Pressure Front Research Articles

Computing Pressure Front Propagation Using the Diffusive-Time-of-Flight in Structured and Unstructured Grid Systems via the Fast-Marching Method

SummaryDiffusive-time-of-flight (DTOF), representing the travel time of pressure front propagation, has found many applications in unconventional reservoir performance analysis. The computation of DTOF typically involves upwind finite difference of the Eikonal equation and solution using the fast-marching method (FMM). However, the application of the finite difference-based FMM to irregular grid systems remains a challenge. In this paper, we present a novel and robust method for solving the Eikonal equation using finite volume discretization and the FMM.The implementation is first validated with analytical solutions using isotropic and anisotropic models with homogeneous reservoir properties. Consistent DTOF distributions are obtained between the proposed approach and the analytical solutions. Next, the implementation is applied to unconventional reservoirs with hydraulic and natural fractures. Our approach relies on cell volumes and connections (transmissibilities) rather than the grid geometry, and thus can be easily applied to complex grid systems. For illustrative purposes, we present applications of the proposed method to embedded discrete fracture models (EDFMs), dual-porosity dual-permeability models (DPDK), and unstructured perpendicular-bisectional (PEBI) grids with heterogeneous reservoir properties. Visualization of the DTOF provides flow diagnostics, such as evolution of the drainage volume of the wells and well interactions.The novelty of the proposed approach is its broad applicability to arbitrary grid systems and ease of implementation in commercial reservoir simulators. This makes the approach well-suited for field applications with complex grid geometry and complex well architecture.

A review of liquid silicone rubber injection molding: Process variables and process modeling

AbstractLiquid silicone rubber (LSR) is an elastomer molded into critical performance components for applications in medical, power, consumer, automotive, and aerospace applications. This article reviews process behavior, material modeling, and simulation of the (LSR) injection molding process. Each phase of the LSR injection molding process is discussed, including resin handling, plastication, injection, pack and hold, and curing; and factors affecting the molding process are reviewed. Processing behavior of LSR is marked by transient interactions between curing, shear rate, temperature, pressure, and tooling. Therefore, current LSR models for curing, viscosity, pressure, and temperature are discussed. Process dynamics and material modeling are combined in LSR injection molding simulations with applications in mold design, troubleshooting process‐induced defects, and management of shear stress and non‐uniform temperatures between LSR and substrates during overmolding. Finally, case studies using commercial simulation software are presented, which have shown cavity pressure and flow front advancement within 3% of experimental values. Optimization of LSR materials, data collection, model fitting, venting, and bonding remain areas of continued interest.

Environmental safety assessment in the influence zone of the existing Zelenchukskaya HPP-PSHS and Krasnogorsk HPP-1, HPP-2 under construction in the area of Ust-Dzhegutinsky reservoir location

«Environmental safety “in the considered spatial limits of the Upper Kuban basin geosystem (F w=11,0*103 km3, W l=3,3*103 km3, Watm=110*103 Wb.g.U.K..=124,3*103 km3) makes up about 21.4% of the spatial limits of the Kuban River, where water resources are formed according to the average long-term data about 3,0 km3, which are used in various sectors of economic activity, including for the electricity generation at Zelenchukskaya HPP-PSHS and Krasnogorsk HPP-1 and HPP-2 under construction with installed power up to 25 mW, located below the HPP-PSHS Zelenchukskaya section and above the alignment (12 km) of the Ust-Dzhegutinsky reservoir. Such HPP-PSHS placement on a 15 km long section of the Kuban Zelenchukskaya riverbed and Krasnogorsk HPP-1 and HPP-2 under construction determined a highly active zone of influence (IV), which is caused by a change in the natural hydrograph of the Kuban River, with the formation of the channel flow movement unsteady hydraulic regime in the considered section of the river to the Ust-Dzhegutinsky reservoir. Unsteady changes in the water level in the Ust-Dzhegutinsky reservoir affects the pressure front structures safety, the functional operation of the water intake system in the Bolshoi Stavropol Canal (BSC) and hydraulic BSC operation. To assess the level of “environmental safety”, it became necessary to develop a mathematical model of the impact of the HPP-PSHS functional work and Krasnogorsk HPP-1 and HPP-2 under construction at the level modes of operation of the Ust-Dzhegutinsky reservoir.

