Effects of energy-input mode on permeability enhancement of reservoir rock via low-amplitude stress waves

Damaged rock mass contains a great number of microstructures and defects at various scales, which could influence wave propagation significantly. In this paper, investigation of low-amplitude stress wave attenuation in rocks with different damage degrees was undertaken to determine the effects of parameters including damage degree, heterogeneity and type of waveforms on wave propagation, attenuation and slowness. The numerical method was validated through comparison with laboratory measurements. The damage was induced by conducting uniaxial compression tests on the intact rocks and was defined with the acoustic emission. The results showed that the attenuation ratio of low-amplitude stress wave increases with increasing rock damage degree. The rock heterogeneity was incorporated into the numerical model using a digital image processing technique with the combination of X-ray micro-computerized tomography. We found that the rock heterogeneity has great influence on the wave amplitude attenuation. Three types of waveforms, i.e., half-sine wave, square wave and exponential decay wave, were utilized to analyze influences of waveforms on attenuation characteristics in damaged rock specimens. The attenuation ratio of the exponential decay wave is the highest for rock specimens with the same damage degree, attributed to its highest dominant frequency. Moreover, detailed spectrum analysis suggested that the dominant frequency of the wave signal decreases with increasing damage degree. Based on derived simulating results, an improved classification scheme for determining rock fracturing degree, which groups the rock into zones of slightly fractures, moderately fractured and strongly fractured, was established by considering the relationship between wave amplitude and damage, since wave amplitude, other than wave velocity, is more sensitive to rock damage. The findings in this paper could facilitate a better understanding of wave propagation through rock and provide another means to determine rock fracturing degree.

The effect of mechanical anisotropy on the transmission of low-amplitude stress waves in brittle rock

During blasting in engineering construction, the surrounding rock becomes unstable and is damaged under the impacts of multiple low-amplitude stress waves. It is of great practical significance to understand the damage evolution characteristics and the attenuation of the mechanical properties of rocks subjected to multiple stress waves. Single impact and repeated impact tests for sandstone were carried out using a split Hopkinson pressure bar (SHPB) loading system. The single impact test results showed that the sandstone materials were strain-rate-dependent, and the dynamic constitutive curve could be divided into four stages, namely the linear elastic stage, the new crack formation stage, the plastic strengthening stage and the unloading stage. The failure pattern mostly indicated splitting tensile failure, and the impact damage threshold was 45 J. The relationship between the damage and stress wave amplitude was D = 0.0029·exp\({\boxed{f_{()}}}\)(5.4127•σ/76.13) − 0.0504. The repeated impact test results showed that the dynamic compressive strength and the dynamic elastic modulus decreased, while the failure strain increased gradually as the number of impacts (n) increased. The sandstone specimen under repeated impacts had only one fracture surface compared with the single impact failure pattern. The cumulative damage presented the development form of ‘rapid rise–steady development–rapid rise’, and the damage evolution law could be expressed by D = 0.265 − 0.328·ln⁡⁡⁡\({\boxed{f_{()}}}\)(ln13.989/n). Finally, a set of methods to determine the Holmquist–Johnson–Cook (HJC) model parameters for sandstone was proposed based on a single impact test, repeated impact test, uniaxial compression test and triaxial compression test. The numerical simulation results of the SHPB test showed that the dynamic constitutive curves of sandstone were in good agreement with the experimental results.

Stress wave emission (SWE) is defined as the elastic waves released within a material as a result of crack propagation due to the application of stress. A SWE in a brittle material is a relatively low-amplitude stress wave that is caused by the release of energy by microstructural events. This type of a phenomenon is best described by the "Griffith Theory of Solids." Since the SWE's are generally thought of as step or delta functions due to the nature of the generating source, their frequency spectrum is very broad. A series of experiments in SWE were conducted to determine the characteristics of the emissions, the instrumentation requirements, and to better understand the phenomenon for its application to brittle materials (glass and ceramic (Al <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> 0 <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</inf> ) thin film substrates). It was found that the detected SWE's released have a frequency spectrum very much like acoustic white noise over the frequency range of 0.1 kHz to approximately 100 MHz. The detection of SWEs in ceramic substrates was accomplished using a broadband 700 kHz piezoelectric transducer whose output amplified and filtered signal indicated the presence of a substrate microcrack. In order to distinguish true signals from faulty ones, a simple likelihood ratio detector was devised, where its threshold was previously experimentally determined.

Permeability enhancement of reservoir rocks loaded by repeated low-amplitude stress waves

The response of a 2‐D glass‐fibre epoxy composite to one‐dimensional shock loading has been investigated as a function of orientation of the fibres to the loading axis, to determine its equation of state. When the shock axis is parallel to a fibre direction, the shock front appears ramped at low stresses, steepening as the impact stress increases. Analysis of these traces suggests that a low stress amplitude wave is transmitted along the fibres at a high velocity, with a lower velocity wave is transmitted through the matrix between those fibres, taking the material to its final stress amplitude.

