Development Of Fracture Network Research Articles

A new approach for characterization and prediction of natural fracture occurrence in tight oil sandstones with intense anisotropy

Natural fractures provide the main path for fluid flow in tight oil reservoirs and can control the flow direction in the subsurface. Tight sandstones commonly have intense mechanical anisotropy, which means that fracture development in such tight formations may vary widely with respect to fracture orientations. However, the prediction of the degree of fracture development for each orientation is challenging. Focusing on the tight sandstones of the Chang 4 and 5 Member in the Jiyuan oil field, Ordos Basin, China, a new approach was presented for better prediction of the tectonic fracture occurrence in different directions based on fracture characterization, controlling factor, and formation mechanism analysis. First, fracture types, characteristics, formation time, and controlling factors were determined using data from outcrops, cores, and image logs. Then, triaxial tests were conducted to measure the mechanical parameters of rock samples in different directions that assessed the mechanical anisotropy of the formation and its impact on the development of fracture networks in the basin. Next, finite element numerical simulations of the paleotectonic stress field during fracturing were performed based on the fracture formation mechanism and controlling factors (lithology, bed thickness, sedimentary microfacies, and rock anisotropy). Finally, according to the failure criteria established using the measured mechanical parameters, the failure ratio and strain energy were calculated. These criteria could be employed to predict the fracture occurrence and the degree of development of each fracture network. The simulation predictions in this work are in good agreement with observed data from outcrops, cores, and image logs.

Investigation of the Effects of Fracture Orientation and Saturation on the Vp/Vs Ratio and Their Implications

Fractures are widely distributed within the Earth’s crust and have a substantial influence on wave propagation. Although many studies have been conducted on fracture characterization based on seismic anisotropic attributes, natural horizontal fractures and hydraulic fracture networks have yet to be characterized. Here, we investigate the influence of the fracture orientation and saturation on the Vp/Vs ratio via laboratory observations and theoretical modeling. Then, we discuss the feasibility of using the Vp/Vs ratio to predict natural horizontal fractures and evaluate hydraulic fracture networks. Our study demonstrates that an abnormally low Vp/Vs ratio, together with a lack of shear wave splitting, could be used to predict horizontal fractures. Fracture network development that results from hydraulic fracturing stimulation is associated with a much higher Vp/Vs ratio and no shear wave splitting; these two characteristics could be used to estimate fracture development and evaluate hydraulic fracturing stimulation.

An integrated approach to determining 4D stress development at Castle Cove

Models for basin evolution and natural fracture development often contain many uncertainties. Multiscale approaches to structural analysis assist in reducing these by providing checkpoints for structural evolution to better constrain the development of paleostress phases through time. In this study, we integrate the analysis of calcite twins, magnetic fabrics, stylolites and natural fractures at Castle Cove in the eastern Otway Basin, producing a five-phase model for stress evolution consisting of: phase 1 ~NW–SE Mid-Cretaceous strike-slip or compression; phases 2 and 3 Late Cretaceous extension, coinciding with the development of ~NW–SE and ~NE–SW striking extensional fracture sets; phase 4 ~NE–SW strike-slip and compression, representing an enigmatic period of stress evolution with respect to the current understanding of the Otway Basin; and phase 5, present day ~NW–SE strike-slip stress. The results contribute to a 4D structural history construction for the eastern Otway Basin and suggest that the evolution of the region may require reassessing in order to determine the timing and nature of the detected ~NE–SW oriented compressional event. This study also demonstrates how the use of a calcite stress inversion technique can assist in providing mechanical checkpoints for the evolution of complex natural fracture networks, which can easily be expanded within the sub-surface.

