Rock Permeability Research Articles

Experimental investigation on the effects of confining pressure and pore pressure on gas/water transport properties of COx argillite

As a preferred host rock for the disposal of high-level long-lived radioactive waste in France, Callovo-Oxfordian (COx) argillite inevitably undergoes gas and water intrusion during long-term sealing. In this study, a series of gas and water permeability tests have been performed on dry, partially saturated, and fully saturated COx argillite to investigate the effects of confining pressure and pore pressure on transport properties of this material. It is first found that gas permeability is very sensitive to the sample damage state for dry samples. Due to the presence of microcracks, the relationship between porosity and gas permeability is not evident, while the Klinkenberg effect is clearly observed in intact samples. Moreover, gas permeability of partially saturated samples is very sensitive to water saturation, especially for highly saturated samples. This indicates that the material has a good gas sealing capacity after the host rock is resaturated during the long-term sealing process. For both low- and highly saturated samples, confining pressure and gas pressure have very different effects on gas permeability of partially saturated argillite. Water permeability of argillite varies between 10−20–10−21 m2 (Kw), suggesting a good water-sealing capacity. The evolution of Kw is more sensitive to the change of pore pressure than to the change of confining pressure, indicating that the effective stress coefficient for water permeability is greater than 1. The intrinsic permeability of this low-permeable clayey rock is discussed at the end of this work.

Osmotic Pressure Gradient Effects on Water Diffusion in Porous Rock: Can They Pervert Permeability Tests?

Abstract Generation of a large network of hydraulic cracks is of key importance not only for the success of fracking of shale but also for the recent scheme of sequestration of CO2 in deep formations of basalt and peridotite, which are mafic and ultramafic rocks that combine chemically with CO2. In numerical simulation of the creation of a fracture network in porous rock, an important goal is to enhance the rock permeability. The objective of this article is to calculate the effect of osmotic pressure gradients caused by gradients of concentration of the ions of Ca, Mg, Na, etc. on the effective permeability of the rock. The basic differential equations are formulated, and their explicit solutions for appropriate initial and boundary conditions are obtained under certain plausible simplifications. The main result is explicit approximate formulas for the critical time before which no water permeation through a test specimen can be observed. Depending on various parameters, this time can be unacceptably long, which is manifested as a zero water outflow. The solution may also explain the unreasonably small permeability values reported for some shales.

Research on a permeability model of coal damaged under triaxial loading and unloading

It is important to research permeability models to clarify the evolution of methane seepage from coal seams and to reduce the occurrence of gas accidents. However, existing permeability models do not fully account for the influences of effective stress and adsorption/desorption on the permeability of coal rocks damaged during the three-dimensional stress loading and unloading process in coal mining. This study proposed a novel coal permeability model by more robustly considering the effects of stress and gas adsorption/desorption based on the generalized Hooke's law and Gibbs' equation. Then, a series of triaxial stress loading–unloading seepage experiments for coal rock were performed to validate the proposed model. The experimental research shows that triaxial stress loading and unloading have a significant effect on coal rock permeability. Before coal rock failure, the gas permeability shows a “V”-shaped development trend. After the deviatoric stress exceeds the compressive strength of the coal, the coal permeability increases sharply. Theoretical verification shows that the established exponential permeability model can effectively reflect not only the dynamic increase in the permeability of damaged coal under triaxial stress loading–unloading conditions but also the decrease in coal rock permeability during the increase in gas pressure with a constant effective stress. Moreover, the influence factors introduced in the permeability model can effectively reflect the influences of effective stress and gas adsorption/desorption on coal permeability under triaxial stress loading–unloading with different gas pressure conditions. These findings elucidate the variation in methane permeability of damaged coal rock under triaxial stress loading and unloading. The established permeability model can be used to carry out other numerical simulation studies to provide theoretical support for the prediction of gas permeability and efficient recovery of coalbed methane for coal mining working faces.