Distinctive features of cascad hydraulic units location in the Republic of Azerbaijan

Purpose. To analyze the reservoir fund of the Republic of Azerbaijan and to establish the distinctive features of the location of artificial water bodies along the hydrological network.
 Methods. Theoretical analysis of the distribution of artificial water bodies along the largest rivers of the Republic of Azerbaijan taking into account their geographical and hydromorphological features.
 Findings. The analysis of the location of artificial water bodies in the territory of the Republic of Azerbaijan and neighboring states has been carried out. Distinctive features of the cascade of waterworks on the Kura river have been identified. The substantiation of the need to develop a methodology for assessing the risk of emergencies at the specified cascade of hydroelectric complexes, taking into account their distinctive features is given.
 Application field of research. The presented results can be used in the field of ensuring the safety of hydrotechnical pressure front structures when carrying out the probabilistic and deterministic assessment of risk of emergencies on them.

2D Hydro‐Mechanical‐Chemical Modeling of (De)hydration Reactions in Deforming Heterogeneous Rock: The Periclase‐Brucite Model Reaction

AbstractDeformation at tectonic plate boundaries involves coupling between rock deformation, fluid flow, and metamorphic reactions, but quantifying this coupling is still elusive. We present a new two‐dimensional hydro‐mechanical‐chemical numerical model and investigate the coupling between heterogeneous rock deformation and metamorphic (de)hydration reactions. We consider linear viscous compressible and power‐law viscous shear deformation. Fluid flow follows Darcy's law with a Kozeny‐Carman type permeability. We consider a closed isothermal system and the reversible (de)hydration reaction: periclase and water yields brucite. Fluid pressure within a circular or elliptical inclusion is initially below the periclase‐brucite reaction pressure and above in the surrounding. Inclusions exhibit a shear viscosity thousand times smaller than for the surrounding. For circular inclusions, solid deformation has a minor impact on the evolution of fluid pressure, porosity, and reaction front. For elliptical inclusions and far‐field shortening, rock pressure inside the inclusion is higher compared to circular inclusions, and reaction‐front propagation is faster. The reaction generates a large change in porosity (∼0.1%–55%) and in solid density (∼2,300–3,500 kg m−3), and the reaction front exhibits steep gradients of fluid pressure and porosity. Reaction‐front propagating increases the weak inclusion's surface and causes an effective, reaction‐induced weakening of the heterogeneous rock. Weakening evolves nonlinear with progressive strain. Distributions of fluid and rock pressure as well as magnitudes and directions of fluid and solid velocities are significantly different. The models mimic basic features of shear zones and show the importance of coupling deformation and metamorphism. The applied MATLAB algorithm is provided.

Fluid Initiation of Fracture in Dry and Water Saturated Rocks

Abstract—We present the results of the laboratory studies on fluid initiated fracture in the samples of porous-fractured rocks that have been initially saturated with a pressure-injected fluid and then tested under increasing fluid pressure in saturated rocks. The tests were conducted at the Geophysical observatory “Borok” of Schmidt Institute of Physics of the Earth of the Russian Academy of Sciences. The laboratory is equipped with electrohydraulic press INOVA-1000. The experiments were conducted on the rock samples with substantially different porosity. The tested samples were made of Buffalo sandstones, granites from the well drilled in a seismically active region and of granites from the well in the Voronezh crystalline massif. The permeability of granite samples was varied by their controlled artificial cracking by successive heating and cooling. The experimental procedure was set up in the following way. A preliminarily dried sample was initially subjected to uniaxial loading in uniform compression (confining pressure). Loading was performed at a constant strain rate until the moment when the growth rate of acoustic emission (AE) activity began to accelerate which indicated that the stress level approaches ultimate strength. Since that, the loading rate was decreased by an order of magnitude, and water was infused into a sample from its top face. The bottom end of a sample was tightly sealed and impermeable to water. After this, the pore pressure in the sample that had got saturated with water to that moment was raised in steps whose amplitudes were varied. The obtained results of the laboratory studies show that the character and intensity of fluid initiation of fracture markedly differ under primary fluid injection into the porous-fractured samples and under the subsequent increases of the pore pressure in the saturated samples. The time delay of acoustic response relative to fluid initiation and the amplitude of the response proved to be larger in the case of water injection into dry samples than in the case of raising the pore pressure in saturated samples. The AE response to the decrease in the pore pressure was also detected in the experiments. The theoretical analysis of fluid propagation in a pore space of an air-filled sample in the model with piston-type air displacement has shown that in the case of water injection into a dry sample, the fluid pressure front propagates more slowly than in the saturated sample.