Chemical controls on the propagation rate of fracture in calcite

Calcite (CaCO3) is one of the most abundant minerals in the Earth’s crust, and it is susceptible to subcritical chemically-driven fracturing. Understanding chemical processes at individual fracture tips, and how they control the development of fractures and fracture networks in the subsurface, is critical for carbon and nuclear waste storage, resource extraction, and predicting earthquakes. Chemical processes controlling subcritical fracture in calcite are poorly understood. We demonstrate a novel approach to quantify the coupled chemical-mechanical effects on subcritical fracture. The calcite surface was indented using a Vickers-geometry indenter tip, which resulted in repeatable micron-scale fractures propagating from the indent. Individual indented samples were submerged in an array of aqueous fluids and an optical microscope was used to track the fracture growth in situ. The fracture propagation rate varied from 1.6 × 10−8 m s−1 to 2.4 × 10−10 m s−1. The rate depended on the type of aqueous ligand present, and did not correlate with the measured dissolution rate of calcite or trends in zeta-potential. We postulate that chemical complexation at the fracture tip in calcite controls the growth of subcritical fracture. Previous studies indirectly pointed to the zeta-potential being the most critical factor, while our work indicates that variation in the zeta-potential has a secondary effect.

Numerical Study of Fracture Network Evolution during Nitrogen Fracturing Processes in Shale Reservoirs

This paper develops a numerical model to study fracture network evolution during the nitrogen fracturing process in shale reservoirs. This model considers the differences of incompressible and compressible fluids, shear and tensile failure modes, shale heterogeneity, and the strength and permeability of both shale matrix and bedding planes through the coupling of mechanical-seepage-damage during fracturing fluid injection. The results show that nitrogen fracturing has a lower breakdown pressure and larger seepage zone than hydraulic fracturing under the same injection pressure. Tensile failure was identified as the major reason for the initiation and propagation of fractures. Ignoring the effect of bedding planes, the fracture initiation pressure, breakdown pressure, and fracturing effectiveness reached their maxima when the stress ratio is 1. Under the same strength ratio, the propagation path of the fractures was controlled by the stronger effect that was casused by the bedding angle and stress ratio. With increasing the strength ratio, the fracture number and shearing of the bedding plane increased significantly and the failure pattern changed from tensile-only mode to tensile-shear mode. These analyses indicated that the fracture network of bedding shale was typically induced by the combined impacts of stress ratio, bedding angle and strength ratio.

Tracking the development of seismic fracture network from The Geysers geothermal field

Underground fluid injections result in rock mass fracturing. The associated environmental hazards in a significant part stem from a possibility for linking these fractures. The resultant crevices may allow for an undesired and hazardous fluid migration. We studied the fracture linking problem on data from a part of The Geysers geothermal field in California, USA. We parameterized seismic events by the distance between hypocenter and injecting well, by the angle between the position vector of hypocentre and the maximum horizontal stress direction and by the angle of rotation required to turn the event’s double-couple mechanism into the prevailing in this area faults’ orientation. To make these parameters comparable, we transformed them to equivalent dimensions. Based on distances between events in the transformed parameter space, we divided the seismic events into clusters. The percentage of potentially linked fractures in clusters was greater at low than at high injection rate.

Latest research progresses in network fracturing theories and technologies for shale gas reservoirs

In view of the bottleneck of shale gas development technologies, the latest progresses in such theories on the fields like fracture network fracability evaluation, fracture network propagation mechanism, stimulated reservoir volume (SRV) evaluation and fracturing fluid research & development were analyzed comprehensively in reference to the existing achievements and experiences in North American shale gas reservoir development and based on the probes, practices and cognitions of shale gas development technologies in China since 2005. First, the study on the shale brittleness develops from the definition of mineral and mechanical semi-quantitative threshold measurement to a comprehensive evaluation which integrates rock constituents, elastic mechanics and development characteristics of natural fractures together. Second, the study on the fracture network development develops from the directional extension theory to the simulation of fracture network formation in the stochastic distribution pattern of natural fractures. Third, the study on SRV develops from the micro-seismic monitoring based instrument and technology to the mathematical theory evaluation method dependent on the discrete fracture network and extended finite element. And fourth, the study on fracturing fluids develops from the wide application of slick water (drag reducing water) and a linear gel fracturing fluid system to the development and application of novel fracturing fluids with little or no water. Then, the theoretical and technological challenges were pointed out, including the comprehensive fracability evaluation of reservoir fracture networks, the deep seated shale gas fracturing, the operation curve diagnosis of shale gas fracturing, the shale gas refracturing theory, and the research & development and flowback control of novel fracturing fluids. Finally, the development trend of related technologies was predicted so as to provide a theoretical and technical guidance for the efficient shale gas development in China in the future.

