Pre-existing Fractures Research Articles

Winter weathering of fractured sedimentary rocks in a temperate climate: observation of freeze–thaw and thermal processes on the Niagara Escarpment, Hamilton, Ontario

AbstractThe Niagara Escarpment is a fractured Palaeozoic sedimentary cuesta, subject to year-round weathering in a temperate climate. We examined the temperature of the rock surface and fractures at threein situsites with varying aspect and lithology, as well as the surface and interior of three control blocks maintained in outdoor conditions between December 2020 and March 2021. The objectives were to examine the interplay between freeze–thaw and thermal weathering in the winter months and to identify potential factors influencing these processes. Both diurnal-scale and prolonged freeze–thaw cycles differing in spatial and temporal extent were identified, coincident with periods of high moisture. We frequently observed rapid temperature changes (>1 °C min−1) at sites with strong insolation, which implies that the temperature regime is suitable for thermal shock and fatigue to occur. Site-specific factors, such as the aspect of the escarpment face and lithology, impact the mechanism and extent of weathering. Southeast-facing sites with high insolation are dominated by diurnal-scale freeze–thaw; west- and east-facing sites with lower insolation experience a more prominent prolonged freeze–thaw cycle. Across all sites there is a gradient between surface and fracture temperature that follows diurnal trends in air temperature and insolation. Variability in the surface-fracture gradient may enhance weathering processes by shifting the orientation and magnitude of stress, and by changing the spatial distribution of freezing and thawing. Our research indicates that site-specific factors and pre-existing fractures moderate the influence of air temperature and insolation on thermal gradients, and ultimately the weathering regime.

High-quality fracture network mapping using high frequency logging while drilling (LWD) data: MSEEL case study

The Marcellus Shale and Energy Environmental Laboratory (MSEEL) provides a comprehensive dataset and field tests that can be used to study the significance of preexisting natural fractures in different subsurface engineering problems such as the effectiveness of the stimulation of an unconventional reservoir, optimized geothermal fluids movement and integrity of the CO2 storage site. Conventionally natural fracture intensity is obtained using sonic and micro-resistivity imaging logs. However, these techniques significantly suffer from two major deficiencies: Human bias in log interpretation and extremely long interpretation time. These two deficiencies are well-recognized in the industry; however, no standard procedures exist to address them. In this study, a new automated machine learning workflow (AMLW) is introduced that uses the LWD high-resolution acceleration data along the horizontal laterals to predict the natural fracture intensities originally obtained using sonic and micro-resistivity imaging. The accuracy and robustness of the new workflow to predict the near wellbore fracture intensities are tested using both regression and classification approaches. Both the regression and classification approaches were able to predict the fracture intensities with high accuracy (average Mean Squared Error of 0.0085 for regression and average accuracy of 0.94 in the confusion matrix for classification). We have shown that only 10%–15% of the labeled resistivity image log is required for training and validation of the machine-learning model. The Automated workflow resulted in K-Neighbors Regressor and classifier algorithms as the best algorithms with a 52.74% and 139.3% improvement in comparison to the Gradient Boosting Regression algorithm (i.e. the fifth best algorithm).

Numerical investigation of laboratory hydraulic fracturing tests in Pocheon granite

This study investigates numerically several hydraulic fracturing experiments that were performed on intact cubic Pocheon granite samples applying different injection protocols. The goal of the laboratory experiments is to test the concept of cyclic soft stimulation which aims to increase permeability sustainably among others. The Irazu 2D numerical code is used to simulate explicitly coupled hydraulic diffusion and fracturing processes under bi-axial stress conditions. Using the hybrid finite-discrete element modelling approach, we test two injection schemes, constant-rate continuous injection and cyclic progressive injection on homogeneous and heterogeneous samples. Our study focuses on the connection between the geometry of hydraulic fractures, fracturing mechanisms and the permeability increase after injection. The models capture several characteristics of the hydraulic fracturing tests using a time-scaling approach. The numerical simulation results show good agreement with the laboratory experiments in terms of pressure evolution characteristics and fracture pattern. Based on the simulation results, the constant-rate continuous and cyclic progressive injection schemes applied to heterogeneous rock sample with pre-existing fractures show the highest hydraulic aperture increase, and thus permeability enhancement.

