Principal Fracture Research Articles

Volcanic weathered crust reservoir analog: Insights from fault-controlled fracture permeability in the trachytic rocks of the Pernambuco Basin, NE Brazil

Naturally fractured volcanic rocks represent a globally significant potential hydrocarbon reservoirs and can also be used for CO2 storage; however, the study of volcanic reservoirs in Brazil is limited. Although numerous volcanic reservoir types have been classified, relatively few studies are dedicated to the weathering of crust-like reservoirs, i.e., volcanic rocks undergoing subaerially alteration by weathering prior to deep burial. This study aims to characterize a volcanic weathered crust reservoir analog located in the coastal zone of the Pernambuco Basin, NE Brazil. We applied a multiscale approach motivated by the expressive potential of volcanic rocks as reservoirs in this marginal basin and considered their implications for CO2 geological storage projects. The deformational aspects observed in this analog have also been observed in other volcanic rocks that crop out in the coastal zone, thus, the data provided here can guide future characterization of both onshore and offshore reservoirs. We focused on an 80 m wide outcrop and employed geometric and topological analysis of two-dimensional fracture networks to investigate the influence of regional fault zone control on the fracture permeability tensors. The parameters of fracture systems were studied using terrestrial gravimetric data, orthophoto mosaic obtained by an unmanned aerial vehicle (UAV), petrography, and outcrop-based structural data. The investigated outcrop includes porphyritic trachytes with euhedral sanidine crystals and minor quartz phenocrysts, sourced from the Ipojuca Magmatic Suite. The residual Bouguer map indicates the influence of at least two regional rift fault zones, i.e., NW-SE (strike-slip) and NE-SW (normal) trending systems, in the studied rocks. These trachytic rocks exhibited a marked dissolution of sanidine phenocrysts and matrix, columnar disjunctions, and natural fractures, causing the porosity and permeability of the reservoir analog to increase. Geometric and topological analyses showed that the south-central sector of the outcrop exhibits heightened connectivity within the fracture network. Furthermore, the principal fracture permeability is controlled by the primary families of the fracture network and regional fault zones. Our results demonstrate that the fracture network observed in the volcanic reservoir analog was influenced by the post-rift tectonic reactivation of Cretaceous structures within the Pernambuco Basin. The secondary porosity resulting from weathering and subsequent dissolution of the primary constituents and fractures of the rocks studied demonstrates the potential for the exploration of these reservoirs as models for known occurrences of volcanic rocks in the onshore and offshore domains of the Pernambuco Basin.

Fracture stratigraphy controls karst localization within a carbonate sequence with terrigenous input: Implications for conduit-seal layer development in a karst system

The present study uses a multidisciplinary approach to investigate the impact of fractures on karst system localization that affects carbonate units containing terrigenous input. The study area is the Lapinha Cave external cliffs in the Neoproterozoic Salitre Formation, São Francisco Craton, Brazil. We performed structural and stratigraphic field investigations of the external cave portions with an unmanned aerial vehicle (UAV) and laser detection and ranging (LiDAR). We also collected uniaxial compressive strength (UCS) and gamma-ray (GR) data and performed petrographic and petrophysical analyses. The fracture stratigraphy characterization results indicate that sedimentary facies controlled the fracture distribution. The studied sequence consists of an impure carbonate sequence that comprises three main sedimentary facies: (i) mudstone (MD), (ii) packstone-grainstone containing mudstone and siliciclastic intercalations (PG), and (iii) mixed carbonate-siliciclastic unit composed of grainstone and sandstone (CS). The MD, PG and CS facies displayed the highest, medium, and lowest fracture intensities and densities, respectively. The MD and PG facies behaved as ideal reservoirs or geofluid conduits related to intense deformation. Two principal fracture sets along these MD and PG facies striking N–S and E‒W, respectively, were documented. In contrast, the heterogeneous and less deformed CS facies displayed a geofluid barrier behavior. Overall, the results produce a guideline for investigating carbonate sequences characterized by terrigenous input affected by different fracture concentrations and distributions.