Quantifying vein attributes in massive mudstones (Triassic, SW England): Implications for progressive evolution of opening-mode fracture networks

Abstract Attributes of gypsum vein arrays in the Triassic massive mudstones of SW England were analyzed to infer the process of vein network evolution. Field data demonstrate that there is no clear correlation between vein orientation and thickness; however, sub-horizontal veins have a higher average thickness than the inclined veins. Vein thickness distribution is best characterized by a log-normal distribution. The fractal dimensions ‘D’ of vein networks analyzed range from 1.44 to 1.71. Vein networks, which contain abundant bridging veins between neighbouring sub-horizontal veins to form an anastomosing pattern, exhibit a higher D value. The fraction of vein volume derived from image analysis ranges from 6.75 to 17.94%. A positive correlation is revealed by plotting the fractal dimensions of the vein networks versus the number of vein-intersection points, suggesting that an anti-clustering distribution of veins could contribute to more interactions and also intersections of the gypsum veins. The vein attribute data presented indicate that mineralization in the massive mudstones is mainly localized to some of the largest veins as a result of mechanical interaction of neighbouring veins during vein propagation, hindering the nucleation of new veins as a consequence. Accommodation of the bulk rock strain could be facilitated mainly by expansion of the large, backbone veins, which follow a positive feedback mechanism of growth. This study reveals a strong link between the geometric parameters of opening-mode fractures with the history of their formation, propagation and intersection.

Joint spacing distribution of granites in Hong Kong

The structural stability of rock masses is dictated by the inherent geological structures present, most notably joints. Joint spacings are one of crucial parameters to control the block volume, permeability, deformability as well as strength of rock masses. While extensive rock engineering projects have been carried out in Hong Kong in the last century, the distribution models of joint spacings in Hong Kong rocks have still rarely been studied systematically and published. This research examines true joint spacings of granites in fourteen different slope outcrops at various localities of Hong Kong. A total number of 1323 joint spacing measurements, belonging to 44 joint sets spanning from 0.005 m to 4.5 m, are obtained. The mean joint spacings among 44 joint sets studied are computed in a range of 0.11 m to 1.33 m. Lognormal Distribution is found to be the dominant joint spacing distribution of granites in Hong Kong. A further in-depth understanding of joint spacing distributions of Hong Kong granitic rocks, which covers about 35% of the total land area, is of profound practical value to the assessment of rock mass excavatability and numerical modelling of rock fracture network development.