Hydraulic Transient Analysis of Sewer Pipe Systems Using a Non-Oscillatory Two-Component Pressure Approach

On the basis of the two-component pressure approach, we developed a numerical model to capture mixed transient flows in close conduit systems. To achieve this goal, an innovative Godunov finite-volume numerical scheme is proposed to suppress the spurious numerical oscillations occurring during rapid pipe pressurization. To dissipate the spurious numerical oscillations, we admit artificial numerical viscosity to the numerical scheme through applying a proposed Harten, Lax, and van Leer (HLL) Riemann solver for calculating the numerical fluxes at the computational cell interfaces. The proposed solver controls the magnitude of the numerical viscosity through adjusting the left and right wave velocities. A wave velocity calculator is proposed to optimally distribute the numerical viscosity over several computational cells around the computational cell in which the pressurization front is located. The proposed solver admits significant artificial numerical viscosity when the pipe pressurization is imminent and automatically reduces it in other places; in this way the numerical diffusion and data smearing is minimized. The validity of the proposed model is justified by the aid of several test cases in which the numerical results are compared with both experimental data and the results obtained from analytical methods. The results reveal that the proposed model succeeds in completely removing the spurious numerical oscillations, even when the pipe acoustic speed is over 1000 m/s. The numerical results also show that the model can successfully capture occurrence of negative pressures during the course of transient flow.

Estimating the net costs of brine production and disposal to expand pressure-limited dynamic capacity for basin-scale CO2 storage in a saline formation

If carbon capture and storage (CCS) needs to be deployed at basin- or larger-scale, it is likely that multiple sites will be injecting carbon dioxide (CO2) into the same geologic formation. This could lead to excessive pressure buildup, overlapping induced pressure fronts, and pressure interference with neighboring uses of the subsurface. Extracting the in situ brine from the storage formation could be necessary to relieve pressure constraints; control migration of the CO2 plume, displaced brine, and the induced pressure front; and sequester more CO2 while reducing potential risks. Such active pressure management could be very costly, and it could present a formidable economic constraint on the feasible scale of deployment of CCS. Alternatively, there may be high-injectivity zones (“storage sweet spots”) where a significant volume of CO2 could be stored without producing brine. For simulated deployment of CO2 storage sites across the Illinois Basin, the results of this study suggest that brine production could be required to sequester 20 % or more of the regional CO2 emissions of major stationary sources in the Mount Simon Sandstone saline formation. In some cases, brine production could expand pressure-limited CO2 storage capacity enough to more than compensate for the additional costs of pressure management, but only if produced brine could be cheaply reinjected onsite for disposal in an overlying geologic formation. With or without brine production, this study found that the lowest-cost deployment option was to inject CO2 only into a potential storage sweet spot of the Mount Simon Sandstone.

