Gas Injection Tests Research Articles

Gas breakthrough in saturated compacted GaoMiaoZi (GMZ) bentonite under rigid boundary conditions

In a geological repository for disposal of high-level radioactive waste, gas breakthrough is an important phenomenon during a gas migration process in the saturated engineered barrier. In this paper, gas injection, swelling pressure, water permeability, and water retention tests were conducted on saturated compacted GaoMiaoZi (GMZ) bentonite to investigate the gas breakthrough mechanism. Results show that, for saturated GMZ bentonite tested under rigid boundary conditions, the gas breakthrough pressure is significantly larger than the swelling pressure and slightly lower than the gas entry pressure obtained from the water retention characteristic and the van Genuchten model. Gas breakthrough pressure deviates from the swelling pressure and approaches the calculated gas entry pressure as the dry density increases. Mechanical and capillary effects are both important to the gas migration process for specimens with lower dry densities, and the capillary effect becomes more important with the increase of dry density. The desaturation and shrinkage of the specimen will result in unexpectedly high and disordered interfacial gas flux. For specimens with higher dry densities, gas will only flow through interconnected larger pores, then result in minor desaturation–shrinkage of the specimen. Finally, a new model with consideration of both mechanical and capillary effects is proposed, which can accurately predict gas breakthrough pressure for a GMZ bentonite specimen.

Experimental investigation on gas migration in saturated Shanghai soft clay

Understanding of gas migration behavior in saturated soft soil is of great importance for designing and construction activities involving applications of pneumatic caisson method and compressed-air tunneling technique in soft ground. For further investigation of mechanical effect on gas migration behavior in such applications, including gas flow rate, gas permeability, as well as the gas breakthrough pressure, step-wisely gas injection tests were conducted on Shanghai soft clay under different vertical loads in this paper. Results show that gas flow rate will increase as the injection pressure increases. The conventional two-phase flow theory, which is mainly governed by capillary pressure, can be used to model gas flow under lower gas injection pressures, but fails to describe gas flow under higher gas injection pressures, due to the significant influence of mechanical stress. As effective stress increases, gas permeability decreases sharply at the initial stage followed by a gradually stabilization stage due to the dissipation of mechanical effect. The mechanical stress has significant influence on gas permeability for specimens with higher compressibilities. Formation of gas flow pathways, either caused by capillary pressure or mechanical stress, can result in gas breakthrough, the corresponding pressure of which is determined according to the evolution of gas flow rate and can be used as the working gas pressure for avoiding significant gas loss in the gas-tight working structures in pneumatic caisson method and compressed air tunneling technique. Gas breakthrough pressure increases with the increase of vertical load.

A comparative experimental study of gas injection in shale plugs by flooding and huff-n-puff processes

The increase in U.S crude oil over the past decade have been primarily driven by tight and shale oil. With the high production decline rates in shale reservoirs, improved oil recovery techniques must be applied to maintain the oil recovery from existing wells. Gas injection has been investigated and demonstrated as the most effective solution to face such challenge. As gas can be injected into the subsurface by two modes: continuous injection (flooding) and cyclic injection (huff-n-puff), this study aims to compare the recovery efficiencies of such two processes in shale core plugs with ultra-low matrix permeability.Eagle Ford core samples were used in this study and saturated with shale oil. Using N2 as the gas source, gas injection tests were operated on the same plug orderly by both modes under the same operating conditions. The sensitivity of soaking time on the huff-n-puff performance was evaluated. Lab-scale numerical models were built to simulate flooding and huff-n-puff processes and to history match the experimental data. It was found that optimization design of huff-n-puff is important to achieve the maximum oil recovery. The results show that the huff-n-puff possess can achieve a higher oil recovery than the flooding process.