Improved vertical stress profiling for unconventional reservoirs

A critical input in a hydraulic fracture stimulation design is the vertical stress profile. From it, designers can determine the variation in the minimum horizontal stress as a function of depth. The depth profile of the minimum horizontal stress (i.e., the vertical stress profile) is a major factor in controlling the geometry of a hydraulic fracture or fracture network development (e.g., Economides and Nolte, 2000). For example, an optimum stimulation design would define the pumping variables and schedule so as to limit height growth to the productive interval while promoting optimum horizontal growth. Height growth into non-productive layers above or below the pay interval reduces the horizontal length in the productive interval. In addition, gravity can concentrate the proppant in the fractures below the producing interval, diminishing fracture conductivity in the producing interval. Both mechanisms decrease production. Therefore, to design an optimal stimulation, an accurate vertical stress profile is required. This requires appropriate and accurate stress models. Petroleum geomechanics typically uses one of two general classes of analytic stress models. One model is based on linear elasticity; the other is based on failure mechanics. Linear elastic models use Hooke’s law and therefore assume the target formation is continuous and homogeneous. In these models, the formation’s elastic constants linearly relate stress with strain. Failure models, such as Mohr-Coulomb, assume a maximum in situ shear stress above which failure occurs and which is controlled by the shear strength of the formation. While these two classes of models use strikingly different assumptions, they often yield similar stress profiles (Thiercelin and Plumb, 1994). Because elastic and failure models predict similar stress profiles plus the belief that the linear elasticity is more appropriate, almost all hydraulic fracture stimulation designs use only linear elastic models. As we show below, this (overused) practice, using only linear elastic models, can be inappropriate and results in inaccurate and misunderstood results. We show this practice results in a compromised vertical stress profile when weak layers exist in the stimulated interval. Weak layers can be stressed beyond their elastic limit, causing additional, unaccounted for strain due to yielding. The increased strain results in an increase in the minimum horizontal stress, exceeding that predicted by linear elasticity. The consequence is an incorrect vertical stress profile and an inaccurate understanding of the fracture geometry from the stimulation. This paper details an improved workflow for formations that include zones or layers whose present state exceeds their linear elastic limit. For example, when weak layers are present, numerical modelling is a reasonable and possibly more accurate alternative to analytical modelling for predicting the vertical stress profile. With today’s computing power, accurate and appropriate numerical models can be built and run in minutes to hours rather than the days to weeks of past decades. Below we develop and illustrate such a workflow. Choosing and implementing the appropriate model, whether it is analytic or numerical, as shown by this work, gives a more accurate vertical stress profile. This will in turn improve completion and stimulation designs and the understanding of the stimulated fracture or fracture network geometry.

In-situ imaging of fracture development during maturation of an organic-rich shale: Effects of heating rate and confinement

The statistics and dynamics of fractures formed during the accelerated maturation of kerogen-rich shale was investigated by heating Green River Shale (R-8 unit, Piceance Basin, northwestern Colorado) core samples while 3D X-ray microtomographic images were acquired. Previous studies have shown that, when there was no confining stress, fractures formed while the kerogen contained in the shale matured, and the produced hydrocarbon was expelled through these fractures. In the present study, X-ray tomographic scans at multiple voxel sizes were conducted on similar samples during heating. In one experiment, the shale sample was tightly fitted in a non-porous ceramic tube to confine it while it was heated. Three unconfined samples were heated and then held at different final temperatures to investigate the effects of the gas production rate on fracturing. 3D image processing was used to survey fracture network development, and time-lapse 2D digital image correlation analysis was used to monitor the development of the displacement and strain fields. The results revealed that fracturing is strongly dependent on the heating rate and the final heating temperature. While most of the fractures were oriented more-or-less parallel to the bedding plane, some were strongly inclined relative to the bedding plane. The formation of inclined fractures is attributed primarily to the shape and orientation of a minority of the flake-like kerogen patches, and also to the effective stress field and fractured zones of weakness. A conceptual model is proposed to explain the dynamics of fluid expulsion and the associate fracturing behavior.

Interaction analysis of propagating opening mode fractures with veins using the Discrete Element Method

Cemented natural fractures (veins) in shale formations generate mechanical heterogeneities which contribute to complex hydraulic fracture network development. Lee et al. (JGR, 2015) experimentally investigated the interaction of propagating tensile fractures with calcite-filled veins in Marcellus shale using Semi-Circular Bend (SCB) tests. In this study, we investigate those experimental results with three-dimensional Discrete Element Method (DEM) modeling to gain further insight in the fracture interaction processes that generate hydraulic fracture complexity. In addition, we quantified the length of fracture diversion distance to show the effect of vein properties, including approach angle, thickness, strength, stiffness, penetration length, on fracture diversion results.The numerical results compared well with the laboratory fracture diversion results. There is a tendency towards fracture diversion as the approach angle becomes smaller and more oblique to the propagating fracture plane. As the stiffness of the vein increased, the microcrack-related damage increased and the bond breakages extended longer to either side of the main fracture path. Samples with thicker veins and increasing penetration length showed a weaker vein response; the induced fracture propagated for a longer distance in veins before kinking back into the rock matrix. In addition, the numerical model confirmed that the opening mode fracture initiated from the notch tip and the intergranular bond failure created microcracks within the vein before the approaching fracture intersected the vein. The failure mode of the bonds between discrete elements in the vein was predominantly tensile mode rather than shear.

