High-pressure Fluid Injection Research Articles

Impact assessment of strain-dependent permeability on reservoir productivity in CSS

Geomechanical analysis is essential to assess the productivity forecast of cyclic steam simulation (CSS) operations in heavy oil reservoirs. The high-temperature and high-pressure fluid injection, as well as depletion in unconsolidated and poorly cemented porous media during CSS, may generate a relevant stress–strain response at levels that can lead to irreversible changes in reservoir permeability. Therefore, it is fundamental to consider permeability dependence on rock strain to properly analyze the impact of geomechanics, pressure, and temperature on reservoir performance. This paper implements a proposed directional strain-dependent permeability model to assess the productivity and compare it with a conventional volumetric permeability model through numerical simulation, considering the effects of wellbore creation on the stress–strain initial state. An explicit coupling between CMG-STARS and the geomechanics in-house simulator GSIM is carried out to perform the simulations using the proposed model. The results of oil production rates and permeability profiles show competitiveness between dilation and compaction periods that modify the structure of the porous media. There is a significant influence of stress state, strain, and injected energy on the permeability parameter. The approximations of this study might be used for feasibility assessment and optimization of CSS when integrating reservoir flow and geomechanical behavior analysis in productivity forecast.

Major influencing factors for the nucleation of the 15 November 2017 Mw 5.5 Pohang earthquake

The occurrence mechanism and key factors to induce the 15 November 2017 MW5.5 Pohang earthquake are investigated through combined interpretation of active fault zone, stress field evolution, earthquake source properties, fault structure, and spatiotemporal seismicity change. Analysis of historical earthquakes suggests the presence of active faults spawning moderate-size earthquakes around the Pohang Enhanced Geothermal System. The instrumental seismicity suggests that the faults were kept loaded until the occurrence of the Pohang earthquake. The Coulomb stress changes induced by the 2016 ML 5.8 (MW5.4) Gyeongju earthquake and the 2011 MW9.0 Tohoku-Oki earthquake suggest additional stress loading on the faults. The spatial distribution and focal mechanism solutions of mainshock and aftershocks illuminate a complex segmented fault structure. The fault geometry suggests that the fluid injection was made at the northeastern fault-segment conjunction. Numerical modeling of stress transfer and rupture dimension suggests that a ML3.1 (MW3.3) earthquake 7 months before the mainshock loaded the stress of tens of kilopascals around the hypocenter of mainshock. A series of the seismic activities including the Tohoku-Oki earthquake and local moderate-size earthquakes as well as high-pressure fluid injection contributed to the occurrence of the Pohang earthquake.

Numerical simulation of fluid injection-induced fault slip in heterogeneous shale formations

We study the impact of reservoir heterogeneity on fault slip in shale reservoirs subject to hydraulic stimulation. We construct a two-dimensional numerical model representing the horizontal cross-section of a faulted heterogeneous shale formation and simulate coupled hydro-mechanical processes in the system during and after high-pressure fluid injection. We conceptualize the natural fault as a geological composite consisting of a fault core and a damage zone that involves a set of subsidiary fractures parallel to the fault strike. We couple the solid deformation and fault displacement with the Darcy flow based on poro-elasticity principles and hydro-mechanical constitutive relations. The heterogeneous nature of the permeability field of the shale reservoir is mimicked as a random field governed by a log-normal probability density function and prescribed correlation lengths. We elucidate the linkage between fault slip and fluid flow field, permitting the capture of the spatio-temporal evolution of preferential flow channels and their consequences on fault slip and induced seismicity. We report a significant role of reservoir heterogeneity in fault reactivation when the hydraulic fracture is away from the fault but at a distance smaller than the correlation length. The results of our research have important implications for many fluid injection-related geoengineering activities.