Investigation on gas migration in saturated materials with low permeability

Investigation of the hydro-mechanical effects on gas migration in saturated materials with low permeability is of great theoretical and practical significances in many engineering fields. The conventional two-phase flow (visco-capillary flow) theory, which regards the capillary pressure as the only controlling factor in gas migration processes, is commonly adopted to describe the gas flow in geo-materials. However, for materials with low permeability, the conventional two-phase flow theory cannot properly describe the gas migration. In this work, hydro-mechanical coupled gas injection tests were conducted. The volumetric variation of the liquid for applying the confining pressure in the specimen cell and the gas flow rate were monitored. Test results indicate that gas migration is influenced by the capillary pressure and the mechanical stress simultaneously. The two key parameters of the gas entry pressure Pentry and the gas induced-dilatancy pressure Pdilatancy are introduced for description of gas migration with respect to the capillary pressure and the mechanical stress effects, respectively. When the gas injection pressure is smaller than the Pentry and the Pdilatancy, the balance between the gas injection pressure and the confining pressure will lead to an intermittent gas flow. Sudden increase of gas flow rate could be observed once the gas injection pressure approaches the Pentry or the Pdilatancy. For higher gas injection pressures, the mechanical stress effects on gas migration could not be neglected. The sudden increase of gas flux under high gas injection pressures could be caused by the mechanical induced-dilatancy of channels, capillary pressure induced-continuous flow pathways, as well as the failure of sealing-efficiency. The failure of sealing-efficiency is closely related to the difference between the gas injection pressure and the confining pressure rather than the properties of the material tested. Monitoring the volume of liquid for applying confining pressure is helpful for detecting the failure of sealing efficiency and the mechanism of gas breakthrough.

An experimental study of the flow of gas along synthetic faults of varying orientation to the stress field: Implications for performance assessment of radioactive waste disposal

AbstractCritical stress theory states that fault transmissivity is strongly dependent upon orientation with respect to the stress tensor. This paper describes an experimental study aimed at verifying critical stress theory using a bespoke angled shear rig designed to examine the relationship between gas flows along a kaolinite‐filled synthetic fault as a function of fault dip. A total of 22 gas injection experiments were conducted on faults oriented 0°, 15°, 30°, and 45° to horizontal; both with and without active shear. Gas flow was seen to be complex; repeat gas injection testing showed a consistent gas entry pressure but considerably different, nonrepeatable, gas peak or breakthrough pressure. Gas flow occurred along discrete, dilatant pathways. The physics governing the pressure at which these features formed was repeatable; however, permeability was dependent on the number, distribution, and geometry of the resultant pathways. The nonrepeatable gas response suggests that the number of pathways was dependent on very subtle variations in gouge properties. No fault orientations were seen to exhibit nonflow characteristics, although critical stress theory predicted that two of the investigated fault angles should be effective seals. However, a small variation in gas entry pressure was seen with fault angle as a result of varying normal and shear stress acting on the gouge material. Shear was seen to enhance gas movement by reducing gas entry pressure and increased permeability once gas became mobile. Therefore, in kaolinite gouge‐filled faults, shear is not an effective self‐sealing mechanism to gas flow.

Pressure transient behavior of immiscible water alternating gas (IWAG) injection well with and without relative permeability hysteresis and capillary pressure effects

Abstract Pressure transient tests can provide useful information for characterization and evaluation of reservoirs under secondary and tertiary processes if they are properly designed and analyzed. There are a number of literature concerning the injection and falloff testing of water or gas injection wells. However, information available on immiscible water alternating gas (IWAG) injection tests is limited to falloff tests without considering the effect of trapped gas and capillary pressure. In this study, the pressure transient behavior of IWAG injection and falloff tests with and without relative permeability hysteresis and capillary pressure effects is investigated using simulated pressure data. The oil, gas, and water saturations distribution show that gas relative-permeability hysteresis improves the displacement of oil in IWAG injection. However, when capillary pressure is included, more oil is left behind the front. The pressure-derivative curves of falloff periods following gas injection period exhibit a long transition period with a half-slope line before the late-time radial flow for all cases, whereas, the pressure-derivative curve of the water injection falloff period after first gas injection for the case without hysteresis and capillary pressure exhibits a quarter-slope line in the long transition period. It is shown that trapped gas (or hysteresis in gas relative permeability) and capillary pressure have significant effects on the pressure and pressure-derivative behaviors of injection tests, but less effect on falloff tests. The findings of the study not only provide new insights to the pressure transient behavior of injection and falloff tests conducted during IWAG injection operations, but also should prove useful when properly interpreting and analyzing such tests.