Numerical simulation of hydraulic fracturing with consideration of the pore pressure distribution based on the unified pipe-interface element model

Understanding hydraulic fracture propagation behaviours is vital for optimizing fracturing design in oil and gas production. It has been observed that the stress and pore pressure distribution are much more complex during fracturing treatment as a result of long-term fluid injection and production in the reservoir. When rock is subjected to internal hydraulic pressure and external mechanical loading, the closing, opening, or other interactions of pre-existing weaknesses or induced new fractures will be altered. In this paper, a number of hydraulic simulation scenarios are performed to investigate the effect of pore pressure distribution on fracture initiation and propagation using the unified pipe-interface element method (UP-IEM). The proposed method for predicting the hydraulic fracture path in the permeable porous medium with non-uniform pore pressure field is validated with experimental results. Considering the arrangement of well patterns and natural fractures in the rock, the problems of hydraulic fracture propagation behaviours with wellbore interaction and pore pressure induced natural fracture activation are simulated. The influence of pore pressure saturation and the injection sequence of wellbores on hydraulic fracturing are researched. A sensitivity analysis is carried out to investigate the parameter effects on pre-existing fracture tortuosity, opening width and breakdown pressure under pore pressure. These numerical examples provide guidance to engineers to estimate the probability of danger or possibilities to improve the injection and production of wells.

The Role of Faults as Barriers in Confined Seismic Sequences: 2021 Seismicity in the Granada Basin (Betic Cordillera)

AbstractFault barriers are key structures for studying seismic hazard in regions of intense brittle deformation. The interaction between fault sets affects their seismogenic behavior, if some of them act as barriers. The Granada Basin, in the Betic Cordillera, is a region affected by shallow brittle deformation, as it was the scenario for the recent Granada 2021 seismic sequence. This seismicity presented a swarm behavior at the beginning of the sequence, followed by mainshock‐aftershock features. Geological and gravity data presented here reveal that the basement is affected by two sets of NW‐SE and NE‐SW normal faults and intensely deformed by vertical NW‐SE joints. Improved relocation of the Granada 2021 seismicity reveals a confined chimney‐shape seismicity caused by the activity of a 2 km long NW‐SE normal fault segment. The confinement of the sequence is associated with the NE‐SW fault set acting as a barrier that restricts the rupture area, limiting the maximum magnitude, and favoring the recurrence of events with smaller magnitude. The chimney‐shape of the seismic sequence suggests that the deformation is propagated vertically to the surface, facilitated by preexisting fractures. The shallow extensional deformation during the uplift of the central Betic Cordillera drove the activity of the local structures obliquely to the regional extensional trends, as evidenced by the seismic sequence. This multidisciplinary study improves the knowledge on the origin of the Granada Basin and underlies the important role of preexisting fractures on fault segmentation and seismic propagation, decreasing the seismic potential of this area.

412.11: New-Onset Osteoporotic Fracture After Kidney Transplantation: A Matched Comparative Nationwide Cohort Study