Modelling fluid injection-induced fracture activation, damage growth, seismicity occurrence and connectivity change in naturally fractured rocks

We develop a fully-coupled hydro-mechanical model to simulate fluid injection-induced activation of pre-existing fractures, propagation of new damages, development of seismic activities, and alteration of network connectivity in naturally fractured rocks. The natural fracture system is represented by a discrete fracture network. The stress and strain fields of the fractured porous media are solved in the framework of a finite element model, which mimics the damage evolution in rock matrix based on an elasto-brittle failure criterion and simulates the normal/shear displacement of natural discontinuities based on a non-linear constitutive law. The coupled geomechanics and fluid flow processes in the fractured rock are computed honouring essential coupling mechanisms such as pore pressure-induced shear slip of pre-existing fractures, poro-elastic response of rock matrix, and stress-dependent permeability/storativity of both fractures and rocks. We use the numerical model developed to investigate the hydro-mechanical behaviour of two cases of deeply buried fractured rock in response to high-pressure fluid injection, one case with fracture density just below the percolation threshold and the other above the threshold. We observe a strong control of natural fracture network connectivity on the damage emergence, seismicity occurrence and connectivity change in the rock mass subject to hydraulic stimulation. We also highlight the strong poro-elastic effect that tends to drive heterogeneous connectivity evolution of fracture systems during fluid injection. The results of our research and insights obtained have important implications for injection-related geoengineering activities such as the development of enhanced geothermal systems and extraction of hydrocarbon resources.

Hydromechanical insight of fracture opening and closure during in-situ hydraulic fracturing in crystalline rock

Six hydraulic fracturing (HF) experiments were conducted in situ at the Grimsel Test Site (GTS), Switzerland, using two boreholes drilled in sparsely fractured crystalline rock. High spatial and temporal resolution monitoring of fracture fluid pressure and strain improve our understanding of fracturing dynamics during and directly following high-pressure fluid injection. In three out of the six experiments, a shear-thinning fluid with an initial static viscosity approximately 30 times higher than water was used to understand the importance of fracture leak-off better. Diagnostic analyses of the shut-in phases were used to determine the minimum principal stress magnitude for the fracture closure cycles, yielding an estimate of the effective instantaneous shut-in pressure (effective ISIP) 4.49±0.22 MPa. The jacking pressure of the hydraulic fracture was measured during the pressure-controlled step-test. A new method was developed using the uniaxial Fibre-Bragg Grating strain signals to estimate the jacking pressure, which agrees with the traditional flow versus pressure method. The technique has the advantage of observing the behavior of natural fractures next to the injection interval. The experiments can be divided into two groups depending on the injection location (i.e., South or North to a brittle-ductile S3 shear zone). The experiments executed South of this zone have a jacking pressure above the effective ISIP. The proximity to the S3 shear zone and the complex geological structure led to near-wellbore tortuosity and heterogeneous stress effects masking the jacking pressure. In comparison, the experiments North of the S3 shear zone has a jacking pressure below the effective ISIP. This is an effect related to shear dislocation and fracture opening. Both processes can occur almost synchronously and provide new insights into the complicated mixed-mode deformation processes triggered by high-pressure injection.

Influence of reservoir geology on seismic response during decameter-scale hydraulic stimulations in crystalline rock

Abstract. We performed a series of 12 hydraulic stimulation experiments in a 20m×20m×20m foliated, crystalline rock volume intersected by two distinct fault sets at the Grimsel Test Site, Switzerland. The goal of these experiments was to improve our understanding of stimulation processes associated with high-pressure fluid injection used for reservoir creation in enhanced or engineered geothermal systems. In the first six experiments, pre-existing fractures were stimulated to induce shear dilation and enhance permeability. Two types of shear zones were targeted for these hydroshearing experiments: (i) ductile ones with intense foliation and (ii) brittle–ductile ones associated with a fractured zone. The second series of six stimulations were performed in borehole intervals without natural fractures to initiate and propagate hydraulic fractures that connect the wellbore to the existing fracture network. The same injection protocol was used for all experiments within each stimulation series so that the differences observed will give insights into the effect of geology on the seismo-hydromechanical response rather than differences due to the injection protocols. Deformations and fluid pressure were monitored using a dense sensor network in boreholes surrounding the injection locations. Seismicity was recorded with sensitive in situ acoustic emission sensors both in boreholes and at the tunnel walls. We observed high variability in the seismic response in terms of seismogenic indices, b values, and spatial and temporal evolution during both hydroshearing and hydrofracturing experiments, which we attribute to local geological heterogeneities. Seismicity was most pronounced for injections into the highly conductive brittle–ductile shear zones, while the injectivity increase on these structures was only marginal. No significant differences between the seismic response of hydroshearing and hydrofracturing was identified, possibly because the hydrofractures interact with the same pre-existing fracture network that is reactivated during the hydroshearing experiments. Fault slip during the hydroshearing experiments was predominantly aseismic. The results of our hydraulic stimulations indicate that stimulation of short borehole intervals with limited fluid volumes (i.e., the concept of zonal insulation) may be an effective approach to limit induced seismic hazard if highly seismogenic structures can be avoided.