Exploratory data analysis of the Large Scale Gas Injection Test (Lasgit) dataset, focusing on ‘second-order’ events around macro-scale gas flows

Abstract The Large Scale Gas Injection Test (Lasgit) is a field-scale experiment designed to study the impact of gas build-up and subsequent migration through an engineered barrier system (EBS). Lasgit has a substantial experimental dataset containing in excess of 26 million datum points. The dataset is anticipated to contain a wealth of information, ranging from long-term trends and system behaviours to small-scale or ‘second-order’ features. In order to interrogate the Lasgit dataset, a bespoke computational toolkit, designed to expose and quantify difficult to observe phenomena in large, non-uniform datasets, has been developed and applied. Presented results focus on the investigation and interpretation of second-order events occurring in close proximity (temporally and spatially) to a known macro-scale gas flow event that occurred during the second gas injection test. The similarity of the investigated event to dilatant flow observed in laboratory experiments is noted, as is the evidence for localized flow pathways in the bentonite EBS. The sensitivity of the toolkit's ability to highlight second-order events is also evaluated.

The experimental in-situ study of gas migration in crystalline rock with a focus on the EDZ

Abstract Centre of Experimental Geotechnics (CEG) participated in the FORGE project within WP4, which was focused on disturbed host rock formations. The Czech deep radioactive repository concept envisages crystalline rock as the host rock environment; therefore the CEG concentrated on such formations. Large-scale gas injection tests were carried out in Josef Underground Laboratory as part of the project aiming to study the behaviour of both the excavation damaged/disturbed zone (EDZ) and the undisturbed rock environment. The project schedule involved the performance of a large number of tests at various pressure levels and flow rates and in different parts of the rock mass. The various phenomena were studied employing gas injection test techniques using both single- and double-packer equipment. The tests performed revealed a significant difference in terms of gas permeability between the EDZ and the undisturbed rock mass. An investigation of the influence of the effect of pressure on gas permeability was also performed in order to study the process of the opening and closing of fractures (pathway dilation).

Numerical interpretation of gas-injection tests at different scales

Abstract To investigate gas-migration processes in saturated low-permeability argillaceous rocks, gas-injection tests under different injection pressures were carried out at different scales: on core samples at the laboratory scale; in the packed-off section of boreholes at the borehole scale (HG-B); and in the sealed microtunnel at the tunnel scale (HG-A) – a 1:2 scale experiment at the Mont Terri Rock Laboratory, Switzerland. All three tests at the Mont Terri Rock Laboratory involved Opalinus Clay. A fully coupled hydromechanical model has been developed that takes account of elastic and plastic anisotropies, anisotropic two-phase flow based on the van Genuchten function, and permeability changes when evaluating the experimental data. Two different flow regimes were studied: two-phase flow under low gas-injection pressure and dilatancy-controlled gas flow under high gas-injection pressure above the confining pressure in the laboratory experiment or the minimal principal stress in situ . When dealing with the dilatancy-controlled gas-flow regime, special consideration was made by applying two permeability approaches in which (i) the permeability change was pore-gas-pressure dependent and (ii) where the permeability change was deformation dependent. Using the parameter values determined by laboratory data, the in situ borehole tests obtained under well-defined hydromechanical conditions could be analysed accordingly. The gas-flow regime in large-scale experiments, as in the case of HG-A, is mainly governed by experimental circumstances: in this case, the excavation-induced fractures around an opening with a permeability four order of magnitude higher than that in the undisturbed rock mass.