Impact of air blast overpressure waves (OPW), or shock wave, with the body wall or body armor produces two types of energy waves: high-frequency low-amplitude stress waves and long-duration low-frequency share waves. These types of energy waves are characterized by different mechanisms of primary tissue injury that mostly affect lung. Systemic inflammation and resultant acute respiratory distress syndrome are known major secondary causative agents of delayed multiple organ failure and subsequent death after OPW exposure. However, association of each pattern of the blast OPW-produced energy waves with postexposure inflammatory events has not yet been delineated. The objectives of the present research were a) establishment of a rat model for assessment of the inflammatory response following lung injury produced by exposure to medium-amplitude (approximately 120 kPa) low-frequency (260+/-5 Hz) OPWs; and b) assessment of the dynamics of alteration in polymorphonuclear leukocyte counts and expression of CD11b adhesion molecules on the surface of polymorphonuclear leukocytes and status of iron-transferrin complexes in peripheral blood after OPW exposure. This study focused on the OPW effects at different time periods, using a sequential approach to postexposure events. Lung injury in rat was induced by OPW generated in a laboratory shock tube. Animals were exposed to OPW (at peak overpressure of 118+/-7 kPa) that produced "moderate" lung injury. Military research institute. Twenty-seven CVF Sprague-Dawley rats were subjected to OPW exposures, and 17 sham-treated animals were used as control. Lung tissue and blood samples were collected at 1, 3, 6, 12, and 24 hrs following OPW exposures and compared with samples collected from nonexposed animals. OPW-induced lung injury caused a 2.7-fold increase in the number of circulatory polymorphonuclear leukocytes as early as 1 hr postexposure, which is indicative of mobilization of the pool of marginated polymorphonuclear leukocytes into the free circulation. Polymorphonuclear leukocyte counts increased through the following 3- and 6-hr periods, when they were, respectively, 5-fold and 3.5-fold higher than in controls. These effects were accompanied by a pronounced expression of CD11b in polymorphonuclear leukocytes and tissue sequestration of blood iron-transferrin complexes during the entire 24-hr period of observations. The increase in circulatory polymorphonuclear leukocytes was accompanied by a decrease in iron-transferrin complex concentrations that apparently reflected implication of blood plasma iron in the inflammatory cell response to OPW-induced injury. The observed dynamics in polymorphonuclear leukocyte alterations in peripheral blood after OPW exposure were similar to those found recently in clinical observations of nonpenetrating injury and in animal models of infectious insults. Therefore, our data suggest that the main pattern of proinflammatory alterations in the rat model of lung injury induced by exposure to long-duration shock wave is similar to patterns that are characteristic of major trauma. The data further suggest that the expression of polymorphonuclear leukocyte CD11b and the response of iron-transferrin complex can be considered as potential surrogate markers in blood for systemic alterations following OPW-induced injury and, therefore, warrant further investigation in a human pilot study.

By using numerical code RFPA2D(dynamic version), numerical model is built to investigate the failure process of rock particle under impact loading, and the influence of different impact loading on crushing effect and consumed energy of rock particle sample is analyzed. Numerical results indicate that crushing effect is good when the stress wave amplitude is close to the dynamic strength of rock; it is difficult for rock particle to be broken under too low stress wave amplitude; on the other hand, when stress wave amplitude is too high, excessive fine particle is produced, and crushing effect is not very good on the whole, and more crushing energy is consumed. Secondly, in order to obtain good crushing effect, it should be avoided that wavelength of impact load be too short. Therefore, it is inappropriate to choose impact rusher with too high power and too fast impact frequency for ore particle.

An experimental study of ultra-high pressure water jet-induced fracture mechanisms and pore size evolution in reservoir rocks

Apparent gas permeability behaviour in the near critical region for real gases

Numerical evaluation of hot dry rock reservoir through stimulation and heat extraction using a three-dimensional anisotropic coupled THM model

Addressing the practical challenges of difficult drilling for blasting-induced permeability enhancement in deep, soft, and high-gas coal seams, where fractures remain underdeveloped and prone to re-compaction, this study proposes blasting operations within the floor strata. This approach aims to enhance the permeability of soft coal seams, thereby extending the duration of effective gas extraction. A bidirectional loading gas–solid coupling blasting simulation system was established in the laboratory, enabling multi-faceted analysis of experimental models through macroscopic crack patterns, internal damage mechanisms, and strain data of coal and rock masses. Comparative experiments were conducted, contrasting various control hole spacings with conventional blasting techniques. The findings reveal that as the blasting stress wave traverses the control hole walls, tensile stress waves are reflected, facilitating crack propagation. The guiding effect of the control holes and the spatial compensation they provide significantly increase the extension distance of explosion-induced cracks, resulting in directional failure of the test specimens and heightened damage in the far field of the blast. After the blasting process, the arrangement of control holes can result in an increase of up to 133% in damage to the coal seam and a reduction of up to 167% in damage to the floorboard compared to the model without control holes. Notably, when the control holes are proximal to the coal-rock interface, the near-end coal body experiences the most pronounced effects, with peak damage and tensile strain in the d = 20 mm model being 1.93 and 1.79 times higher, respectively, than those in models without control holes. Conversely, for control holes located further from the interface, the distal coal body experiences the greatest influence, exhibiting 1.53 and 1.55 times higher peak damage and tensile strain, respectively, in the d = 80 mm model compared to uncontrolled counterparts. Field observations at the C13 coal seam of a mine within the Huainan mining area corroborate these findings, where the volume of gas extraction and its concentration experienced a rapid increase following blasting and penetration enhancement. Optimum permeability enhancement occurs when the blasting hole is situated 4 m from the extraction point, resulting in a 131% increase in gas extraction purity from 0.15 × 10–3 m3/min to 1.97 × 10–3 m3/min. Furthermore, gas concentration soars by 373%, from 5.86% to 21.86%. These research outcomes offer valuable insights and hold considerable reference significance for blasting-induced permeability enhancement in deep, soft, and high gas coal seams.