Assessment of True Formation Resistivity and Water Saturation in Deeply Invaded Tight-Gas Sandstones Based on the Combined Numerical Simulation of Mud-Filtrate Invasion and Resistivity Logs

The process of mud-filtrate invasion involves immiscible fluid displacement and salt mixing between mud filtrate and formation fluids in porous and permeable rocks. Consequently, the post-invasion spatial distribution of fluids and electrolyte concentration around the borehole affects resistivity measurements with different depths of investigation (DOI). In the presence of deep mud-filtrate invasion, the assessment of water saturation in the uninvaded zone based on the deep resistivity log can be inaccurate. Deep and electrically conductive filtrate invasion coupled with shoulder-bed effects can artificially increase water saturation (Sw) estimations by 20 saturation units (s.u.) in the Barik reservoir, resulting in pessimistic estimates of hydrocarbon pore volume if no corrections are applied. The Barik sandstone reservoir, which is characterized by low porosity ( up to 14%), low-to-medium permeability (up to 40 md), and high residual gas saturation (40 to 50%), exhibits low storage capacity to admit the critical filtrate volume necessary for building an impermeable mudcake. Combined with multiple days of overbalanced exposure to saline water-based mud (WBM), mud-filtrate invasion results in deep and smooth radial transition zones where the uninvaded formation is far beyond the depth of investigation of laterolog tools. Deep resistivity values are, therefore, lower than the true formation resistivity. Additionally, numerical simulations of resistivity logs show that the resistivity reduction by conductive invasion is further aggravated by shoulder-bed effects when individual reservoir thickness falls below 2.5 m. This paper describes the implementation of a compositional fluid-flow simulator to numerically model WBM-filtrate invasion and mudcake buildup in vertical boreholes. The algorithm allows the simulation of physical dispersion and fluid displacement around the borehole in a multilayer model. Time-dependent radial profiles of Sw and salinity are combined with core-calibrated porosity and electrical properties to compute electrical resistivity via Archie’s formulation. Subsequently, numerically simulated logs are generated using vendor-specific forward model processing and compared against field measurements. This workflow was extensively tested in various reservoir intervals with a wide range of petrophysical rock types and drilling conditions. Results show that the deep laterolog exhibits low sensitivity to conductive filtrate invasion when reservoirs’ porosities are lower than 8%. Above that threshold value, invasion length is a nontrivial process involving multiple variables. Even though exposure time to openhole conditions is a key factor leading to deep invasion, certain reservoir characteristics can lead to deeper invasion at short exposure times and significantly increase uncorrected Sw estimates.

New Iterative Resistivity Modeling Workflow Reduces Uncertainty in the Assessment of Water Saturation in Deeply Invaded Reservoirs

A new iterative modeling workflow has been designed to reduce the uncertainty of water saturation (Sw) calculations in the tight Barik sandstone in the Sultanate of Oman. Results from this case study indicate that Sw can be overestimated by up to 20 s.u. if the as-acquired deep resistivity is used in volumetric calculations. Overbalanced drilling causes deep invasion of water-based mud (WBM) filtrate into porous and permeable rocks, leading to the radial displacement of in-situ saturating fluids away from the wellbore. In low-porosity reservoirs drilled with WBM, the inability of the filtration process to quickly build impermeable mudcake translates into long radial transition zones. Under certain reservoir and drilling conditions, deep resistivity logs cannot reliably measure true formation resistivity and are, therefore, unable to provide an accurate assessment of hydrocarbon saturation. The effect of mud-filtrate invasion on resistivity logs has been extensively documented. Processing techniques use resistivity inversion and tool-specific forward modeling to provide uninvaded formation resistivity logs, which are much better suited for in-place resource volume assessment. However, sensitivity analysis shows that the accuracy of invasion-corrected logs dramatically decreases as the depth of invasion increases, whereby the inversion process needs to be further constrained. The new workflow is designed to reduce the non-uniqueness of true formation resistivity models so that they honor multiple and independent petrophysical data. The inversion routine utilizes a Bayesian algorithm coupled with Markov-Chain Monte Carlo (MCMC) sampling. Inversion results are iteratively modified based on two rock property models : one derived from rock-core data (helium expansion porosity and Dean-Stark saturations) and the other using an equivalent log interpretation of thick reservoir intervals from oil-based mud (OBM) wells. Simulated borehole resistivity is compared to field logs after each validation loop against rock property models. The new inversion-based workflow is extensively tested in the unconventional tight Barik Formation across water-free hydrocarbon and perched water intervals, and inversion-derived Sw models are independently validated by capillary-pressure-derived saturation-height models and fluid inflow rate from production logs.