Simultaneous gas and water flow in a damage-susceptible bedded argillaceous rock

A mathematical model that couples the governing and constitutive equations of two-phase flow and mechanical equilibrium has been developed to simulate gas injection tests for both laboratory- and field-scale experiments. The model takes into consideration the inherent anisotropy of sedimentary rocks due to bedding by including an anisotropic elastoplastic model for the mechanical process and using an anisotropic permeability tensor for the flow processes for both water and gas. The gas and water flow rates are assumed to follow Darcy’s law. The relative permeability of each phase and their respective degrees of saturation are represented by the Van Genuchten’s functions. We simulated laboratory and field gas injection experiments in Opalinus clay, a candidate geological formation for the geological disposal of radioactive wastes. The numerical results show good agreement with the experimental data measured in these tests in terms of two-phase flow regimes and hydromechanical response at various monitoring locations. Damage zones, either pre-existing due to excavation or induced by high gas injection pressure, are shown to clearly influence the gas flow rates and directions and would need special consideration in the design and safety assessment of the repository system.

Modelling fluid flow in Opalinus Clay excavation damage zone. A semi-analytical approach

Abstract The HG-A in situ test , located at the Mont Terri Underground Rock Laboratory (URL), was analysed as part of the FORGE project. This test investigated the behaviour of the excavation damage zone (EDZ) around a backfilled microtunnel by a series of water- and gas-injection tests. Prior to testing, the backfilled microtunnel was sealed with a hydraulic megapacker system. A key aspect was the investigation of crack opening and closure along the EDZ in response to water and gas injections in the context of radioactive waste disposal. In the model, the intrinsic permeability of the EDZ was assumed to depend on deformation, and additional simplifying assumptions were considered: axisymmetry about the tunnel axis; no gravity; soil slices orthogonal to the tunnel axis move independently and in plane strain; liquid and gas flows along the EDZ parallel to the tunnel axis; and undrained saturated conditions for the Opalinus Clay. As a result, the field equations were reduced to differential equations for liquid and gas pressures defined in a one-dimensional (1D) domain representing the EDZ. The main trends of the pressure evolution observed in the test section were reproduced. A 2D axisymmetric model confirmed the validity of the simplifying assumptions, except for small zones of the EDZ near the megapacker ends.

Hydromechanical properties of bentonite–sand block assemblies with interfaces in engineered barrier systems

Abstract Deep underground repositories for radioactive waste generally rely on a multibarrier system to isolate the waste from the biosphere. It consists of the natural geological barrier provided by the repository host rock and its surroundings, the waste container and an engineered barrier system (EBS): that is, the backfilling and sealing of shafts and galleries to block any preferential path for radioactive contaminants. Bentonite emplaced in compacted block form is the preferred option for the clay buffer for most waste management organizations. In assessing the performance of bentonite block masonries, conductive discrete interfaces inside the sealing elements (i.e. contacts between blocks) and to the host rock may act not only as mechanical weakness planes but also as preferential fluid pathways. We performed hydraulic tests on prefabricated bentonite–sand block assemblies (60:40). The results document that despite existing interfaces, the investigated bentonite block assembly behaves no different to that of the homogenous matrix during the saturation of the buffer. This has been confirmed by gas-injection tests on the former interface, as well by shear tests. The outstanding observation is that our results convincingly demonstrate that interfaces between bentonite bricks may ‘heal’ (not only seal), as was physically verified by confirmation of cohesion after presaturation.

Evidence of localised gas propagation pathways in a field-scale bentonite engineered barrier system; results from three gas injection tests in the large scale gas injection test (Lasgit)

Three gas injection tests have been conducted during a large scale gas injection test (Lasgit) performed at the Äspö Hard Rock Laboratory, Sweden. Lasgit is a full-scale experiment based on the Swedish KBS-3 repository concept, examining the processes controlling gas and water flow in highly water-saturated compact buffer bentonite. Three preliminary gas injection tests have been performed. The first two tests were conducted in the lower array of injection filters (FL903). Both of these tests showed similar behaviour that corresponded with laboratory observations. The third gas test was conducted in an upper array filter (FU910), which gave a subtly dissimilar response at major gas entry with an initial pressure drop followed by a secondary gas peak pressure. Lasgit has confirmed the coupling between gas, stress and pore-water pressure for flow before and after major gas entry at the field scale. All observations suggest mechanisms of pathway propagation and dilatancy predominate. In all three gas tests the propagation was through localised features that tended to exploit the interface between the copper canister and the bentonite buffer. Considerable evidence exists for the development of a highly-dynamic, tortuous network of pressure induced pathways which evolves both temporally and geospatially within the clay, opening and closing probably due to local changes in gas pressure and or effective stress.