Generation of Risk Information Based on Comprehensive Real-Time Analysis of Flooding and Landslide Disaster Occurrence Hazard and Social Vulnerability

It is important to discern in real time the risk level of rain-related disasters such as floods and landslides in order to maintain readiness against heavy-rainfall disasters and to decide on the suitable response measures. In this study, we developed an information processing technology that employs hazard information which indicates the risk of inundation or landslides. It also presents indices of social vulnerability and applies the spatial resolution functions of a geographic information system (GIS) to extract in real time the highly exposed and vulnerable areas that are faced with an increased risk of flooding or landslides. The technology’s validity was verified using a case study – namely, the heavy rainfall that accompanied a pressure front in August, 2019. The results show that, with respect to flood risks, we were able to extract in real time specific areas where flooding may be taking place, thus demonstrating the possibility of applying the technology to decide priorities in disaster response measures. Future issues are related to information dissemination, including the specific labeling and expressions that are easy for the user to understand as well as improving the user interface so as to facilitate delivery of relevant risk information in real time.

Geometric modeling and analysis of detonation cellular stability

A geometric model with a low computational complexity capable of simulating detonation behavior in physical systems is proposed. In support of the geometric model development, a series of cylindrical 1D simulations with a variable size initiation kernel are performed in hydrogen-oxygen mixtures. From these 1D simulations a detonation cell stabilization mechanism is identified. The stabilization mechanism is predicated on the size of the gap between the pressure and temperature fronts at the point where the average pressure front velocity along one cell length is equal to the CJ velocity. This gap, in a multidimensional detonation, is the ignition kernel of a subsequent blast, and dictates the formation of the subsequent cell. Serial analysis of blasts in this context leads to a unique stable blast kernel size for any mixture, which, within the uncertainty of the initial kernel state, can predict the experimental cell length for mixtures considered in this study. Using a tabulation of the 1D simulations as an input, a formulation and sample results of the geometric model are shown. The geometric model can reproduce both qualitative and quantitative features of experimental detonation cellular structure.

Detonation development from a hot spot in methane/air mixtures: Effects of kinetic models

Natural gas is a promising alternative fuel which can be used in internal combustion engines to achieve low carbon emission and high thermal efficiency. However, at high compression ratio, super-knock due to detonation development might occur. In this study, the autoignitive reaction front propagation and detonation development from a hot spot were investigated numerically and the main component of natural gas, methane, was considered. The objective is to assess the performance of different kinetic models in terms of predicting hot spot–induced detonation development in methane/air mixtures. First, simulations for the constant-volume homogeneous ignition in a stoichiometric methane/air mixture were conducted. The ignition delay time, excitation time, critical temperature gradient, thermal sensitivity and reduced activation energy predicted by different kinetic models were obtained and compared. It was found that there are notable discrepancies among the predictions by different kinetic models. Then, hundreds of one-dimensional simulations were conducted for detonation development from a hot spot in a stoichiometric CH4/air mixture. Different autoignition modes were identified and the detonation regimes were derived based on the peak pressure and reaction front propagation speed. It was found that even at the same conditions, different propagation modes can be predicted by different kinetic models. The broadest detonation development regime was predicted by the reduced GRI mechanism, while a relatively narrow regime was predicted by the recent kinetic models such as FFCM-1 and Aramco 3.0. The present results indicate that super-knock prediction strongly depends on the kinetic model used in simulations. Therefore, significant efforts should be devoted to the development and validation of kinetic models for natural gas at engine conditions.

Bilinear pressure diffusion and termination of bilinear flow in a vertically fractured well injecting at constant pressure

Abstract. This work studies intensively the flow in fractures with finite hydraulic conductivity intersected by a well injecting or producing at constant pressure, either during an injection or production well test or the operation of a production well. Previous investigations showed that for a certain time the reciprocal of flow rate is proportional to the fourth root of time, which is characteristic of the flow regime known as bilinear flow. Using a 2D numerical model, we demonstrated that during the bilinear flow regime the transient propagation of isobars along the fracture is proportional to the fourth root of time. Moreover, we present relations to calculate the termination time of bilinear flow under constant injection or production well pressure as well as an expression for the bilinear hydraulic diffusivity of fractures with finite hydraulic conductivity. To determine the termination of bilinear flow regime, two different methods were used: (a) numerically measuring the transient flow rate in the well and (b) analyzing the propagation of isobars along the fracture. Numerical results show that for low dimensionless fracture conductivities the transition from bilinear flow to another flow regime (e.g., pseudo-radial flow) occurs before the pressure front reaches the fracture tip, and for high dimensionless fracture conductivities it occurs when the pressure front arrives at the fracture tip. Hence, this work complements and advances previous research on the interpretation and evaluation of well test analysis under different reservoir conditions. Our results aim to improve the understanding of the hydraulic diffusion in fractured geologic media, and as a result they can be utilized for the interpretation of hydraulic tests, for example to estimate the fracture length. Highlights. The reciprocal of flow rate is proportional to the fourth root of time. The migration of isobars in the fracture is proportional to the fourth root of time. For low dimensionless fracture conductivities, bilinear flow ends before the pressure front reaches the fracture tip. For high dimensionless fracture conductivities, bilinear flow ends when the pressure front reaches the fracture tip. Isobars accelerate when they approach the fracture tip.