Hydraulic fracture propagation in a heterogeneous stress field in a crystalline rock mass

Abstract. As part of the In-situ Stimulation and Circulation (ISC) experiment, hydraulic fracturing (HF) tests were conducted in a moderately fractured crystalline rock mass at the Grimsel Test Site (GTS), Switzerland. The aim of these injection tests was to improve our understanding of processes associated with high-pressure fluid injection. A total of six HF experiments were performed in two inclined boreholes; the surrounding rock mass was accessed with 12 observation boreholes, which allows for the high-resolution monitoring of fracture fluid pressure, strain, and microseismicity in an exceptionally well-characterized rock mass. A similar injection protocol was used for all six experiments to investigate the complexity of the fracture propagation processes. At the borehole scale, these processes involved newly created tensile fractures intersecting the injection interval, while at the cross-hole scale, the natural network of fractures dominated the propagation process. The six HF experiments can be divided into two groups based on their injection location (i.e., south or north to a brittle–ductile shear zone), their similarity of injection pressures, and their response to deformation and pressure propagation. The injection tests performed in the south connect upon propagation to the brittle–ductile shear zone. Thus, the shear zone acts as a dominant drain and a constant pressure boundary. The experiments executed north of the shear zone show smaller injection pressures and larger backflow during bleed-off phases. From a seismic perspective, the injection tests show high variability in seismic response independently of the location of injection. For two injection experiments, we observe reorientation of the seismic cloud as the fracture propagated away from the wellbore. In both cases, the main propagation direction is normal to the minimum principal stress direction. The reorientation during propagation is interpreted to be related to a strong stress heterogeneity and the intersection of natural fractures striking differently than the propagating hydraulic fracture. The seismic activity was limited to about 10 m of radial distance from the injection point. In contrast, strain and pressure signals reach further into the rock mass, indicating that the process zone around the injection point is larger than the zone illuminated by seismic signals. Furthermore, strain signals indicate not just single fracture openings but also the propagation of multiple fractures. Transmissivities of injection intervals increase about 2–4 orders of magnitudes.

Kinematic dilation during the hydraulic stimulation of pre-fractured rocks

A single planar fracture geometry dominates the process of hydraulic fracturing in homogeneous, isotropic and cohesive materials. However, this fracture geometry cannot explain the high recovery efficiency observed in shale gas and enhanced geothermal energy. Experimental and numerical studies reported here demonstrate that pre-fractured, structured reservoirs experience extensive geometric distortion and dilation around the main opening-mode discontinuity generated by high-pressure fluid injection. This kinematic dilation may decrease at high confining stresses because blocks deform and split. Parameters such as the dominant fracture set orientation, block size and slenderness and blocks overlap length characterise a pre-structured medium and determine its deformation pattern and hydromechanical behaviour. Kinematically controlled dilational distortion greatly improves fluid conductivity in the pre-structured medium. A sixth-power relationship is anticipated between the enhanced hydraulic conductivity and the roundness of the main opening.

Correction to: Management of industrial high-pressure fluid injection injuries (IHPFII): the Water Jetting Association (WJA) experience with water driven injuries.

The original article can be found online.