Structural Damage Evolution of Mesoscale Representative Elementary Areas of Mudstones

The mechanical behavior of mudstones is significantly affected by their mesostructure. This study investigates the damage evolution of red mudstone mesostructures under uniaxial compression through U-Net image segmentation, mesoscopic representative elementary area (mREA), meso-element equivalent method, and linearly superimposed model. Results show that the mREAs can be simplified as a binary structure comprising nonclayey minerals and a porous matrix. The inclusion-matrix interfaces constituted by the dominant inclusions, such as large inclusions and a series of inclusions with specific arrangements, are the weakest regions at the postpeak stage. The shear failure of these interfaces leads to general shear failure with a penetrating principal fracture or local shear with several nonpenetrating local fractures in the mREAs. The damage evolution and failure mode are sensitive to the inclusion geometric features, e.g., inclination, grain size, and spacing. However, the uniaxial compressive strength (UCS) is mainly determined by the included angle between the dominant inclusion inclination and the loading direction. With consistent composition, an increase in the included angle from 0 to 90° results in an increase in the UCS of the muddy facies and a decrease in the sandy facies, with the UCS of the sandy facies being more sensitive to changes in the included angle. Our findings indicate that the structural failure of red mudstones from mesoscopic to macroscopic is driven by the accumulation and expansion of shear failure at multiple scales due to the presence of hard–soft interfaces. This study enhances our understanding of the structural failure mechanism of mudstones and serves as a practical reference for the mesomechanical testing of clay rocks.

Differences in Pilon Fractures According to Ipsilateral Fibular Injury Patterns: A Clinical Computed Tomography-Based Mapping Study.

Preoperative verification of fracture morphology is essential for determining the definitive fixation strategy in the management of a pilon fracture. This study aimed to determine the correlation between fibular injury patterns and fracture morphologies and introduce clinical implications. Computed tomography scans of 96 pilon fractures were retrospectively analyzed and divided into three types: intact fibula, simple fracture, and multifragment fracture. The principal fracture line and comminution zones were illustrated on a plafond template and diagrammatized on a 6 × 6 grid using PowerPoint software as fracture mapping. Correlations between fibular injury patterns and fracture morphologies, including comminution zones and principal fracture lines, were analyzed. The thickest comminution zone was most often located in the anterolateral quadrant. According to fibular injury patterns, the comminution zone of the multifragment group was placed two grids more lateral than that of other groups. Lateral exits of the principal fracture line in the multifragment group were much more concentrated within the fibular incisura. In pilon fractures, a more complex fibular fracture pattern was related to the valgus position. Moreover, the articular fracture pattern of pilon fractures differed according to coronal angulation and fibular fracture pattern. These differences should influence the operative approach and placement of the plate.

Numerical Study on the Use of Alternating Injection Hydraulic Fracturing Technology to Optimize the Interaction Between Hydraulic Fracture and Natural Fracture

Hydraulic fracturing is a key technology for shale gas production. Activating the natural fracture (NF) system in shale reservoirs and forming a complex fracture network can greatly improve the effect of fracturing. The effect of fracturing is mainly influenced by geological factors and operational parameters of a reservoir. Effectively reforming a reservoir under unfavourable geological conditions and maximizing the activation of NFs in the reservoir can substantially increase its reformed volume and the production of shale gas. Alternating fluid injection (AFI) fracturing technologies using multiple fracturing fluids with different viscosities can activate closed NFs while retaining the high conductivity of the principal fracture to achieve a larger stimulated reservoir volume (SRV). In this paper, a hydraulic-mechanical coupling finite element method (FEM) of a reservoir is established, and AFI fracturing technology is numerically simulated using the continuum method. In addition, the fracture propagation stage and path propagation for AFI technology are discussed. The results show that AFI fracturing technology can form principal fractures with high conductivity and activate NFs in a reservoir to form comparatively complex fracture propagation paths.

Damage Model for Reservoirs with Multisets of Natural Fractures and Its Application in the Simulation of Hydraulic Fracturing

The presence of natural fractures can significantly affect the quality of hydraulic fracturing operations in tightsand and shale or oil/gas formations. This paper describes the procedure used to model natural fractures with continuum damage tensor and the resulting orthotropic permeability tensor. A damage model that uses damage variable in tensor form is presented. In the procedure presented, a nonnegligible angle is assumed to exist between directions of principal stresses in the formation and in the natural-fractures-related damage tensor, and this difference in orientation is modeled by introducing local directions in the model. A damage-dependent permeability tensor in tabular form is then proposed. A second case scenario when the directions of principal stresses and natural fractures align is also analyzed. Numerical results of fracture distribution are presented for both cases, and differences can be seen from the computed contour of the damage variable. The results indicate that the model can effectively simulate the fracture propagation phenomena during hydraulic fracturing.