Badania biomonitoringowe oraz kontrolne prace badawcze dla potrzeb podziemnego magazynowania gazu i obecnie eksploatowanych złóż gazu ziemnego

This paper refers to several aspects of research studies supporting the oil and gas industry – in particular underground gas storage (UGS) in depleted deposits and salt caverns – and focuses on questions related to the formation of hydrogen sulphide contamination in reservoir conditions as well as on methods for limiting unfavourable biogenic phenomena. The main problem found at gas storage facilities is the activity of sulphate reducing bacteria (SRB). The elimination or limitation of H2S generation in microbiologically contaminated environments have been the subject of many extensive studies. Biocides, biocorrosion inhibitors and H2S scavengers are widely applied to protect reservoir structure, gas storage infrastructure as well as water-based drilling fluids from the negative effects of bacterial activity. One of the most popular biocidal products, recommended for oil and gas industry are triazine derivatives, laboratory tested in the presented biomonitoring studies. Triazine products prove very effective in biomass reduction and elimination of anaerobic bacteria, especially SRB. Before any industrial operation (based on technology of using biocides), it was necessary to analyse the sulphur compounds in the stored natural gas in different exploitation gas wells of UGS. It was also necessary to investigate the selection of a specific biocidal product and its proper concentration. A concentration that is too low may even stimulate the microbial growth; since the substance is not toxic for microorganisms, they may start to metabolise it. Moreover, the wrong choice of biocides may even generate an economic loss or environmental hazard. Generally, the application of biocides, H2S scavengers and nitrate-based treatment are one of the most effective world strategies to decrease microbiological contamination, which affects various areas of the oil and gas industry. These products have also been successfully applied to control bacterial growth in Polish natural gas wells. The issue of the influence of microorganisms and biomass on the permeability of reservoir rocks was also presented. In addition, the paper refers to biodegradation processes, that take place in the environment of drilling fluids. Also, the issue of choice of biocide/H2S scavenger preparations for industrial applications is presented. The choice of chemicals includes efficiency tests of nanoparticles in contaminated media.

The Pressure‐ and Temperature‐Dependence of Thermal Pressurization in Localized Faults

AbstractThermal pressurization (TP) is predicted to be an important dynamic frictional weakening mechanism during earthquakes. The prevailing models, though, assume that the physical, thermal and hydraulic properties of the fluid‐saturated rock in the fault zone are constant during TP (the constant case) which inherently involves temperature and pressure changes. We solve the governing equations for TP in their general form with pressure‐ and temperature‐dependent physical, thermal and hydraulic parameters of a saturated fault zone with a one‐dimensional numerical model (the variable case). The model considers slip on a plane at a constant rate, and so does not account for dynamic frictional rupture scenario. We test a wide range of medium permeabilities of 10−22–10−16 m2 and porosities 0.5%–17%, based on experimental data for Frederick diabase, Westerly granite and the Hanaore Fault gouge. We find that the predicted shear stress drop and temperature rise is similar between the two cases for low permeability (<10−20 m2) and low porosity (<1%) fault rocks, owing to an increase in the fluid pressurization factor and hydraulic diffusivity. Differences between the two models are evident in more permeable and porous fault rocks. The increase in hydraulic diffusivity during TP results in a diffusional length which scales with time0.7 in the variable case. In addition, our calculations show that it is important to apply the constant case model with the ambient initial conditions for the modeled fault zone and not time‐averaged values that aim to represent the temperature and pressure changes that occur during TP.