Experimental investigation of the variations in hydraulic properties of a fault zone in Western Shandong, China

Coal remains the largest contributor to China’s energy structure. However, coal production has frequently been threatened by groundwater inrush incidents induced by fault zones. The variations in the hydraulic properties of fault zones under high water pressure in relation to groundwater inrush risk are poorly understood. This paper presents high-pressure fluid injection tests used to investigate the variations in hydraulic properties of a fault zone at depth. The analysis of experimental results shows that the injection rate and hydraulic conductivity increase as nonlinear functions of injection pressure. The evolution process induced by high water pressure can be divided into three flow phases, including a Darcy flow phase, a flow transition phase and an unstable flow phase. Two thresholds of injection pressure for the onset of nonlinear flow and the onset of substantial fracture dilation and hydraulic fracturing were identified. The hydraulic conductivity of the fault zone is closely related to the water pressure, with an evident increase in permeability resulting from fracture extension and enhancement of fracture connectivity, which were attributed to water injection. Finally, a conceptual model was developed to demonstrate the textural and hydraulic evolution of the fault zone.

Management of industrial high-pressure fluid injection injuries (IHPFII): the Water Jetting Association (WJA) experience with water driven injuries

BackgroundIndustrial high-pressure fluid injection injuries (IHPFII) are largely occupational in nature, where these injuries are most often sustained by male manual workers. Such traumatic injuries are largely sustained with water, grease, paint, gasoline or paint thinner. IHPFII are extremely serious injuries with life and limb-threatening potential carrying the risk of life-long disability.MethodsWe reviewed the Water Jetting Association© adverse incident database of advisory alerts detailing cases from around the world that have been brought to the association’s attention and the English-language literature on high-pressure hydrostatic injuries from 1937 to 2018.ResultsAccidents involving high-pressure water jets in the industry are uncommon. The clinical impact in all of the cases reviewed and the effects of water jet impacts range from instant fatalities at scene to loss of limb function and amputation. The majority of observed fatalities are due to major hemorrhage (exsanguination) secondary to the direct dissection of great vessels or high-energy blunt soft tissue injury and traumatic brain injury.ConclusionsAs with any other trauma, IHPWJI commonly result in amputation or death. Nonetheless, a lack of comprehension of the potential severity of injuries and range of infective complications appears to be largely due to the apparent benignity of the initial presentation of the wound. This in turn leads to delays (both avoidable and unavoidable) in the transfer to appropriate medical facilities and definitive care. There is an identifiable need for education (including for health care providers across multiple levels), training and the availability of personal trauma kits for the timely and effective management of IHPWJI from the initial jet impact on the scene, as well as a need for an established referral system.

Neutron imaging: a new possibility for laboratory observation of hydraulic fractures in shale?

Hydraulic fracturing, the creation of fractures by high-pressure fluid injection into a solid medium, is of interest to enhance the permeability of rocks. This complex three-dimensional hydro-mechanical process, however, has only been studied in the laboratory by boundary measurements or acoustic techniques with low spatio-temporal resolutions until now. In this paper, direct, high spatial resolution, and near real-time visualisation results of hydraulic fracture generation and propagation in prismatic specimens of Marcellus shale rock under in situ conditions (70 MPa, plane strain) are presented. Poly-methyl methacrylate specimens are also tested under the same conditions to highlight the importance of rocks' internal structure on the response of the tested rock. The results reveal a complex interaction among the injected fluid, the pre-existing natural fractures in shale structure, and the hydraulically induced fracture highlighting the governing role of rock fabric even under high stresses. These measurements are possible due to the unique sensitivity of neutrons to water. Besides the intrinsic interest of the results presented, this exploratory investigation highlights the potential of neutron imaging in elucidating the evolution of fluid flow and fluid-driven fractures, as X-rays have done for the evolution of solid structure only. Further, understanding of the mechanics of fracking will lead to development of more accurate hydro-mechanical constitutive models thus enabling the design of field operations with higher efficiencies.

Comprehensive geological dataset describing a crystalline rock mass for hydraulic stimulation experiments

High-resolution 3D geological models are crucial for underground development projects and corresponding numerical simulations with applications in e.g., tunneling, hydrocarbon exploration, geothermal exploitation and mining. Most geological models are based on sparse geological data sampled pointwise or along lines (e.g., boreholes), leading to oversimplified model geometries. In the framework of a hydraulic stimulation experiment in crystalline rock at the Grimsel Test Site, we collected geological data in 15 boreholes using a variety of methods to characterize a decameter-scale rock volume. The experiment aims to identify and understand relevant thermo-hydro-mechanical-seismic coupled rock mass responses during high-pressure fluid injections. Prior to fluid injections, we characterized the rock mass using geological, hydraulic and geophysical prospecting. The combination of methods allowed for compilation of a deterministic 3D geological analog that includes five shear zones, fracture density information and fracture locations. The model may serve as a decameter-scale analog of crystalline basement rocks, which are often targeted for enhanced geothermal systems. In this contribution, we summarize the geological data and the resulting geological interpretation.