Stress Perturbation From Aseismic Slip Drives the Seismic Front During Fluid Injection in a Permeable Fault

AbstractFluid pressure changes affect fault stability and can promote the initiation of earthquakes and aseismic slip. However, the relationship between seismic and aseismic fault slip during fluid injection remains poorly understood. Here, we investigate, through 3‐D hydromechanical modeling, the spatiotemporal evolution of seismicity and aseismic slip on a permeable, slip‐weakening fault subjected to a local injection of fluid, under different prestress conditions. The model results in an expanding aseismic slip region, which concentrates shear stress at its edge and triggers seismicity. The aseismic slip dominates the slip budget, whatever the initial fault stress. We find that the seismicity is collocated with the aseismic rupture front rather than with the fluid pressure diffusion front. On faults initially far from failure, the aseismic rupture front is located behind or at the pressure front. On faults initially closer to failure, the model predicts that both the rupture front and the seismicity outpace the pressurized zone, resulting in a sharp increase of the migration velocity and released moment of the seismicity. Insights gained from this modeling study exhibit various features that are observed in sequences of induced earthquakes in both field experiments and natural reservoir systems and can help guide the interpretation of past and future observations of induced seismicity.

Improving well productivity – Ways to reduce the lag between the diffusive and convective time of flight in shale wells

This short communication explains how the key to improve shale well productivity lies in reducing the gap between the diffusive and convective time of flight. Diffusive time of flight in a hydrocarbon reservoir controls which reservoir regions will be affected by the pressure transient at a certain time after the onset of production. The convective time of flight is a measure of tracer front advance as well as for fluid withdrawal rates due to a well system draining the reservoir. The difference between the advance rate of a diffusive pressure front and a convective tracer front has been long recognized. However, only recently has it become clear that a thorough grasp of the two concepts is crucial for better decisions about fracture and well spacing in low permeability reservoirs. Although the diffusive time of flight is highlighted in the traditional pressure depletion models, the convective time of flight has a more direct impact on well productivity because it controls the advance rate, to the well system, of hydrocarbon fluids to be produced. The two concepts, diffusive and convective time of flight, are briefly highlighted below, with emphasis on their relevance for decision-making in shale field development solutions.

Consequence prediction for dust explosions involving interconnected vessels using computational fluid dynamics modeling

Combustible dust explosions continue to present a significant threat toward operating personnel and pneumatic conveyance equipment in a wide variety of processing industries. Following ignition of suspended fuel within a primary enclosure volume, propagation of flame and pressure fronts toward upstream or downstream interconnected enclosures can result in devastating secondary explosions if not impeded through an appropriate isolation mechanism. In such occurrences, an accelerated flame front may result in flame jet ignition within the secondary vessel, greatly increasing the overall explosion severity. Unlike an isolated deflagration event with quantifiable reduced pressures (vent sizing according to NFPA 68 guidance), oscillation of pressure between primary and secondary process vessels leads to uncertain overpressure effects. Dependent on details of the application such as relative enclosure volumes, relief area, fuel type, suspended concentration, duct size, and duct length, the maximum system pressure in both interconnected vessels can be unpredictable. This study proposes the use of FLame ACceleration Simulator (FLACS) computational fluid dynamics (CFD) modeling to provide reliable consequence predictions for specific case scenarios of dust deflagrations involving interconnected equipment. Required minimum supplement to the originally calculated relief area (Av) was determined through iterative simulation, allowing for reduced explosion pressures (Pred) to be maintained below theoretical enclosure design strengths (Pes).