CO2 Leakage Scenarios in Shale Overburden

Potential CO2 leakage from deep geologic reservoirs requires evaluation on a site-specific basis to assess risk and arrange mitigation strategies. In this study, a heterogeneous and realistic numerical model was developed to investigate CO2 migration pathways and uprising time in a shaly overburden, located in the Malaysian off-shore. Fluid flow and reactive transport simulations were performed by TOUGHREACT to evaluate the: (1) seepage through the caprock; (2) CO2-rich brine leakage through a fault connecting the reservoir with seabed. The effect of several factors, which may contribute to CO2 migration, including different rock types and permeability, Fickian and Knudsen diffusion and CO2 adsorption in the shales were investigated. Obtained results show that permeability mainly ruled CO2 uprising velocity and pathways. CO2 migrates upward by buoyancy without any important lateral leakages due to poor-connection of permeable layers and comparable values of vertical and horizontal permeability. Diffusive flux and the Knudsen flow are negligible with respect to the Darcy regime, despite the presence of shales. Main geochemical reactions deal with carbonate and pyrite weathering which easily reach saturation due to low permeability and allowing for re-precipitation as secondary phases. CO2 adsorption on shales together with dissolved CO2 constituted the main trapping mechanisms, although the former represents likely an overestimation due to estimated thermodynamic parameters. Developed models for both scenarios are validated by the good agreement with the pressure profiles recorded in the exploration wells and the seismic data along a fault (the F05 fault), suggesting that they can accurately reproduce the main processes occurring in the system.

Hydrogen's organic genesis

Natural hydrogen exploration has been restricted in scope due to the predominance in thinking that various rock interactions with water in cratonic settings offer the best natural hydrogen sources. The limited exploration findings in these areas in conjunction with advances in the understanding of hydrogen generation via anthropogenic methods suggest that other source alternatives such as organic hydrogen generation need to be revisited. The ideas on the maturation of organic matter may need to be reassessed with respect to hydrogen. It is suggested that an overlapping thermo-catalytic set of processes occurs to produce hydrocarbons and hydrogen. Initially clay reacts with kerogen producing hydrocarbons, hydrogen and amorphous carbon, the alteration of clays releasing hydronium. During the late catagenic phase thermo-catalysis of hydrocarbons by amorphous carbon create shorter chained hydrocarbons and hydrogen whilst amorphous carbon degrades to carbon black. During expulsion of hydrocarbons and hydrogen the permeability of the source rock becomes more heterogeneous, isolating some reactants creating pyrobitumens and other carbonaceous materials. At higher temperatures during metagenesis isolated pyrobitumens and other carbonaceous materials in shales are turned to graphite releasing gaseous hydrocarbons and eventually diatomic hydrogen in a second phase by thermo-catalytic reaction with carbon black. This matches the temperatures and results at which laboratory experiments and petrochemical processes used to generate hydrogen are observed. The current conclusion of the hydrocarbon generation story at the methane preservation limit should be recognized as the start of hydrogen generation and graphitization as the end of the process. Instead of basin exploration focused solely on hydrocarbons and stopping due to concerns of over maturity of source and reservoir, exploration may continue deeper in search of organic hydrogen. This should be noted as a primary hydrogen generation mechanism globally and provide a suitable model to aid hydrogen exploration and lead the energy transition into a hydrogen economy.

Impact of stress-driven crack growth on the emergence of anomalous transport in critically connected natural fracture networks