Investigation of the hydraulic properties of deep fractured rocks around underground excavations using high-pressure injection tests

Groundwater inrush during excavation is one of the greatest challenges in modern underground engineering. The characteristics of groundwater inrush induced by high-pressure water in fractured rocks are poorly understood, and the risk assessment of water inrush hazard associated with deep geological conditions remains to be improved. Here, we present an in situ high-pressure fluid injection experimental investigation of the hydraulic properties of deep natural fractured rocks. We observe that the injection rate increases as a nonlinear function of the injection pressure, and the process can be divided into an initial flow phase and a flow mutation phase, which indicates flow regime variations during fluid injection. Fluid injection primarily triggers fracture dilation and then results in an evident increase in rock permeability. A conceptual model involving multiple physical processes is derived to describe the permeability evolution and flow behavior of fractured rocks throughout the high-pressure fluid injection tests. The results provide valuable insight for evaluating the evolution of the hydraulic properties of the surrounding rocks in relation to water inrush and dewatering designs.

Observation of transient seismic velocity variations allow monitoring of field scale pressure changes during high-pressure fluid injection

Observation of transient seismic velocity variations allow monitoring of field scale pressure changes during high-pressure fluid injection

Two-dimensional magmons with damage and the transition to magma-fracturing

Magma-fracturing during melt migration is associated with the propagation of a pore-generating damage front ahead of high-pressure fluid injection, which facilitates the transport of melt in the asthenosphere and initiates dike propagation in the lithosphere. We examine the propagation of porous flow in a damageable matrix by applying the two-phase theory for compaction and damage proposed by Bercovici et al. (2001a) and Bercovici and Ricard (2003) in 2-D. Damage (void generation and microcracking) is treated by considering the generation of interfacial surface energy by deformational work. We examine the stability of 1-D solitary waves to 2-D perturbations, and study the formation of finite-amplitude, two-dimensional solitary waves with and without solenoidal (rotational) flow of the matrix. We show that the wavelength and growth rate of the most unstable perturbations are dependent on both background porosity and the presence of solenoidal flow field. The effect of damage on finite amplitude 2-D solitary waves is then examined with numerical experiments. Stably propagating circular waves become flattened (elongated perpendicular to gravity) for small porosity, or elongated (parallel to gravity) for large porosity with increased damage. We show that the weakening of the matrix due to damage leads to these changes in wave geometry, which indicates a transition from magmatic porous flow to dike-like or sill-like magma-fracturing as magma passes through a semi-brittle/semi-ductile zone in the lithosphere.

Modeling porous rock fracturing induced by fluid injection

In this paper, we describe and implement a theoretical model that captures the physical processes occurring during hydraulic fracturing of saturated and unsaturated, porous rock. The model is based on a poro-elastic–plastic rheology with a Mohr–Coulomb yield function and unassociated flow rule, and includes frictional hardening, cohesion weakening, damage and mobilized dilatancy effects. In addition, fluid injection is described using the Richard׳s approximation. We numerically implement the model using a finite difference scheme on a staggered grid, and compare the model results with laboratory experiments published in Stanchits et al. (2011) [16]. From various experiments performed by Stanchits et al., we use three different laboratory configurations for comparing our model results and observations: (a) triaxial compression of a drained rock sample, (b) low pressure fluid injection into a drained, critically stressed rock, and (c) high pressure fluid injection into a saturated rock sample. We find good agreement between model and observations for all cases, indicating a proper formulation and implementation of the dominant physical mechanisms acting. The model reproduces experimental observations of macroscopic fracture, fluid front localization, and the stress–strain response curves. Matching observations from laboratory scale experiments establishes a benchmark and a calibrated model for numerically simulating experiments performed at the larger, field scale.

