Targeting the incompatibility issue between traditional Portland cements and non-aqueous drilling muds, a mud solidification technique based on geopolymer cements was recently introduced to the industry. This technique was achieved by incorporating non-aqueous fluids (NAF) such as synthetic-based or oil-based drilling mud in a geopolymer cement that is formed by blending an aluminosilicate powder with an alkaline activating solution. In a previously completed study, the consolidated mud, or "geopolymer hybrid", was evaluated for its compressive strength, thickening time and stability for applications including primary cementing as well as lost circulation control. In an effort to further develop the geopolymer hybrid system as a new generation well cementing material, the present work comprehensively studies their mechanical properties, self-healing capabilities and cement-to-pipe bond strength. One of the concerns currently preventing the use of geopolymers for well cementing is its low early compressive strength. This study shows that although the early age uniaxial compressive strength of geopolymer is lower than that of Portland cement, with an applied confining stress, geopolymers have more than sufficient strength for primary cementing applications. Furthermore, the abilities of cement to properly bond with steel casing and rock formations, as well as to withstand subsurface stress changes over time are of paramount importance for long-term cement integrity. The triaxial tests performed in this study show that geopolymers are less brittle than Portland cement, and can therefore accommodate stress changes without localized failure. The geopolymer bond strength measurements showed that geopolymer could bond to steel even in presence of non-aqueous drilling muds. In addition, the triaxial tests revealed that geopolymers exhibit true self-healing capabilities, characterized by samples fully regaining their mechanical competence with 28 day post-failure strength exceeding the native 7-day strength. Portland cement, in comparison, retained lower ultimate strength than its 7-day strength. The self-healing capability of geopolymers is ideally suited to adapt to subsurface stress changes, making such materials ideally suited for use as permanent barriers in plug and abandonment operations.

CO2 geological sequestration is one of the most efficient and direct ways to reduce the CO2 emissions to the atmosphere. Understanding reservoir capacity and geomechanical implications of CO2 injection are necessary for safe and effective storage. Large pore pressure increases from injection may induce fault reactivation and seal failure, and compromise storage integrity. Several studies examined the Frio brine pilot CO2 sequestration project conducted in Dayton, Texas. However, geomechanical implications were not fully evaluated at that time besides calculations required for a class V well permit. This article presents a comprehensive case study to evaluate pore pressure increase due to CO2 injection at the Frio injection site. The work includes new laboratory experiments and well-logging data analyses. The results are consistently assimilated into a numerical reservoir model which includes compositional flow. A fine-scale reservoir model of the Frio pilot test is developed in our in-house reservoir simulator IPARS (Integrated Parallel Accurate Reservoir Simulator). We perform history matching of the pressure transient data of Frio pilot test, and the history-matched reservoir model is used to predict the implications of larger CO2 injection volumes. Reservoir simulation results show that injection rates adopted during the actual pilot test very likely did not upset the mechanical integrity of the storage complex. Higher and sustained injection rates may increase pore pressure beyond limits for fault reactivation and hydraulic fracturing. A change of over a few-hundreds-psi could cause the sand to reach the shear yield stress. However, tested rock samples exhibit ductile deformation at in-situ effective stresses suggesting that reaching shear yield stress in neighboring faults may not jeopardize the sealing integrity of faults.