We study the role of in-situ stresses in fluid flow and mass transport through a two-dimensional fractured rock. The fracture network is based on a real outcrop with a connectivity state around the percolation threshold. We simulate stress-dependent fracture propagation and deformation using a geomechanics model based on the hybrid finite-discrete element method. The hydrodynamic transport through the deformed rock mass is then modeled based on particle tracking simulations. By exploring various scenarios of different stress ratios and orientations, we systematically analyzed how the superimposed multiple geomechanical processes (i.e., compression-induced closure, shear-induced dilation, and stress-induced propagation) control the aperture patterns, flow fields, and transport behaviors of the fracture network. Our results show that an increased stress ratio tends to promote fracture shear dilation and thus enhance flow channelization, resulting in an anomalously early arrival of solute transport. Furthermore, the increased differential stress attempts to drive the growth of wing cracks that bridge pre-existing fractures and form new flow pathways. We observe a qualitative transformation of the flow structure from a disconnected to a connected regime accompanied by a significant alteration of its overall transport properties. This phenomenon is attributed to the superimposed effect of stress-driven crack propagation and shear-induced fracture dilation. By quantitative parameter analyses of global flow and transport properties, we further found that the breakthrough time median of particles is well correlated with the equivalent permeability of the fractured rock when the effect of new cracks is removed. However, this correlation is greatly reduced when fracture propagation is considered; instead, the breakthrough time shows a good correlation with mean tortuosity. Our findings have important implications for the strategy design of geothermal production and waste disposal in critically connected fractured geological media under various stress conditions.

A comprehensive review of enhanced in-situ CO2 mineralisation in Australia and New Zealand

The urgent need to accelerate permanent anthropogenic carbon dioxide (CO2) abatement has led to a growing interest in in-situ CO2 mineralisation globally. This review outlines the progress of in-situ CO2 mineralisation in Australia and New Zealand, while focusing on potential sink reservoirs for mineralisation, including magnesium-rich and calcium-rich ultramafic and mafic rocks. These rock types are abundant in various regions across both countries. The review first begins by highlighting the current status and progress of Australian and New Zealand emissions and national abatement goals. It then previews the currently funded carbon capture, utilisation, and storage (CCUS) projects across both countries and discusses where in-situ CO2 mineralisation stands to complement other CO2 storage methods. Secondly, it details the different mineralisation mechanisms within selected rock types/complexes in Australia and New Zealand, followed by a discussion of the limitations and advantages of each selected major rock type/complex within both countries, including the relative distances between deposits and major emission hotspots. Finally, within this review we examine the governmental funding and perception of enhanced in-situ CO2 mineralisation within the two countries. An example in Mount Keith Ultramafic Complex (MKUC) is also presented, which shows the relationship between rock permeability, expected CO2 injection, and the estimated cost per ton for direct pumping into low permeability Peridotite deposits. The results show an inversely proportional relationship between permeability and injection rate, and thus the cost per ton of CO2 stored.The estimated cost calculations clearly demonstrate that the limiting factor of in-situ mineralisation is not the availability of sink reservoirs within Australia and New Zealand but rather the need for significant government incentives to present an attractive breakeven cost that can compete in the CO2 storage market. The results also show that New Zealand currently has a slight advantage due to the 2-fold higher carbon pricing compared to Australia.

Experimental Investigation of the Fractal-Permeability Properties of Locally Fractured Coal Bodies around Gas Extraction Boreholes

To investigate the permeability characteristics in the in-situ fractured coal body around the perimeter of gas extraction boreholes, the steady-state permeability of fractured coal bodies with different gradations was tested using the fractured rock permeability test system. By controlling the axial displacement and permeability pressure, the permeability parameters were obtained under different porosities. The interactions between the permeability parameters and the process of permeability destabilisation are discussed. The results show that the permeability characteristics of the broken coal body obey the Forchheimer relationship: As the axial displacement increases, the permeability resistance of the fluid increases and the non-Darcy property of the sample becomes more significant. With the decrease in the porosity of the sample and the increase in the power index n, the permeability k decreases and the non-Darcy factor β increases. The final fractal structure of the sample will be changed by particle fragmentation and migration during the loading process of the sample with different levels, and the internal pore structure of the sample will further affect the penetration of the penetration channel. A critical characteristic value for the seepage instability in broken coal bodies is given, and an expression for determining the seepage instability by permeability and non-Darcy factors is proposed. The results indicate that a negative non-Darcy factor is not a necessary condition for permeability instability, and the critical Reynolds number for the permeability instability in broken coal bodies was determined from the perspective of the Reynolds number. The conclusions of this study can provide theoretical support for the theoretical study of permeability and the permeability of pre-smoking coal seams.