Tracking Fluid Flow in Shallow Crustal Fault Zones: 2. Insights From Cross‐Hole Forced Flow Experiments in Damage Zones

AbstractCross‐hole fluid injection tests were performed on shallow (0.5 km depth) crustal fault zones to characterize their internal flow structures on scales of 3–30 m. The data were acquired at the Grimsel Rock Laboratory, Switzerland, after the zonal isolation of damage zones in boreholes equipped with multipacker systems. We show that 80% of the pressure responses detected evolved as a power function of time due to fracture‐controlled flow and were best described by a fractional model with a flow dimension (n) in the range of [1.3,1.5]. The scaling of characteristic times (tc) and intermediate transitions in the flow dimension is shown to be inconsistent with the trend expected for normal diffusion, indicating a diffusion slowdown process caused by the spatial integration of structurally different damage zones. An analysis of the entire data set points to a subdiffusive regime where the mean squared displacement of pressure fronts, ⟨r2(t)⟩, scales as ⟨r2(t)⟩∝t0.59, similar to an anomalous diffusion process on synthetic fractal systems. We characterize this diffusion slowdown by calculating a network connectivity exponent (θ) of 1.4, from which we estimate a fractal dimension of df=2.23 for the tested fault zones. Such a value close to 2.0 supports the view of fault damage zones as hierarchical systems promoting flow channeling. Our results provide further support to theoretical models of fractional flow and their application to fractured media in nature.

A133 DEVELOPMENT OF TYPE 3 ACHALASIA WITH DISTANT PANCREATIC MALIGNANCY

Abstract Background Pseudoachalasia is often caused by malignant involvement at the gastroesophageal junction (GEJ) leading to dysphagia. Aims We describe a case of type 3 achalasia presenting in a woman with metastatic pancreatic cancer with no direct involvement at the GEJ, fundus or cardia. Methods A case report and literature review were performed. Results A 53-year-old woman presented with a 2-month-history of progressive abdominal pain, nausea and vomiting with a 30-pound weight loss. She had a remote history of breast cancer in remission after surgery and chemoradiation. On presentation, she denied chest pain, reflux, dysphagia or odynophagia. Abdominal exam revealed focal epigastric tenderness and jaundice. Abdominal CT showed a 6.7 x 5.8 cm conglomerate mass involving the hepatic hilum, pancreatic head, duodenum, common bile duct, and portal vein with gastric outlet and biliary obstruction. This mass was confirmed to be a pancreatic adenocarcinoma on pathology. She then underwent nasogastric tube decompression. Initial esophagogastroduodenoscopy (EGD) confirmed a stenotic area at the distal duodenal cap. A duodenal stent and common bile duct stent were placed during a second EGD. The esophagus and GEJ were unremarkable on both endoscopic exams. She was started on chemotherapy with gemcitabine and abraxane. Two weeks after her stent placement, she rapidly developed severe retrosternal squeezing discomfort and choking occurring with swallowing. CT chest and abdomen were negative for any intrathoracic and diaphragmatic involvement with stability of the mass. A barium swallow study demonstrated tertiary contractions in the thoracic esophagus with marshmallow hold-up in the distal esophagus. She then underwent a high-resolution esophageal manometry study that demonstrated an elevation of integrated residual pressure (IRP) of the lower esophageal sphincter (LES) and absence of peristalsis, with the distal 2/3rds of the esophagus showing a simultaneous and prolonged pressure front consistent with type 3 achalasia, Chicago classification v3.0 [Figure 1]. All contractions had a distal contractile integral (DCI) of >8000 mmHg-cm-s. She experienced significant symptom improvement with pinaverium bromide, a gut-specific calcium channel antagonist. A review of the literature revealed that there have been 4 English-language cases published on pseudoachalasia associated with pancreatic cancer, with all cases describing direct infiltration of pancreatic cancer in the GEJ, cardia or fundus with manometric features of type I achalasia. Conclusions We report the first case of type 3 achalasia with no evidence of direct malignant infiltration at the GEJ on radiographic and endoscopic evaluations. Possible mechanisms to explain this phenomenon include paraneoplastic antibody-mediated impairment of enteric neurons that decrease nitric oxide availability, or microscopic disease involvement at the GEJ. Funding Agencies None