Characterization of pore-fracture networks and their evolution at various measurement scales in coal samples using X-ray μCT and a fractal method

The characteristics of pore-fracture networks, as well as their evolution in coal during loading, are important for exploration of coalbed methane. In this study, the evolution of fracture networks in coal during loading was investigated using X-ray micro-Computed Tomography (X-ray μCT) with a uniaxial compression facility. The cross-sectional slices of the coal sample were obtained by μCT scanning at ten loading levels. The physical and geometrical parameters, such as porosity, fracture aperture, fracture density, and connectivity, were obtained by processing the 3D μCT data. Along with the self-similar characteristics of the 3D fracture networks, a simplified Sierpinski-like fractal model was proposed to further examine the evolution of the pore-fracture networks with higher measurement accuracy. The results showed that the developed fractal model had the ability to characterize the geometrical characteristics of the pore-fracture networks at various measurement scales. With the decreasing measurement scale r, the porosity and volumetric fractal dimensions of the pore-fracture networks increased, which were approximately linear and logarithmic curves with respect to log (1/r), respectively. At the measurement scale of 50 μm, the longitudinal cleavage fractures, which were parallel to or perpendicular to the joints, were generated during uniaxial compression. The fracture porosity, as well as the fracture density, increased with increasing axial stress. The fracture growth process could be divided into three stages: the initial stage, the stable growth stage, and the accelerated growth stage. At the measurement scale of 0.008 μm, the pore-fracture evolution in the coal sample under uniaxial compression occurred in four phases, namely, the pore compaction phase, the production phase of new fractures, the stable development phase of main fractures, and the accelerated production and development phase of the fractures. At the measurement scale of 0.5 μm, the coal permeability varied in a V-shaped curve with axial stress.

Study on the Fractal Characteristics of Fracture Network Evolution Induced by Mining

The evolution and distribution of fracture network induced by mining is essential to determine the mechanical properties and permeability of disturbed rock mass. In this paper, the similar material model tests are employed to simulate the stress variation, cyclic breaking, and fracture formation and distribution status of the overlying strata with different loading conditions, rock properties, and mining process. The fractal dimension of mining‐induced fracture network varied with mining advancing, and the evolvement laws of fracture network with mining advancing and different mining advancing footage are concerned and obtained. By establishing the relationship between the fractal dimension and the mining length in different horizontal and vertical zones, it demonstrates that the fractal dimensions in horizontal and vertical zones have a self‐similar characteristic, and the distribution of the fractal dimension of the mining‐induced fractures shows generally the “W”‐type trend.

Directional semivariogram analysis to identify and rank controls on the spatial variability of fracture networks

In this study, the directional semivariogram is deployed to investigate the spatial variability of map-scale fracture network attributes in the Paradox Basin, Utah. The relative variability ratio (R) is introduced as the ratio of integrated anisotropic semivariogram models, and R is shown to be an effective metric for quantifying the magnitude of spatial variability for any two azimuthal directions. R is applied to a GIS-based data set comprising roughly 1200 fractures, in an area which is bounded by a map-scale anticline and a km-scale normal fault. This analysis reveals that proximity to the fault strongly influences the magnitude of spatial variability for both fracture intensity and intersection density within 1–2 km. Additionally, there is significant anisotropy in the spatial variability, which is correlated with trends of the anticline and fault. The direction of minimum spatial correlation is normal to the fault at proximal distances, and gradually rotates and becomes subparallel to the fold axis over the same 1–2 km distance away from the fault. We interpret these changes to reflect varying scales of influence of the fault and the fold on fracture network development: the fault locally influences the magnitude and variability of fracture network attributes, whereas the fold sets the background level and structure of directional variability.