Background: Successful kidney transplantation (KT) has brought better survival and quality of life to patients with end-stage kidney disease (ESKD), although long-term immunosuppressive agent-related side effects such as osteoporosis remain unresolved. We aim to explore the incidence risk of de novo osteoporotic fractures after KT compared with matched dialysis patients. Method: We constructed a nation-wide retrospective cohort consist of 141,674 ESKD patients from 2008 to 2020 using the National Health Insurance System database of Korea. After 1-year of wash out periods, patients younger than 18-year-old, having previous history of any kind of cancer, experiencing parathyroidectomy, re-transplantation, osteoporotic fracture, or taking osteoporosis-treating medication were excluded. Then, we extracted KT recipients and 1:1-matched dialysis patients according to the age, sex, era, and the presence of hypertension or diabetes. Osteoporotic fractures are defined as fractures associated with low bone mineral density including hip, spine, forearm, and humerus. We compared the de novo osteoporotic fracture incidence between the two groups. Result: After exclusion of 26883 patients in wash-out periods, 483 children, 6953 having cancers, 336 experiencing parathyroidectomy, 8907 preexisting osteoporotic fractures, 3755 established osteoporosis medications, and 110 re-transplantations, we finally identified 12760 matched KT-dialysis pairs. Their average age was 51.0 ± (10.1) (standard deviation (SD)) years and 7951 (62.3%) were male. Patients with having hypertension and diabetes were 11800 (92.5%) and 4548 (35.6%), respectively. About 30.5% KT patients received their graft preemptively. New-onset osteoporotic fractures after inclusion occurred in 631 (4.9%) of dialysis patients and 588 (4.6%) of KT recipients, respectively. The most common fracture site was forearm in both groups. During overall 4.7 ± 3.1 years of follow up, KT recipients showed similar risk for the development of new-onset osteoporotic fracture compared with matched dialysis controls even after adjustment of matching variables, previous use of steroid or anti-depressants, vitamin D or its analog, calcium-based phosphate binders. In subgroup analysis, they showed a lower risk of hip fracture (adjusted hazard ratio 0.51, 95% CI 0.38-0.67, p <0.001), but similar risks of other types of osteoporotic fractures including spine, forearm, or humerus compared to dialysis controls. Conclusion: In this study, overall new-onset osteoporotic fracture risk of KT recipients was not different from matched dialysis controls except a lower risk of hip fractures.

Parametric effects on fracture geometries from multi-fracture propagation emanating from neighbouring wellbores in quasi-brittle rocks

An integrated approach is presented using field data input from measured geological information into numerical simulation for understanding effects of induced stress on geometries of multiple fracture propagation. We establish a benchmark study based on comparison of field result with numerical computations. The comparison then acts as reference measures for studying effects of changing in-situ stress, fracturing fluid viscosity and fracture spacing on propagation and geometries of multiple fractures between neighbouring wellbores in an undisclosed gas field. This leverages understanding of more complexities associated with inter-well multiple fracture growth that are idealized as straight from certain perspectives. Although some studies focus on stress interference from pre-existing fractures, actual fracture propagation geometries may be far from the idealized scenarios. Therefore, the stress shadow effects between growing multiple hydraulic fractures, if not taken into account, can lead to unrealistic estimation of hydraulic fracture trajectories. Consequently, more attention should be paid to the actual propagation of hydraulic fractures. Actual field geologic information is provided through well-logging and field mapping data. Very short fractures were propagated in these wells before the operation was terminated due to technical problems. The reservoir depth in the area is about 2170 m. At such depth, quasi-brittleness of shale should be accounted for by using relevant methods that capture rock ductility such as traction separation law. Abaqus commercial software is used to conduct the numerical computation using extended finite element method based on cohesive zone modeling. Application of this technique is further validated using Kristianovich-Geertsma-de Klerk analytical solution. This study is important in field implementation of infill drilling with well-informed mechanism of fracture interference in the inter-well region.

CO2 and heat energy transport by enhanced fracture permeability in the Monterotondo Marittimo-Sasso Pisano transfer fault system (Larderello Geothermal Field, Italy)