A new wormhole mechanistic model for radial acid flow geometry using novel 3D flow correlations

In carbonate reservoirs, matrix acidizing is used to improve well performance by creating conductive channels called wormholes. Predicting wormhole propagation rate in radial acid flow geometry up to the real wellbore scale is the main problem regarding designing field operations in previous studies which are still challenging. In this study, a new mechanistic approach is presented to predict the radial flow acidizing up to near wellbore scale by considering the vital effects of porous media size and geometrical parameters of wormhole patterns.Accordingly, wormhole tip velocity and wormhole wall fluid loss rate will be introduced as the scale transferring parameters between different porous media sizes which are correlated in terms of rock diameter and wellbore height, wormhole size and density. These novel correlations are extracted from the large databank of flux distribution between rock matrix and wormhole channel obtained in 60 different 3D FEM flow simulations for various patterns of wormhole propagating and different wellbore sizes. In addition, an HPHT radial acid flooding setup is designed and constructed to perform radial acid coreflood on ultra-low permeable rocks (k«1md) for model validation. During the experiments, the breakthrough pore volume, pressure behavior and CT scan of samples are closely monitored. The results showed that due to the tight window of pore size distribution, faster breakthroughs with 1.23 and 1.45 pore volumes occurred in these tight samples compared to those reported already which are confirmed by developed model prediction. Besides, the model results showed that increasing the rock height (open-hole interval) would increase wormhole tip velocity and decrease acid breakthrough time. In addition, the higher temperature shifted the acid response curve to the right, toward higher acid breakthrough volume and higher acid concentration shifted it toward lower breakthrough volumes. The model is validated using radial acid experiments and previous radial acid models for different rock sizes with good agreement (R2 = 0.9547, MSE = 0.02161 and RMSE = 0.14701). The new radial acidizing model is capable to quickly predict wormhole propagation rate up to wellbore scale without requiring excess up-scaling data due to considering wide ranges of porous rock sizes from (25.4–152.4 cm) diameter and (5–25.4 cm) pay zone height (sufficient size for considering repetitive wormhole patterns). This technique can be utilized for tuning parameters of various radial cases to predict the acid tests results leading to reduce the number of required experiments.

Experimental Study on the Effects of Vibrational Frequency on the Permeability of Gas-Containing Coal Rocks

Experimental Study on the Effects of Vibrational Frequency on the Permeability of Gas-Containing Coal Rocks

Uncertainty quantification of the convolutional neural networks on permeability estimation from micro-CT scanned sandstone and carbonate rock images

Rock permeability is one of the most crucial properties affecting subsurface fluid flow behaviors. To accurately and robustly estimate the permeability, Digital Rock Physics, including micro-CT scanning technology and direct flow simulations on scanned images, has prevailed in recent years. Besides, machine learning techniques such as convolutional neural networks (CNNs) have been widely adopted and achieved success in permeability estimations directly from rock images. However, existing ML methods used for permeability estimation from rock images lack uncertainty quantification that causes unreliable predictions and overconfident estimations on out-of-distribution (OOD) samples. In this work, we propose a PI3NN-CNN framework to address this problem. PI3NN-CNN consists of a CNN model for absolute permeability estimation and a PI3NN method to quantify the estimation uncertainty. It is able to quantify the uncertainty for in-distribution (InD) data with a desired confidence level, and identify OOD samples to avoid overconfident predictions. We demonstrate the method using micro-CT scanned images from two sandstone and two carbonate rocks. We found that PI3NN-CNN generates accurate predictions for InD samples, while producing high-quality prediction uncertainties regardless of the prediction accuracy. Meanwhile, PI3NN-CNN identifies OOD samples using its special network initialization scheme. The unique feature of PI3NN-CNN makes it applicable to more complex real-world image-based data for robust learning and predictions without overconfident estimations when the ground-truth information is unavailable.