Morphology and Tensile Properties as a Function of Welding Current in Thermoplastic Induction Welds

Abstract Compared to other conventional joining methods, induction welding offers the superiority of avoiding mechanical degradation and satisfying the need for weight reduction in the aircraft industry. In this paper, a metal mesh was adapted as an induction component in the induction welding of polyetheretherketone (PEEK) with various currents. The effect of welding current on the microstructure and mechanical properties of the induction welding joint was further investigated. The results indicate that induction welding joints with the narrow thickness of the fusion zone and high tensile strength can be attained in the welding current range of 7.05 A to 11.05 A. However, when the current exceeds 13.91 A, the excessive heat input leads to the unsteady flow of PEEK or even thermal oxidative degradation and thermal decomposition, which increases the thickness of the fusion zone and reduces the tensile strength of the joint. In addition, the principal fracture mode presents cohesive failure, thereby promoting the tensile strength of the joint.

Numerical Simulation of Shale Reservoir Fluid-Driven Fracture Network Morphology Based on Global CZM

There are a large number of natural fractures in shale reservoirs, which create great challenges to hydraulic fracturing. Activating the natural fractures in reservoirs can form a complex fracture network, enhance fracturing effects, and increase shale gas production. Reservoir geological conditions (low in situ stress, natural fracture distribution, and cement strength) and operation parameters (fracturing fluid viscosity and injection rate) have an important influence on fracture network propagation. In this article, a two-dimensional hydraulic fracturing fluid-mechanic coupling numerical model for shale reservoirs with natural fractures was established. Based on the global cohesive zone model, the influence of geological conditions and operation parameters on the propagation of the hydraulic fracture network and fracturing process is investigated. The numerical simulation results show that when the horizontal in situ stress difference, approach angle, and cement strength are low, it is easier to form a complex fracture network. Research on the construction parameters indicated that when the viscosity of the fracturing fluid is low, it is easier to form a complex network of fractures, but the length of the fractures is shorter; in contrast, the fractures are straight and long. In addition, increasing the injection rate is beneficial for increasing the complexity of the fracture network while increasing the initiation pressure and width of the principal fracture reduces the risk of sand plugging. This article also proposes an optimization solution for hydraulic fracturing operations based on numerical simulation results.

Semi-Analytical Rate Decline Solutions for a Refractured Horizontal Well Intercepted by Multiple Reorientation Fractures with Fracture Face Damage in an Anisotropic Tight Reservoir

Refracturing treatment is an economical way to improve the productivity of poorly or damaged fractured horizontal wells in tight reservoirs. Fracture reorientation and fracture face damage may occur during refracturing treatment. At present, there is still no report on the rate decline solution for refractured horizontal wells in tight reservoirs. In this work, by taking a semi-analytical method, traditional rate decline and Blasingame-type rate decline solutions were derived for a refractured horizontal well intercepted by multiple reorientation fractures with fracture face damage in an anisotropic tight reservoir. The accuracy and reliability of the traditional rate decline solution were verified and validated by comparing it with a classic case in the literature and a numerical simulation case. The effects of fracture reorientation and fracture face damage on the rate decline were investigated in depth. These investigations demonstrate that fracture face damage is not conducive to increasing well productivity during the early flow period and there is an optimal matching relationship between the principal fracture section angle and permeability anisotropy, particularly for the reservoirs with strong permeability anisotropy. The fracture length ratio and fracture spacing have a weak effect on the production rate and cumulative production while the fracture number shows a strong influence on the rate decline. Furthermore, multifactor sensitivity analysis indicates that fracture conductivity has a more sensitive effect on well productivity than fracture face damage, implying the importance of improving fracture conductivity. Finally, a series of Blasingame-type rate decline curves were presented, and type curve fitting and parameter estimations for a field case were conducted too. This work deepens our understanding of the production performance of refractured horizontal wells, which helps to identify reorientation fracture properties and evaluate post-fracturing performance.

Study on influencing factors of microbial induced carbonate precipitation in a cross fracture network