Carbon dioxide is one of the most important gasses naturally released from geothermal systems. Establishing the processes and pathways that regulate the CO2 diffuse degassing can provide valuable information for exploration and exploitation purposes of geothermal reservoirs. In this work, a high-resolution CO2 flux (with records up to 2927 g m–2 d–1) and soil temperature (with records up to 98.8 °C) survey was carried out along with detailed fracture parameters measurements in a selected area of the Monterotondo Marittimo-Sasso Pisano transfer fault (Larderello geothermal system, Tuscany, Italy). The main aim was to define the behavior of diffuse CO2 through the fault system and investigate how the soil CO2 flux and steam change with respect to the architecture of the fault damage zone (i.e., volumetric fracture intensity, permeability, and persistence of the fractures). The presence of multiple populations of CO2 flux suggested that three different transport mechanisms control soil degassing: i) purely diffusive, ii) mixed diffusive-advective, and iii) purely advective, characterized by efflux values of <20, between 20 and 300 and >300 g m–2 d–1, respectively. The spatial distribution of these fluxes well agrees with the fracture distribution and features of the Jurassic radiolarite (Diaspri Fm) dissected by NNE-striking faults. Areas with high CO2 emissions are indeed able to reveal major upflow areas from deep reservoirs through deep-reaching permeable fault zones. The interaction between pre-existing fractures and fracture-related fault-zone locally enhances the secondary rock permeability as highlighted by the correlation between Discrete Fracture Network (DFN) modeling and advective flux. Eventually, by normalizing the CO2 output to the fault strip (1350 m2), a release of CO2 equal to ∼155 t d–1 km–2 was estimated.

3D digital outcrop model-based analysis of fracture network along the seismogenic Mt. Vettore Fault System (Central Italy): the importance of inherited fractures

The Mt. Vettore area is located in the Central Apennines (Italy), a region characterized by intense seismic activity that has recorded multiple moderate-to-high magnitude seismic sequences. The seismic activity is due to the presence of normal fault systems, among which is the Mt. Vettore Fault System (VFS), which was last activated during the 2016-17 Central Italy seismic sequence.. Moreover, the region has experienced three major tectonic phases over geological history, thus it is important to unravel their contribution to the current fracture network. Based on the integration of field observation with Unmanned Aerial Vehicle - Digital Photogrammetry data, we aim to analyze the fracture network on eight different outcrops located at different structural positions with respect to VFZ. Results show that the Late Miocene−Early Pliocene compressional phase deeply affected the present-day fracture pattern, which is especially related to the evolution of the Mt. Sibillini regional thrust and its related anticline. The present-day Quaternary extensional phase, and the associated normal faults, mostly reactivate some of the pre-existing fracture sets.

Hotspot volcanism along a leaky fracture zone contributes the formation of the 85°E Ridge at 11°N latitude, Bay of Bengal

The 85°E Ridge is a prominent linear structure in the northeastern Indian Ocean. Its nature and origin are still controversial due to the coverage of thick Bengal Fan sediments. Here we present a newly collected 400-km-long 2-D seismic reflection profile which crosses the 85°E Ridge near 11°N and shows the basement morphology and internal structures of the ridge. The ridge has a thickened crust characterized by two basement highs rising up to ~2–3 km above the surrounding oceanic crust and a Moho depression with amplitude of ~3–4 km. Typical volcanic structures formed by hotspot volcanism are identified in the ridge. Deep-rooted faults played important roles in the formation of the 85°E Ridge. They acted as the preferred conduit for the upwelling of hotspot melts, but their syn- and post-volcanic reactivations induced the collapse of volcanic sequences and the relative uplift of the East Ridge. These observations support that the 85°E Ridge near 11°N having formed by hotspot magmatism drained along a leaky fracture zone. Combined with previous results, a new single-hotspot formation model of the 85°E Ridge is proposed, in which the entire 85°E Ridge was formed by a weak and pulsating mantle plume acting on the northward drifting Indian Plate. Its along-strike variations in location, morphology, and structure in different segments are interpreted to reflect different elastic strength of the oceanic lithosphere and the existence of a pre-existing fracture zone which laterally redistributed the shallow melts.