Hydrothermal alteration of basalts in the ultramafic-associated Tianxiu Vent Field, Carlsberg Ridge

The Tianxiu vent field (TVF) is the first active ultramafic-associated hydrothermal field to be discovered at the termination of a detachment fault on the slow-spreading Carlsberg Ridge. Most hydrothermal chimneys in the TVF are developed on the basaltic apron. This paper presents mineralogical, petrological, and geochemical data for altered basalts in and surrounding the TVF. The basalt samples can be classified into seafloor-weathered and hydrothermally altered (i.e., slightly saponite-altered, highly saponite-altered, and silicified) types. At the periphery of the TVF, the basalts have experienced seafloor weathering, which has resulted in slight mineralogical and geochemical changes. In the TVF, most basalts have interacted with diffuse low-temperature hydrothermal fluids, leading to variable degrees of saponite alteration. As the distance from the vents decreases, basaltic glass, olivine, and pyroxene are progressively replaced by saponite; Zn, Cu, Co, and U are enriched; and Ca and Mn are depleted. Magnesium, Fe, and Li have been lost due to olivine replacement by saponite, and have been gained due to pyroxene replacement by saponite. In the fluid flow conduits, the basalts have interacted with focused medium- to high-temperature fluids, which has led to the replacement of saponite by opal; depletion in Mg, Fe, Li, and rare earth elements; and enrichment in Zn, Cu, Co, and U. We propose a new hydrothermal alteration model for basalts in an ultramafic-associated vent field. Based on a comparison with mafic-hosted vent fields along mid-ocean ridges, we propose that: (1) the permeable host rocks promote lateral fluid flow, widespread mixing with cold seawater, and pervasive low-temperature hydrothermal alteration; and (2) H2S-poor reducing fluids reduce the solubility of U and inhibit the precipitation of Fe, leading to high Cu/Fe and Zn/Fe ratios, and high U concentrations. The hydrothermal alteration model for the TVF is applicable to similar vent fields and can guide exploration for seafloor massive sulfide deposits along slow-spreading ridges.

Insights into the activation characteristics of natural fracture during in-fracture temporary plugging and diverting fracturing

In-fracture temporary plugging and diverting fracturing (ITPDF) has proved to be an efficient way to stimulate naturally fractured reservoirs by creating complex fracture networks. In this work, a 2D fluid–solid fully coupling finite element model is established to investigate the activation characteristics of natural fractures influenced by various factors. This model is verified against the published experimental and numerical results, and the considered factors include the approaching angle, the tensile strength of natural fracture, the horizontal stress contrast, and the injection rate. Innovatively, the plug model is proposed to plug hydro-fractures during the simulation. The mechanical properties of the natural and hydraulic fractures are designed based on the real tight reservoir in Tarim Oilfield, West of China. This work finds out the most sensitive factors to the activation of the NFs and reveals the influencing pattern of various factors. Moreover, this work presents the NF activation process and reveals the dynamic change in the fracture width. During ITPDF, the HF firstly activates the NF acute and then crosses the NF when the approaching angle is ≤ 60°, and the HF firstly crosses the NF and then activates the NF acute branch when the approaching angle is > 60. The difference in the activation pressure between the two NF branches can reach up to 35 MPa. Moreover, The NF acute branch cannot be activated at the IPT distance beyond 5 m, the IPT permeability beyond 670 mD, and the rock permeability beyond 0.8 mD. This study guides candidate well selection and diverter recipe optimization during ITPDF operations in Tarim Oilfield.

Measuring gas permeability in tight cores at high pressure: Insights into supercritical carbon dioxide seepage characteristics.

This study introduces a new, more accurate instrument for measuring gas permeability in tight cores at high pressure, surpassing the accuracy of conventional methods. Using the steady-state method, the study tested the permeability of carbon dioxide (CO2) in tight cores at various pressure conditions. We observed that, similar to nitrogen, CO2 permeability initially conformed to the Klinkenberg slip theory, but began to deviate as back pressure increased. Supercritical CO2 exhibited a slight increase in permeability, which contrasts with both nitrogen behavior and the Klinkenberg slip theory. Accurate measurement of CO2 permeability in tight rocks is crucial for assessing the feasibility and efficiency of CO2 injection and storage, which could help reduce greenhouse gas emissions and drive energy transformation. The innovative measurement approach can assist in identifying the ideal production pressure drops, injection pressures, and flow rate parameters, thus improving the recovery rate of tight oil reservoirs. Nevertheless, the study had some limitations, including prolonged stable flow time, lack of real-time flow rate display, and reliance on theoretical viscosity calculations, all of which could affect the accuracy of permeability measurements. Further research is needed to investigate CO2 permeability under diverse conditions and to deepen our understanding of CO2 transport and storage in tight cores, which could lead to improvements in CO2 sequestration and the efficiency of enhanced oil recovery.