Adaptive mesh refinement for compressible thermal flow in porous media

We present the implementation of a detailed model for compositional flow in porous media into an adaptive mesh refinement (AMR) framework. This model is applicable to a number of thermal problems. The basic numerical approach is an efficient shock-capturing method designed to minimise the number of expensive phase equilibrium calculations needed per time step. This is implemented in an AMR framework, using a multigrid method for general parabolic equations. The AMR method is applicable to a general class of hyperbolic–parabolic problems which includes the porous media model. We test the method on one-dimensional and two-dimensional thermal gas injection test cases, including non-uniform propagation in a heterogeneous medium.

A coupled hydro-mechanical model for simulation of gas migration in host sedimentary rocks for nuclear waste repositories

In a deep geological repository (DGR) for nuclear wastes, several mechanisms such as waste form degradation and corrosion could lead to gas generation. The produced gas can potentially overpressurize the repository, alter the hydraulic and mechanical properties of the host rock and affect the long term containment function of the natural (host rock) and engineered barriers. Thus, the understanding of the gas migration within the host rock and engineered barriers and the associated potential impacts on their integrity is important for the safety assessment of a DGR. In this paper, a coupled hydro-mechanical model for predicting and simulating the gas migration in sedimentary host rock is presented. A detailed formulation coupling moisture (liquid water and water vapor) and gas transfer in a deformable porous medium is given. The model takes into account the damage-controlled fluid (gas, water) flow as well as the coupling of hydraulic and mechanical processes (e.g., stress, deformation). The model also considers the coupling of the diffusion coefficient with mechanical deformation as well as considers the modification of capillary pressure due to the variation of permeability and porosity. The prediction capability of the developed model is tested against laboratory scale and in situ experiments conducted on potential host sedimentary rocks for nuclear waste disposal. The model predictions are in good agreement with the experimental results. The numerical simulations of the laboratory and field gas injection tests provide a better understanding of the mechanisms of gas migration and the potential effects of excessive gas pressure on the host sedimentary rocks. This research work has allowed us to identify key features related to gas generation and migration that are considered important in the long term safety assessment of a DGR in sedimentary host formations.

Self-sealing experiments and gas injection tests in a backfilled microtunnel of the Mont Terri URL

Abstract This paper describes a large-scale experiment on gas transport and hydromechanical processes around underground structures as part of a long-term geoscientific research programme at the Mont Terri Underground Rock Laboratory in the Jura Mountains of Switzerland. A horizontal microtunnel with a diameter of 1 m and a length of 13 m was drilled in an overconsolidated claystone formation. After installing monitoring instruments in the open tunnel, the end of the tunnel was backfilled with sand (test section) and a large hydraulic packer was emplaced in the seal section. The packer was inflated and subsequently the test interval was saturated with a synthetic pore-water. Following saturation an extended programme of hydraulic testing was performed over a two year period. A series of gas injection tests was then performed over a period of approximately 1.5 years. Following this first series of gas injections, a long post-gas hydraulic test has been initiated. The paper presents data and interpretation of the gas injections and subsequent hydraulic testing. The ability of the excavation damage zone to transport gas at pressures below fracturing is demonstrated. The post-gas hydraulic performance is considered and related to the self-sealing of the damage zone observed during saturation and hydraulic testing.