A Physiotherapist-Led Paediatric Fracture Clinic was a Safe and Efficient Alternative to Traditional Outpatient Orthopaedic Care: A Preliminary Evaluation

Background: Demand for publicly funded orthopaedic outpatient services is growing rapidly which affects a health service’s ability to provide timely care. The current study describes the implementation and preliminary evaluation of a novel physiotherapist-led paediatric fracture clinic, which was introduced to provide more timely care to children. The study evaluated 1) the length of time patients spent in the outpatient department per appointment, 2) did-not-attend rates and 3) unplanned re-presentations to the clinic for the same problem. Methods: The study was conducted at a large regional hospital in Victoria, Australia. A physiotherapist-led paediatric fracture clinic was introduced in 2013 to provide care to children with acute simple limb fractures. Descriptive data for all patients seen in the clinic between 1st January 2013 and 31st December 2018 were collected prospectively. The performance of the new service was evaluated by comparing it to the pre-existing orthopaedic surgeon-led fracture clinic for the six months prior to the introduction of the new service for 1) the total time patients spent in the outpatient department per appointment and 2) the number of patients that failed to attend an appointment. The pre-existing surgeon-led clinic was also compared to the surgeon-led clinic for the six months following the introduction of the physiotherapist-led clinic. Total time spent in the department per appointment was compared between groups using linear regression, controlling for age, diagnosis and appointment type (new or review). The proportion of patients who did not attend an appointment were compared between groups using the chi square test. The number of unplanned representations to either clinic was assessed between 01/01/2013 and 31/12/2018. Results: Between 2013 and 2018, there were 4337 attendances at the physiotherapist-led clinic. Total time in the department was significantly less (p&lt;.05) for patients seen in the physiotherapist-led clinic when compared to the pre-existing surgeon-led clinic (median difference 82 minutes). Total time in the surgeon-led clinic also reduced following the introduction of the new service (median difference 21 minutes). Did-not-attend rates (10.3%) and unplanned re-presentations (0.6%) were similar (p&gt;.05) between the groups. Conclusions: The physiotherapist-led clinic improved the timeliness of care for paediatric patients without evidence of compromised patient safety. KEYWORDS: Paediatrics, musculoskeletal health, service evaluation, service redesign, workforce reform, fractures, orthopaedics.

Novel method to select chemical particulates for near-wellbore fluid diversion during hydraulic fracturing treatment

The application of chemical particulates as diverter is an evolving technology and has become popular among E&P operators. The primary objective of Near-Wellbore Diversion (NWD) is uniform distribution of treatment slurry within the target zone. The mixture of bead, powder and flakes composed of Polylactic Acid (PLA) are commonly employed for NWD application to plug temporarily already stimulated perforations and pre-existing fractures. The successful NWD application largely depends on the selection of diverter chemical, particle size distribution, proportions of each particle sizes, and total diverter quantity. The advantages of chemical diverters are simple injection without any pumping system modification and negligible increase in treatment time. Their chemical composition can be tailored according to the reservoir properties.The diverter works on jamming and sealing mechanisms. The popular Abrams rule for drilling fluid selection and Ideal Packing Theory for obtaining minimum pack permeability are customized here to be used for diverting particles selection. The diverter selection parameters are investigated in laboratory by performing plug-stability experiments on simulated perforations. It was learned that the size of coarse particles should be greater than 0.4 times the size of perforation diameter. The medium and fine particle diameters should be greater than 0.15 times the size of coarse and medium particles respectively. It is also recommended to use a minimum of 0.5 ppa diverter concentration, and Ideal Packing Theory modified for diverter application to determine the proportions of available particle sizes. The behaviour of pressure build-up and fluid leakoff during experiments were compared to evaluate the performance of various diverter blends.The present novel method will help to select the proper diverter for a successful NWD application that will aid to uniformly distribute treatment fluid during hydraulic fracturing treatment. The guidelines for its successful application are provided to help engineers to increase the effectiveness of NWD.