A new description method of the position of combustion front in dry linear fire flooding process

Abstract The description methods of combustion front are required to be fast and accurate for in-situ combustion (fire flooding), and the current methods can't fully conform that requirement. However, the description method based on conventional data monitoring is a way to solve this problem. The experimental results of dry combustion indicates that the position of combustion front has a nearly linear correlation with the total acid numbers (TAN) of oil produced for in-situ combustion (ISC). Meanwhile, the tendency of pressure variation between neighboring air injection wells can determine the state and the time of combustion chamber connected in linear ISC process. Therefore, based on the relationship between TAN, injection pressure and combustion front, the acid numbers-pressure(ANP) method is established，which is suitable for dry linear fire flooding. Further, this method is applied to the calculation of the position of the combustion front in HQ1 ISC pilot area in Xinjiang over the years. Compared with other methods, the ANP method can rapidly calculate the position of combustion front and confirm the connectivity of combustion chamber. In addition, the combination of the ANP and other method is a good solution to confirm the position of combustion front for dry linear ISC process.

Changing Flow Paths Caused by Simultaneous Shearing and Fracturing Observed During Hydraulic Stimulation

AbstractWe monitored the seismohydromechanical rock mass response to high‐pressure fluid injection during a decameter‐scale hydraulic stimulation experiment in crystalline rock at the Grimsel Test Site, Switzerland. Time series recorded at two pressure monitoring locations show abrupt pressure increases that change in amplitude and appearance between subsequent stimulation cycles. Induced seismicity correlates with the propagation of one of these pressure fronts. Deformation data along the same shear zone shows permanent fracture dislocation preceded by strong transient fracture opening. We interpret these observations as nonlinear pressure diffusion along flow channels that reorganize in response to hydromechanical effects during stimulation. Combining these observations with the in situ stress field estimated before stimulation, we argue that the underlying hydromechanical processes involve mixed‐mode stimulation with both Mode I and II/III fracture dislocation.

Tectonic and Anthropogenic Microseismic Activity While Drilling Toward Supercritical Conditions in the Larderello‐Travale Geothermal Field, Italy

AbstractThis study investigates the seismic activity occurring at the Larderello‐Travale geothermal field, central Italy, from June 2017 to January 2018. We deployed a network composed of nine broadband stations around the Venelle 2 well drilling for supercritical fluids. During the experiment, we recognize two types of seismic events (type 1 and type 2). Type‐1 events have clear and wave arrivals and occur in clusters both above and below the K‐horizon, which is a seismic reflector marking a debated transition zone at depth. The distribution and evolution of the seismic sequences suggest that the K‐horizon could be interpreted as a fluid‐rich region at near‐lithostatic pressures. Type‐2 events usually occur in swarms and show a periodic pattern, a narrow frequency band, and almost identical waveforms. Their source is estimated to be located near the well, and their occurrence ceases after about 3 weeks from the conclusion of the drilling. We propose a causal link with the drilling operations where pressure fronts inside the well may promote phase changes and fluid flow across the drilled formations. Our study sheds light on the fluid‐driven tectonic and anthropogenic seismic activity at the Larderello‐Travale geothermal field. More generally, we show that microseismic activity occurring during drilling in high‐pressure and high‐temperature conditions can remain at low magnitudes and that geothermal wells targeting geothermal fluids in such systems may be handled safely despite the critical conditions encountered at depth. The drilling of the Venelle 2 well is an encouraging example for the development of geothermal energy in critical conditions.