Tailored-pulse loading is a gas well stimulation technique that is promising for particular situations. The key factor in determining the probability of success is the interaction of the loading history with the material properties and in situ stresses of the geologic formation. On the basis of theoretical and experimental studies, tailored-pulse loading is expected to be successful if (a) the rock is fairly competent, (b) the yield stress is greater than about 100 MPa, (c) the overburden pressure is less than about 100 MPa, (d) the natural fracture or joint spacing is less than several meters.

As a non-explosive low-disturbance rock breaking technology, carbon dioxide phase transition blasting (CDPTB) is widely used in rock breaking projects such as pressure relief and permeability enhancement in coal mines, open-pit mining, road subgrade excavation, foundation pit excavation, etc. In this paper, the principle and equipment of CDPTB are systematically analyzed, and the characteristics of a reusable fracturing tube and disposable fracturing tube are determined. Different energy calculation methods are analyzed to determine the magnitude or equivalent explosive equivalent of CDPTB. According to the characteristics of impact stress wave and high-pressure gas, the cracking mechanism of CDPTB is proposed. Under the action of medium-impact stress, rock mass will produce multi-point cracking, and high-pressure gas will produce a gas wedge effect in the initial fracture, which determines the comprehensive action path of the stress wave and high-pressure gas. In terms of fracture characteristics, the fractal method is used to evaluate the macroscopic crack and fragment, microscopic fracture and pore characteristics. In terms of vibration characteristics, the attenuation law of CDPTB vibration with distance is statistically analyzed, and the Hilbert–Huang transform method is used to analyze the time–frequency characteristics of CDPTB. This rock breaking technology can be widely used in different projects, and the existing problems and future challenges are put forward.

&amp;lt;p&amp;gt;Volcanic rocks have been known to be reservoirs for more than a century but due to their often complex geological settings, they are commonly avoided.&amp;lt;/p&amp;gt;&amp;lt;p&amp;gt;The North Atlantic Igneous Province (NAIP) is one of the largest igneous provinces in the world. Large ranges of rock types comprise the NAIP, including tholeiitic and alkali basalts, nepheline- and quartz-syenites, nephelinites and carbonatites. The province is Paleocene in age and covers large parts of the North Atlantic region today. Parts of the NAIP outcrop onshore the Faroe Islands, on the western and the eastern side of Greenland, on Iceland and on the British islands.&amp;lt;/p&amp;gt;&amp;lt;p&amp;gt;In the Faroe Islands region volcanic settings serve as shallow geothermal energy systems, shallow groundwater aquifers and hydrocarbon reservoirs. These settings have been studied by core data and wire-line logs and examples will be presented. The aim of this study is to examine the key important parameters governing the reservoir properties and occurrences.&amp;lt;/p&amp;gt;&amp;lt;p&amp;gt;In all settings microfractures are important as porosity and permeability enhancers and are often linked to lava emplacement pathways and specific lava types such as subaqueous hyaloclastites and pillow lavas.&amp;lt;/p&amp;gt;&amp;lt;p&amp;gt;Studies on water movement from onshore the Faroe Islands on the islands of Streymoy have shown that the water in the volcanic settings in the area is being transported through large fractures and weathered flow tops and bases. This has also been seen in e.g. similar large igneous volcanic provinces such as the Columbia River Basalt Group, USA, and the Deccan Traps, India. Six influx zones were identified in the three approximately 200 meter deep geothermal holes on Streymoy, the Faroe Islands. Three with visible macrofractures striking north-south dipping east, two through weathered units, while the sixth influx zone did not show any visible fractures or weathered zones (Eidesgaard et al., 2019).&amp;lt;/p&amp;gt;&amp;lt;p&amp;gt;&amp;lt;strong&amp;gt;Reference:&amp;lt;/strong&amp;gt; Eidesgaard, &amp;amp;#211;.R., Schovsbo, N.H., Boldreel, L.O. and &amp;amp;#211;lavsd&amp;amp;#243;ttir, J. 2019 Shalllow geothermal energy system in fractured basalt: a case study from Kollafj&amp;amp;#248;r&amp;amp;#240;ur, Faroe Islands, NE-Atlantic Ocean. Geothermic vol. 82, p. 296-314.&amp;lt;/p&amp;gt;