Observations of Pore Pressure in Clay-rich Materials; Implications for the Concept of Effective Stress Applied to Unconventional Hydrocarbons

The concept of effective stress is a well-established relationship where the stress acting on a rock can be viewed as the total stress minus the pore water pressure. In clay-rich rocks this relationship has been seen to be imperfect and a Biot coefficient is added to account for the material properties of the clay matrix. Large, stable pressure differentials and gradients were observed in several argillaceous materials during water and gas injection testing for a number of experimental geometries, including triaxial (Callovo-Oxfordian claystone), shear (kaolinite and Opalinus Clay) and full-scale testing (bentonite). Pore-pressure during water injection appeared to be evenly distributed on the sample scale, whereas in full-scale demonstration a complex distribution was seen, which may partly be due to hydraulic disequilibrium. During gas injection testing all observations suggested that transport was predominantly by dilatancy flow and the formation of micro-fissures. This led to localized pore pressure variations and a complex temporally and spatially varying pore pressure distribution. Isolated pockets of increased gas pressure could be seen to be stable. The nature of pore-pressure distribution, both hydraulic and gaseous, and the stability of pore pressure differentials means that the description of a meaningful average pore pressure was difficult and thus the use of effective stress with a single χ value might misrepresent local stresses within the rock. Localized deformation in the formation of dilatant pathways was dominated by the local gas pressure and not the bulk pore pressure. Therefore the law of effective stress on the micro-scale will be valid, whereas on a bulk scale could lead to errors in model predictions. This also has implications on the release of gas from shale due to the localized influence of stress around a fracture. Flow along fractures was localized with only a proportion of the fracture surface playing a part in both water and gas flow.

Phenomena exposure from the large scale gas injection test (Lasgit) dataset using a bespoke data analysis toolkit

Abstract The Large Scale Gas Injection Test (Lasgit) is a field-scale experiment designed to study the impact of gas buildup and subsequent migration through an engineered barrier system. Lasgit has a substantial experimental dataset containing in excess of 21 million datum points. The dataset is anticipated to contain a wealth of information, ranging from long-term trends and system behaviours to small-scale or ‘second-order’ features. In order to interrogate the Lasgit dataset, a bespoke computational toolkit, designed to expose difficult to observe phenomena, has been developed and applied to the dataset. The preliminary application of the toolkit, presented here, has resulted in a large number of phenomena being indicated/quantified, including highlighting of second-order events (small gas flows, perturbations in stress/pore-water sensors, etc.) and quantification of temperature record frequency content. Localized system behaviour has been shown to occur along with systematic aberrant behaviours that remain unexplained.

Laboratory gas injection tests of compacted bentonite buffer material for TRU waste disposal

Abstract The Radioactive Waste Management Funding and Research Center (RWMC) is leading a research programme to evaluate the gas transport mechanisms through a TRU (TRans-Uranium) waste disposal facility in Japan and acquire information on gas migration properties. In this paper we describe a series of laboratory gas injection tests using the bentonite adopted for use in Japanese TRU disposal, as well as an attempted visualization of the gas migration path inside the bentonite used as a buffer. By building a conceptual model from the results of these tests, the characteristics of gas migration through to breakthrough for bentonite can now be better understood in the context of Japanese TRU disposal. Thanks to the outcome of this research project, advanced knowledge may be applied to the conceptualization of TRU waste disposal.

Analysis of flow path around the sealing section HG-A experiment in the Mont Terri Rock Laboratory

As part of the HG-A experiment in the Mont Terri Rock Laboratory, large-scale in situ water/gas injection experiments was conducted in a microtunnel. This research work focuses on the numerical analysis of the experimental data and the in situ observations. Concerning a temporary change of the hydromechanical properties of Opalinus Clay during experimental operations, three phases were numerically interpreted. These included the generation of excavation damaged zone during tunnel excavation, in which highly permeable flow paths around the tunnel have been formed; the self-sealing effect during water tests; and the pressure evolution during a long-term gas injection test. A coupled two-phase flow and mechanics model, taking into account the strong anisotropic properties of Opalinus Clay, was developed to interpret the measured data. The hydraulic anisotropy was described by a transversely isotropic permeability tensor. An elasto-plastic model was established to consider both stiffness anisotropy and strength anisotropy. Anisotropic plasticity was studied using the microstructure tensor method.