Study of D-limonene as novel green hydraulic fracturing surfactant in shale gas reservoir

This study focuses on rock-fluid interaction during hydraulic fracturing in shale gas reservoir. Hydraulic fracturing is a well stimulation technique used to create fracture network in reservoirs by connecting the main planar hydraulic fracture with the pre-existing natural fracture. Hydraulic fracturing fluid additives such as surfactants are added into the hydraulic fracturing fluid system to enhance oil and gas productivity by assisting fluid recovery after fracturing. The main objective of this study is to evaluate the effectiveness and formation damage parameters of the proposed novel green surfactant as hydraulic fracturing fluid additive where commercial-chemical based surfactant was studied for comparison purpose. The green surfactant used was D-limonene whereas Sodium Alpha Olefin Sulfonate (AOS) was used as commercial-chemical based surfactant. Thermogravimetric analysis (TGA) was done to ensure the D-limonene can withstand the high reservoir temperature in which the D-limonene had inflection point and onset temperature of 410 °C and 380 °C respectively. Soaking test was then conducted where the shale samples were soaked in hydraulic fracturing fluid for seven days. The outcome of the soaking test was then studied in Scanning Electron Microscopy (SEM) and wettability analysis. The SEM test showed that the average pore size of the soaked shale samples were similar to the original shale sample which indicated minimal to no formation damage caused by the hydraulic fracturing fluid. Besides that, the contact angle test resulted in the distilled water and D-limonene forming a contact angle of 90° and 0° respectively on the shale samples. The AOS formed contact angles between 65° and 75° for the tested concentrations. From the results, the shale sample was found to be oil wet and showed a high affinity towards the D-limonene followed by AOS and distilled water. Higher affinity of the surfactants promoted the desorption of the hydraulic fracturing fluid from the shale surface.

Impact of Gas Saturation and Gas Column Height at the Base of the Gas Hydrate Stability Zone on Fracturing and Seepage at Vestnesa Ridge, West-Svalbard Margin

The Vestnesa Ridge, located off the west Svalbard margin, is a &gt;60 km long ridge consisting of fine-grained sediments that host a deep-marine gas hydrate and associated seepage system. Geological and geophysical observations indicate the predominance of vertical fluid expulsion through fractures with pockmarks expressed on the seafloor along the entire ridge. However, despite the apparent evidence for an extended free gas zone (FGZ) below the base of the gas hydrate stability zone (BGHSZ), present-day seafloor seepage has been confirmed only on the eastern half of the sedimentary ridge. In this study, we combine the relationships between aqueous phase pressure, capillary pressure, sediment clay fraction, porosity, and total stress to simulate how much gas is required to open preexisting fractures from the BGHSZ towards the seafloor. Data from four specific sites with different lithology and pressure regime along the ridge are used to constrain the simulations. Results demonstrate that fracturing is favored from the FGZ (with gas saturations &lt; 0.1 and gas column heights &lt; 15 m) towards the seafloor. Neglecting the capillary pressure overpredicts the size of the gas column by up to 10 times, leading to erroneous maximum gas vent volume predictions and associated ocean biosphere consequences. Further parametric analyses indicate that variations in the regional stress regime have the potential to modify the fracture criterion, thus driving the differences in venting across the ridge. Our results are in line with independent geophysical observations and petroleum system modeling in the study area, adding confidence to the proposed approach and highlighting the importance of the capillary pressure influence on gas pressure.

The role of pre-existing faults and fractures in shaping polygonal impact craters and its tectonic implications in the southern Margaritifer Terra region, Mars

The role of pre-existing faults and fractures in shaping polygonal impact craters and its tectonic implications in the southern Margaritifer Terra region, Mars

Numerical Simulation of Influencing Factors of Hydraulic Fracture Network Development in Reservoirs with Pre-Existing Fractures