Characterization of shallow karst using electrical resistivity imaging in a limestone mining area

The Guangxi area is famous for its high degree of karstification due to a long period of groundwater erosion and the development of fracture networks. Karst collapse appeared during the mining process and caused many environmental problems. Applying electrical resistivity imaging (ERI) is the most cost-effective method to study the characteristic of the subsurface karst environment. In this area, more than 24 km of ERI profiles around a mining area is carried out to present the development of karst which will be used to evaluate the effects on the ground water and the surrounding environment. The area represents a classical limestone rock which is of high quality and is used for cement production. In this study, the ERI is used to determine the geometry of the karst range at depth. The results are in good agreement with drilling experiments. The ERI survey provided near-surface resistivity information, which is very useful for establishing the geometry and the position of potential karst.

The study on fracture network development in shale fracturing with considering capillary effect

Natural fractures widely exist in shale reservoirs with different scales, ranging from nano scale to micro scale, even to macro and engineering scale. These multi-scale natural fractures have a great influence on the fracture network propagation during a fracturing treatment. Meanwhile, the fracturing fluid flowing into these natural fractures will result in the co-existence of fracturing fluid and natural gas. As a result, the capillary force occurs at the gas-liquid interface in natural fractures. If the sizes of these natural fractures are different, the capillary force in the natural fractures is different as well. In particular, when the natural fracture is small, the capillary force will be very large and even affect the natural fracture initiation and propagation. As a consequence, the fracturing fluid flowing in natural fractures and the fracture network development will be significantly affected by these multiscale natural fractures. In this paper, a model about fracture network propagation was built which considered natural fractures sizes and fracturing fluid wettability. Besides, different methods were adopted to calculate the stress intensity of hydraulic fracture and natural fracture, respectively. In addition, the competitive growth theory of multi fractures is considered to describe the fracture network propagation. The studies found that with long length, small width natural fractures and strong wettability fracturing fluid, natural fractures were more likely to propagate, which resulted in a large fracture network. Besides, when the sizes of natural fractures were different, a more complex fracture network was tended to come out. An experimental study about fracture network propagation in shale was introduced from Suarez-Rivera’s work, in which a “fish bone” shape fracture propagation was observed. By comparing the numerical simulation with Suarez-Rivera’s work, it could be concluded that the simulation was in accordance with shale fracturing experiments results.

Synchrotron tomographic quantification of strain and fracture during simulated thermal maturation of an organic‐rich shale, UK Kimmeridge Clay

AbstractAnalyzing the development of fracture networks in shale is important to understand both hydrocarbon migration pathways within and from source rocks and the effectiveness of hydraulic stimulation upon shale reservoirs. Here we use time‐resolved synchrotron X‐ray tomography to quantify in four dimensions (3‐D plus time) the development of fractures during the accelerated maturation of an organic‐rich mudstone (the UK Kimmeridge Clay), with the aim of determining the nature and timing of crack initiation. Electron microscopy (EM, both scanning backscattered and energy dispersive) was used to correlatively characterize the microstructure of the sample preheating and postheating. The tomographic data were analyzed by using digital volume correlation (DVC) to measure the three‐dimensional displacements between subsequent time/heating steps allowing the strain fields surrounding each crack to be calculated, enabling crack opening modes to be determined. Quantification of the strain eigenvectors just before crack propagation suggests that the main mode driving crack initiation is the opening displacement perpendicular to the bedding, mode I. Further, detailed investigation of the DVC measured strain evolution revealed the complex interaction of the laminar clay matrix and the maximum principal strain on incipient crack nucleation. Full field DVC also allowed accurate calculation of the coefficients of thermal expansion (8 × 10−5/°C perpendicular and 6.2 × 10−5/°C parallel to the bedding plane). These results demonstrate how correlative imaging (using synchrotron tomography, DVC, and EM) can be used to elucidate the influence of shale microstructure on its anisotropic mechanical behavior.