Microbial mineralized precipitation has been widely used in anti-seepage of fractured rock mass due to its environmentally friendly characteristics. The mineralization and sedimentation of the fractured network with vertical and horizontal crosscutting directly affect the anti-seepage of its engineering. Based on the Navier-Stokes equations, solute transport equations, biochemical reaction equations, solution transport, and carbonate precipitation law of the cross fracture networks in rock mass were analyzed. The influence of the Reynolds number (Re) at the inlet, Angle (α) between principal fractures and branch fractures, and fracture aperture degree on the calcium carbonate precipitation were investigated. The results showed that under the condition of two branch fractures, when 1≤Re≤5, there is a weak nonlinear relationship between flow rate (Ra) and Re, where Ra is unaffected by the change of α. When 5<Re≤100, the nonlinear relationship between Re and Ra increased sharply. For 45°≤α≤180°, Ra and α showed a positive correlation due to the gradual increase of the inertial force. After Re>100, Ra gradually became steady. The influence of Re and Ra on the calcium carbonate precipitation in the cross fracture is mainly realized by controlling the change of Ra. For the α=45° model, when Re=1, the amount of calcium carbonate precipitation in the two branch fractures is approximately the same. When Re=100, the maximum concentration of calcium carbonate in the left branch fracture was 60.2 mol/m3, significantly higher than that in the right branch fracture. The research results provide a useful reference in MICP seepage prevention technology applications in a fractured rock mass.

Fracture variability in basalts and its effect on river erosion: a case study in the Paraná Volcanic Province

The variation in the structural characteristics (cooling joints and tectonic fractures) of basaltic flows implies potential variability in the intensity of erosion by plucking. The erosive behavior of the rivers that sculpt these areas depends on their interaction with the diverse fracture systems. In view of this, we analyzed the effect of fracture variability in basalts on erosion in a bedrock river reach located in the Continental Volcanic Province of the Paraná Basin, southern Brazil. The 120-m-long reach is influenced somewhat by a possible fault that crosses it near one end. The fracture density and fracture direction were evaluated through field photogrammetry in seven sample areas distributed along the reach. The fracture direction and main erosion axes were also surveyed by remote piloted aircraft (RPA) aerial imaging. Tectonic fractures were identified in the field; they do not always appear in the survey of the sample areas but are evident in the RPA survey. The main erosion axes coincide with the principal fracture directions (tectonic fractures), which are disposed obliquely to the channel flow direction, making an average angle of 50°. The more abundant and multidirectional cooling joints act to control the plucking process and not to determine the erosion direction. The fracture density decreases with increasing distance from the fault crossing zone (from 9.62 to 3.73 m/m²), although the lower value is influenced by the presence of an amygdaloidal basalt zone. The higher fracture density favors more intense plucking.

Multiaxis three-dimensional (3D) glass fiber preform/cementitious matrix concrete composites: Experimental characterizations by panel test

Multiaxis three-dimensional (3D) alkali-resistant continuous glass fiber/cementitious matrix concrete composites were developed using the “winding and molding” method. The panel properties of the concretes were experimentally investigated and compared with the neat concrete. The experiment revealed that the load-bearing capability, strength and energy absorption properties of the multiaxis three-dimensional concretes were significantly influenced by the placement and orientation of the reinforcement fiber. The newly developed concrete composite was found to have the strain hardening behavior which attributed to an improved energy absorption performance of the concrete. It was also identified that four directional glass structure (ARG-4D) demonstrated somewhat better energy absorption performance compared to that of the biaxial (ARG-2D) and uniaxial (ARG-1D) structures because of the fiber orientation in the multiaxial direction in the concrete. Further investigation of the fractured four directional glass fiber/cementitious matrix concrete (ARG-4D) showed that the fiber-matrix interfacial debonding in each yarn set, local matrix breakages, yarn bridging due to tensile strengthening, tensile pull-out of outer filament and stick-slip frictions between the filament TOWs and the cementitious matrix were the principal fracture mechanisms. The micro-cracks occurred in multiaxis concretes were restricted by the multiaxis 3D fiber architecture. The multiaxis three-dimensional alkali-resistant continuous glass fiber/cementitious concrete can be considered as the damage tolerant structures with regard to neat concrete.

Fracture Properties of Nash Point Limestone and Implications for Fracturing of Layered Carbonate Sequences

Carbonate reservoirs accommodate a significant proportion of global hydrocarbon reserves. However they are often tight and permeability is therefore usually dependent on either flow through existing fractures or through those produced by hydraulic stimulation. Hence, understanding how fracture networks develop in carbonate reservoir rocks is key to efficient and effective production. However, despite their prevalence as reservoir rocks, there is a paucity of data on key fracture properties of carbonate rocks, particularly in more than one orientation. Here, therefore we report measurements of both the tensile strength and fracture toughness of Nash Point limestone in the three principal fracture orientations to determine what effect any mechanical anisotropy might have on fracture propagation. We find Nash Point limestone to be essentially isotropic in terms of both its microstructure and its fracture properties. When comparing the fracture toughness of Nash Point limestone with that of others limestones, we find that fracture toughness decreases with increasing porosity, although this dependency is not as strong as found in other porous rocks. Finally, as many so-called carbonate reservoirs actually comprise layered sequences, we extend our analysis to consider the layered sequence of limestones and shales at Nash Point. We find that the fracture toughness of Nash Point limestone is higher than Nash Point shale but that the fracture energy is lower. We therefore discuss how the implications of fracturing through multi-layered sequences could be explored in future work.