The intersection behavior of hydraulic fractures and single natural fractures has been studied in detail; however, in fractured reservoirs, natural fractures are numerous and interlaced and the intersection of hydraulic fractures and multiple natural fractures occurs during the fracturing process. This intersection behavior is more complex and there is a lack of research on this topic at present. In this study, a numerical simulation model of the interaction between hydraulic fractures and a series of natural fractures was established, the main factors that affect the formation scale of a fracture network during the hydraulic fracturing of a fractured reservoir were studied using the numerical simulation method, and the parameters were also studied. The results showed that the natural fracture trend, in situ stress difference, and injection flow rate have an impact on the scale of a fracture network. The larger the in situ stress difference, the smaller the scale of the fracture network, which gradually changes from multiple clusters of fractures to single fractures. The larger the injection flow rate, the larger the scale of the fracture network. In the uniform stress field, the direction of a natural fracture is closer to the direction of principal stress, so the lower the fracture extension pressure, the smaller the scale of the network. On the contrary, the farther away from the principal stress direction, the lower the fracture extension pressure and the higher the extension pressure, the larger the scale of the fracture network.

Present status and future directions: vertical root fractures in root filled teeth.

Vertical root fracture (VRF) is a common reason for the extraction of root filled teeth. The accurate diagnosis of VRF may be challenging due to the absence of clinical signs, whilst conventional radiographic assessment is often inconclusive. However, an understanding of the aetiology of VRFs, and more importantly, the key predisposing factors, is crucial in identifying teeth that may be susceptible. Thorough clinical examination with magnification and co‐axial lighting is essential in identifying VRFs, and although CBCT is unable to reliably detect VRFs per se, the pattern of bone loss typically associated with VRF can be fully appreciated, and therefore, increases the probability of correct diagnosis and management. The prevalence of VRFs in root filled teeth is significantly greater than in teeth with vital pulps, demonstrating that the combination of loss of structural integrity, presence of pre‐existing fractures and biochemical effects of loss of vitality is highly relevant. Careful assessment of the occlusal scheme, presence of deflective contacts and identification of parafunctional habits are imperative in both preventing and managing VRFs. Furthermore, anatomical factors such as root canal morphology may predispose certain teeth to VRF. The influence of access cavity design and root canal instrumentation protocols should be considered although the impact of these on the fracture resistance of root filled teeth is not clearly validated. The post‐endodontic restoration of root filled teeth should be expedient and considerate to the residual tooth structure. Posts should be placed ‘passively’ and excessive ‘post‐space’ preparation should be avoided. This narrative review aims to present the aetiology, potential predisposing factors, histopathology, diagnosis and management of VRF and present perspectives for future research. Currently, there are limited options other than extraction for the management of VRF, although root resection may be considered in multi‐rooted teeth. Innovative techniques to ‘repair’ VRFs using both orthograde and surgical approaches require further research and validation. The prevention of VRFs is critical; identifying susceptible teeth, utilizing conservative endodontic procedures, together with expedient and appropriate post‐endodontic restorative procedures is paramount to reducing the incidence of terminal VRFs.

Deep Convolutional Network Improves Completion Design

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 201545, “Completion Design Improvement With a Deep Convolutional Network,” by Liwei Li, Nasser Nasrabadi, and Timothy Carr, SPE, West Virginia University. The paper has not been peer reviewed. Maximizing stimulated natural and hydraulic-fracture networks is a primary concern for economic production from a horizontal shale gas well. Geomechanical facies and preexisting fractures in each stage are identified based on similarities in formation characteristics. This often requires analyzing large volumes of drilling, logging-while-drilling (LWD), and measurement-while-drilling (MWD) data. In the complete paper, the authors develop a methodology that calculates mechanical specific energy (MSE) using real-time drillstring acceleration signals. Introduction The Marcellus Shale Energy and Environmental Laboratory (MSEEL) aimed to improve stimulation efficiency by collecting, integrating, and analyzing real-time LWD and MWD data. In addition, engineered completions were combined with fiber-optic distributed-temperature-sensor and distributed-acoustic-sensor data. To reduce issues with data quality measured using surface sensors, advanced MWD and LWD tools are proposed to collect data close to the bit. In the analysis of nonstationary data such as drillstring vibration measurements, the conventional Fourier transform is limited to frequency spectrum without time information. To address this problem, new signal-processing methods have been developed to provide frequency features localized in time. Deep convolutional neural networks (CNNs) are a promising approach for image classification. Feature extractions can be performed by adding convolution and pooling filters, reducing spatial resolutions, and increasing depth to build a deep learning network. The authors propose a decision chart that labels rock laterals using calculated MSE values and interpreted natural fracture intensities to improve stage-by-stage perforation efficiency. The borehole-imaging tool is used to achieve a cost-effective solution. This tool collects both high-resolution acoustic amplitudes and triaxial accelerations.