Mechanical stratigraphy controls fracture pattern and karst epigenic dissolution in folded Cretaceous carbonates in semiarid Brazil

Carbonate reservoirs are often affected by fractures and karst that increase rock's permeability. An accurate characterization of reservoir architecture, fracture attributes and distribution of karst features are critical to modeling carbonate reservoirs, predicting flow behavior and estimating geofluids recoveries. In this contribution, we describe a case study of epigenic karst dissolution concentrated in a fold hinge zone and controlled by stratigraphic and structural processes in Cretaceous layered carbonates in the Potiguar Basin, Brazil. The stratigraphic setting and petrographic/petrophysical properties of host rocks were defined before fracture analysis. Two main joint sets were recognized: master joints (persistent fold hinge parallel set) and cross-joints (orthogonal to fold hinge and confined within master joints). The quantitative mesoscale fracture analysis was performed on a hinge-parallel joint set in 10 field sites distributed across the fold using scanlines to determine two parameters: Fracture Spacing Ratio in individual beds and the Fracture Spacing Index of bed packages. Fracture density and intensity of cross-joints were analyzed in one site using digital scanlines and scanareas on drone images. Results indicate that (i) systematic fractures consist of strata-bound joints, mostly oriented parallel to fold hinge, (ii) fracture intensity does not depend on intrinsic petrophysical properties of individual beds, and (iii) fracture intensity increases within a ∼2.5 km-wide fold hinge zone, consistent with previous studies performed via remote sensing analysis. By integrating our new results with published data, we provide a conceptual model that considers the role of regional-scale tectonic process and local-scale mechanical stratigraphy in controlling the size, shape and distribution of epigenic karst dissolution. Our findings indicate that facture permeability anisotropy, inferred from the karst pattern, is imparted by the combination of bed's attributes (mechanical stratigraphy) and fold-related fracturing. These results are an effective tool to predict the meso- and macro-scale porosity systems in fractured and karstified hydrocarbon reservoirs.

Induced hydraulic fractures in underground block caving mines using an extended finite element method

Block caving has become an attractive underground rock mining method that requires effective preconditioning of the orebody to initiate the progressive rock collapse by its weight. Hydraulic fracturing is commonly used in cave mining as a preconditioning method to induce artificial fractures in the orebody rock mass. This paper presents the numerical model of hydraulic fracture propagation in underground caving mines. An extended finite element method based on phantom nodes is adopted to simulate the propagation of multiple fractures along arbitrary paths. A poroelastic constitutive model governs the behavior of orebody rock mass, while a cohesive zone model controls the hydraulic fracture propagation. The adopted method is validated against analytical solutions for penny-shaped hydraulic fracture. The stress shadowing effect on hydraulic fracturing is investigated using representative rock properties from the “El Teniente mine”, obtained by calibrating a hydraulic fracturing field test. Several analyses of multiple hydraulic fractures considering a sequential fracturing scheme are simulated to investigate the influence of in-situ stresses, fracture spacing, injection flow rate, and Young's modulus and permeability of the matrix. The numerical results show that small fracture spacing, in-situ stress, and rock permeability locally change the principal stress magnitude, resulting in a more extensive fracture propagation. Also, the injection of new fractures induces stress shadows that reinitiate the propagation of the previous ones. This behavior is reduced by increasing the fracture spacing and rock permeability. Finally, this paper provides insights into the main parameters that affect fracture extension, which can be used to optimize the preconditioning process of underground block caving mines.