Two-phase damage models of magma-fracturing

Damage and fracturing in two-phase and porous flows are relevant for geological process such as magma-fracturing during melt migration, which is associated with the propagation of a pore-generating damage front ahead of high-pressure fluid injection. We therefore examine the propagation of porous flow in a damageable matrix by applying the two-phase theory for compaction and damage proposed by Bercovici et al. (2001a) and Bercovici and Ricard (2003). The movement of the fluid and the solid is governed by the two-phase flow laws, while damage (void generation and microcracking) is treated by considering the generation of interfacial surface energy by deformational work. Calculations of one-dimensional (1-D) flow of fluid migrating buoyantly through compacting and damageable matrix show that damage is mitigated in steady-state largely because of the loss of the velocity gradient at the fluid front. However, in time-dependent flows, linear stability analysis shows that the propagation velocity of porosity waves is strongly dependent on damage. In the damage-free case porosity waves are dispersive in that wave-speed decreases with wavenumber (inverse wavelength); however with damage the dispersion flattens and beyond a critical damage reverses (the wave speed increases with wavenumber). Since normal dispersive behavior balances breaking in the nonlinear wave case, such reversed dispersion implies that damage has a profound effect in the nonlinear limit by facilitating wave front steepening and higher wave velocities. Nonlinear solitary wave solutions are obtained numerically and show that the transmission of porosity waves induces high stress and damage that can push the damage front forward. With damage the porosity waves sharpen and calculations suggest that they can transform from shape-conserving solitary waves into faster high amplitude waves, which is also predicted by the linear theory. Such pulse-like sharper waves may prove effective at promoting fluid migration through magma-fracturing.

Water-jet dissector for endoscopic submucosal dissection in an animal study: outcomes of the continuous and pulsed modes

Endoscopic submucosal dissection (ESD) allows en bloc resection of early neoplastic lesions of gastrointestinal tract. Lesions are lifted by submucosal fluid injection before circumferential incision and dissection. High-pressure fluid injection using water jet (WJ) technology is already used for lifting and dissection in surgery. The study was designed to assess WJ for ESD submucosal lifting and dissection. An experimental, randomized comparative, "in vivo" nonsurvival animal study on 12 pigs was designed. Stomach mucosal areas were delineated and resected using three ESD techniques: technique A-syringe injection and IT knife dissection; technique B-WJ continuous injection and IT knife dissection; technique C-WJ injection and WJ pulsed dissection. Injection and dissection speeds and complications rates were assessed. Water jet continuous injection is faster than syringe injection (B faster than A, p = 0.001 and B nonsignificantly faster than C, p = 0.06). IT knife dissection is significantly faster after WJ continuous injection (B faster than A, p = 0.003). WJ pulsed dissection is significantly slower than IT knife dissection (C slower than A and B, both p < 0.001). The overall procedure speed was significantly higher and the immediate bleedings rate was significantly lower for technique B than A and C (overall procedure speed p = 0.001, immediate bleedings p = 0.032 and 0.038 respectively). There were no perforations with any technique. Water jet fluid continuous injection speeds up ESD, whereas pulsed WJ dissection does not.

Assessment of Stimulated Area Growth During High-Pressure Hydraulic Stimulation of Fractured Subsurface Reservoir

Hydraulic stimulation is performed by high-pressure fluid injection, which permanently increases the permeability of a volume of rock, typically transforming it from the microdarcy into the millidarcy range. After a period of stimulation, fluid injection and recovery boreholes are introduced into the stimulated rock volume, and heat is extracted by water circulation. In the present study a simplified mathematical model of non steady-state hydraulic stimulation is proposed and analyzed. Fluid flow is assumed to be radial, injected flow rate constant; and fluid density, rock porosity, and permeability depend on fluid pressure. The conventional boundary of the growing stimulated rock volume is introduced as a surface where the porosity and permeability of the stimulated rock exhibit a sharp decline and remain constant within the undisturbed area. The problem is solved analytically by a modified method of integral correlations. As a result, approximate close-form solutions for pressure distributions in the stimulated and nonstimulated (undisturbed) areas are obtained, and an equation for the moving boundary of the stimulated volume is derived. The correctness of the approximate solution is validated by comparison to an exact self-similar solution of the problem obtained for the particular case when the well’s radius is assumed to be equal to zero.