Mechanisms for the Generation of Complex Fracture Networks: Observations From Slant Core, Analog Models, and Outcrop

We use observations of hydraulic fractures in core, outcrop attributes of natural hydraulic fractures, and analogue models, to address how hydraulic fracture networks evolve. A slant core from the Wolfcamp Formation—an unconventional shale hydrocarbon reservoir in the Permian Basin of West Texas—collected within 18 m and 30 m of two hydraulically stimulated horizontal wells, provided an opportunity to examine hydraulic fractures directly. In approximately 183 m of core, 309 calcite-sealed natural opening-mode fractures and 375 hydraulic fractures were identified. Many hydraulic fractures in the core show complex morphology, including twist-hackle segmentation, diversion, and bifurcation; these structures most commonly develop at lithological bed boundaries and mechanical heterogeneities such as natural fractures and concretions. An outcrop of bed-parallel pavements in the Cretaceous Boquillas Formation in West Texas contains opening-mode fractures that likely formed by natural hydraulic fracturing. Fracture traces provide evidence of twist-hackle segmentation, and are typically associated with bed boundaries and preexisting bed-parallel stylolites. A laboratory study of hydraulic fracturing of 33 synthetic blocks of gypsum and hydrostone revealed fracture steps, diversions, twist hackles, and multiple overlapping fractures together with information on fracture growth directions. These complexities in the fracture network were dominantly nucleated at inclusions used to simulate pre-existing fractures, and as a result of mechanical heterogeneity introduced by the wellbore and perforations. Collectively, our results show that complex fracture networks are produced in hydraulic fracturing of self-sourced reservoir strata. Mechanical stratigraphic boundaries and other heterogeneities are likely to enhance fracture network complexity through the processes of segmentation, diversion, and bifurcation. These processes create multiple fracture strands, resulting in an increased number of hydraulic fractures over those initiated, thereby increasing total fracture surface area. Our study provides insight into hydraulic fracture network propagation, and has applications for evaluation, completion, production, and fracture modeling of unconventional reservoirs.

Study on fracture initiation pressure of a horizontal well considering the influence of stress shadow in tight sand reservoirs

Multi-stage hydraulic fracturing (HF) is an effective stimulation technique for enhanced hydrocarbon recovery in tight sand reservoirs, and the fracture initiation pressure (FIP) is a key parameter for evaluating the difficulty of HF. Recent studies have shown that the stress shadow caused by pre-existing hydraulic fractures considerably affects fracture initiation. However, few studies have fully analyzed the FIP considering the stress shadow effect. Additionally, factors affecting stress shadow have not been quantitatively evaluated. Therefore, the aim of this study was to establish an efficient model for investigating the impact of stress shadow on the FIP. We developed a comprehensive stress distribution model for the perforated horizontal wellbore and analyzed the fracture initiation mechanism. Subsequently, we applied the displacement discontinuity method (DDM) to simulate the stress shadow generated by hydraulic fractures. Next, we validated the DDM against the analytical solution and analyzed the variation of the FIP along the horizontal wellbore considering the effect of stress shadow. Finally, we evaluated the key parameters affecting the FIP using a well-trained back-propagation artificial neural network. The results showed that stress shadow increased the magnitude of the FIP during the multi-stage HF operation. The fracture net pressure and the offset distance from the pre-existing hydraulic fracture are crucial parameters that affect the FIP. This study provides an efficient approach for calculating the FIP for multi-stage HF and can aid engineers in developing tight sand reservoirs.