Determination of some characteristic fracture of concrete of notched beam in bending test

This paper presents the identification some principal fracture parameters of concretes by experiment and simulation on notched beam in bending. The comparison between experimental and simulation results allows to determinate the Critical stress intensity factors KIC, fracture energy Gf and characteristic lengths of fracture process zone (FPZ) lch of 6 class of concrete with the compression resistance varying from 20 MPa to 50 MPa. These are important parameter in the model for predicting the timelife of concrete structure exposed in coastal area.

Characterization of Anisotropic Fracture Properties of Silurian Longmaxi Shale

Fracking is widely applied to enhance shale gas mining, and insight into the fracture behaviors of shale rocks is important. To characterize the fracture properties of Lower Silurian Longmaxi shale, a chevron-notched deep beam specimen, which inherits the advantages of notched deep beam and chevron-notched specimens, is introduced, and several three-point bend tests were conducted on the Longmaxi shale specimens in three principal fracture orientations: the divider, short-transverse, and arrester orientations; the anisotropy in the critical shape coefficient, fracture toughness and fracture energy were then examined and highlighted. The results demonstrate that the critical shape coefficient depends on the anisotropic elasticity of the investigated shale, and a maximum error in the calculated fracture toughness of 9.2% could occur if a critical shape coefficient for isotropic rock is adopted. For the tested shale, the mode I fracture toughness in the divider orientation is close to that in the arrester orientation, with both values being approximately 1.5 times that in the short-transverse orientation; in addition, a progressive decrease in fracture energy from the arrester orientation to the short-transverse orientation is apparent. The fracture surface morphology was observed by scanning electron microscopy, and obvious material deterioration was found near the fracture surfaces for the arrester and divider samples.

Productivity analysis for a horizontal well with multiple reorientation fractures in an anisotropic reservoir

Reorientation fractures may be formed in soft and shallow formations during fracturing stimulation and then affect well productivity. The principal focus of this study is on the productivity analysis for a horizontal well with multiple reorientation fractures in an anisotropic reservoir. Combining the nodal analysis technique and fracture-wing method, a semi-analytical model for a horizontal well with multiple finite-conductivity reorientation fractures was established to calculate its dimensionless productivity index and derivative for production evaluation. A classic case in the literature was selected to verify the accuracy of our semi-analytical solution and the verification indicates this new solution is reliable. Results show that for a fixed fracture configuration the dimensionless productivity index of the proposed model first goes up and then remains constant with the increase of fracture conductivity, and optimal fracture conductivity can be determined on derivative curves. Strong permeability anisotropy is a negative factor for well production and the productivity index gradually decreases with the increase of anisotropic factor. As principal fracture angle goes up, horizontal well’s productivity index increases correspondingly. However, the effect of reoriented fracture angle on the productivity index is not as strong as that of principal fracture angle. When reoriented fracture angle is smaller than principal fracture angle, reoriented factor should be as low as possible to achieve optimal productivity index. Meanwhile, well productivity index rises up with the increase of fracture number and fracture spacing, but the horizontal well has optimal reorientation fracture number and fracture spacing to get the economical productivity. Furthermore, the influence of the rotation of one central reorientation fracture on productivity index is weaker than that caused by the rotation of one external reorientation fracture. In addition, the asymmetrical distribution of one or more reorientation fractures slightly affects the productivity index when fracture conductivity is high enough.

Assessment of CNT-doping and hot-wet storage aging effects on Mode I, II and I/II interlaminar fracture toughness of a UD Graphite/Epoxy material system

The interlaminar fracture toughness of two unidirectional Graphite/Epoxy composite material systems has been experimentally assessed. The systems studied were prepreg composite and prepreg composite treated with carbon nanotubes (CNT). The fracture toughness has been quantified in Mode I, Mode II and Mode I/II by performing DCB, ENF and MMB tests according to relevant ISO and ASTM standards. The effect of aging by storage under hot-wet conditions has been quantified by studying these systems at room temperature without aging and at 70 °C after aging treatment. Experimental data are reported in a 3- or 5-specimen batch mode, indicating non-linear behavior and sensitivity to imperfections in coupons alignment and load application. Moreover, intermediate variables required for the estimation of fracture toughness are presented in order to be used as a reference guide for principal fracture test data evaluation. In the case of the RT systems, measured data have been compared with analytical solutions and finite element model predictions yielding good correlation for DCB and ENF tests and considerable deviation in the case of MMB tests. Main findings include that CNT-doping leads to an increase of fracture toughness in all modes, especially in Mode II, and that aging leads to less variation in measurements for both systems, indicating a more uniform matrix response.

Natural fractures in tight-oil sandstones: A case study of the Upper Triassic Yanchang Formation in the southwestern Ordos Basin, China

Natural fractures are important storage spaces and fluid-flow channels in tight-oil sandstones. Intraformational open fractures are the major channels for fluid flow in tight-oil sandstones. Small faults may provide fluid-flow channels across different layers. According to analogous outcrops, cores, and borehole image logs, small faults and intraformational open fractures are developed in the tight-oil sandstones of the Upper Triassic Yanchang Formation in the southwestern Ordos Basin, China. Among them, high dip-angle intraformational open fractures are the most abundant. Northeast-southwest–trending fractures are the principal fractures for fluid flow because that is the present-day maximum horizontal compressive stress direction. Combined with production data, horizontal wells, striking normal to or at a large angle relative to the major flow pathways, are beneficial for tight-oil production improvement. Fractures with high dip angles are the main factor that influences initial oil production. Linkage and tip damage zones are more favorable for oil production improvement than wall damage zones. This study provides an example of natural fracture characterization and unravels fracture contributions to reservoir physical properties and oil production of tight-oil sandstones, which could provide a geological basis for oil exploration and development in tight sandstones.

Fracture zones detection for groundwater exploration integrating Resistivity Profiling and Very Low Frequency electromagnetic methods (Errachidia basin, Morocco)

Abstract Resistivity Profiling and Very Low Frequency (VLF) electromagnetic methods were introduced to study fracture zones detection in Zaouia Jdida locality, within the Errachidia basin. The Horizontal Profiling was conducted in Wenner-α array, with AB = 300 m and profile lines oriented NW–SE and NE–SW. The resistivity measurements were taken using MAE advanced geophysics instruments. The VLF profiles were implanted with the length reaches 1000 m and profile lines oriented in NE–SW direction. The VLF measurements were collected using T-VLF iris instrument and the data filtering was done using KHFFILT software. Two filters, Karous-Hjelt and Fraser, were applied to the real component of the secondary electromagnetic field. The qualitative interpretation of resistivity results, showed the presence of subsurface targets; fracture zones were detected at 70m, 240m and 450m positions along the profile P1, at 180m, 340m and 450m positions from the profile P2. The semi-quantitative interpretation of VLF results revealed the presence of two principal fracture zones at L3 and L5 locations, oriented NW–SE, at a depth range of 30 m to 60 m. The VLF anomaly observed at L3 location is confirmed by the resistivity measurements from the profile P1 (at 70m station). The identified fractures represent the potential zones for groundwater supply and then will have an implication on storage and movement of groundwater in the prospect area.

Role of fracture opening in triggering microseismicity observed during hydraulic fracturing

Hydraulic fracturing in low-permeability hydrocarbon reservoirs creates/reactivates a fracture network leading to microseismic events. We have developed a simplified model of the evolution of the microseismic cloud based on the opening of a planar fracture cavity and its effect on elastic stresses and pore pressure diffusion during fluid injection in hydraulic fracturing treatments. Using a material balance equation, we compute the crack tip propagation over time assuming that the hydraulic fracture is shaped as a single penny-shaped cavity. Results indicate that in low-permeability formations, the crack tip propagates much faster than the pore pressure diffusion front thereby triggering the microseismic events farthest from the injection domain at any given time during fluid injection. We use the crack tip propagation to explain the triggering front observed in distance versus time plots of published microseismic data examples from hydraulic fracturing treatments of low-permeability hydrocarbon reservoirs. We conclude that attributing the location of the microseismic triggering front purely to pore pressure diffusion from the injection point may lead to incorrect estimates of the hydraulic diffusivity by multiple orders of magnitude for low-permeability formations. Moreover, the opening of the fracture cavity creates stress shadow zones perpendicular to the principal fracture walls in which microseismic triggering due to the elastic stress perturbations is suppressed. Microseismic triggering in this stress shadow region may be attributed mainly to pore pressure diffusion. We use the width, instead of the longest size, of the microseismic cloud to obtain an enhanced diffusivity measure, which may be useful for subsequent production simulations.