The effect of magmatic intrusions on coalbed methane reservoir characteristics: A case study from the Hoskissons coalbed, Gunnedah Basin, Australia

Magmatic intrusions can deteriorate coalbed methane reservoir quality by precipitating minerals in natural fracture and cleat system. To date, the effect of magmatic intrusions on coal rank and maturity has been studied extensively. However, their impact on fluid flow capacity and gas content is poorly investigated. This study evaluates the impact of magmatic intrusions on reservoir characteristics of the Hoskissons coal interval in the Gunnedah Basin (eastern Australia) where numerous coal-intrusion associations exist. Drill stem test (DST), borehole image logs and core data are used to determine fluid flow characteristics, gas content and quality in 14 wells across the Gunnedah Basin. The integration of borehole image logs and DST data analysis enables us to determine the existence, openness, and hydraulic conductivity of natural fracture and cleat systems in the Hoskissons coal interval. In addition, available desorption canister data, gas composition data, and conventional well logs are interpreted to investigate probable thermal effect on gas content.Our analyses of different datasets reveal that the thickness of intrusions and their positions with respect to the Hoskissons coal interval are variable in the studied wells. Permeability varies from 1091mD down to zero owing to heterogeneous fracture and cleat systems. Interpreted natural fracture/cleat systems are well correlated with measured permeability from DST data analysis. This highlights the role of open natural fractures/cleats in fluid flow characteristics of the Hoskissons coal interval. Results indicate that the mineralizing effect of hydrothermal fluids derived either from magmatic intrusions or coal itself is not a controlling factor in fluid flow capacity of the Hoskissons coal interval in the studied wells. This is described by either the distance between coal section and major intrusions in some wells or perhaps emplacement of intrusive bodies prior to development of cleat and natural fracture networks. The destructive impact of intrusions on permeability is observed in one of the studied wells in which in-situ stress perturbation is large (due to presence of magmatic intrusions in sedimentary rocks). Variable in-situ stress orientation can decrease fracture connectivity and consequently fluid flow properties are affected. Gas content largely varies in heat affected coal intervals. This signature is the result of thermal effect fading with distance and is more pronounced when intrusions are in close proximity to coal intervals. Gas composition is variable in the studied wells. Gas composition data indicate that high quality desorbed gas with methane concentration higher than 90% could be found even in coal intervals which are heavily intruded.

Investigation of Fracturing Network Propagation in Random Naturally Fractured and Laminated Block Experiments

Researchers have recently realized thatsilty laminas are very developed in naturally fractured continentalsedimentary formations in the Ordos Basin(China). Studies have shown that silty laminas are significant to improve the physical properties and gas storage capacity, and the natural fractures interact with the hydraulic fractures to maximize the fracture network during hydraulic fracturing. However, the influence of silty laminas withrandom fractures on the created hydraulic fracture networkis not well understood. Laboratory experiments are proposed to investigate the evolution of fracture networks in naturally fractured formations with model blocks that contain laminas and random fractures. The influence of dominating factors was studied and analyzed, with an emphasis on stress ratio, injection rate, and laminae strength. Macroscopic failure morphology descriptions combined with meso 3-D laser scanning techniques are both used to reveal the evolution of fracture networks. It is suggested that high injection rate, medium laminae strength, and low stress ratio tend to increase the stimulated reservoir volume (SRV). The interactions between the silty laminae and random natural fractures affect the effect of hydraulic fracturing effectiveness. This work strongly links the production technology and fracability evaluation in the continental shale formation. It can aid in the understanding and optimization of hydraulic fracturing simulations in silty laminae shale reservoirs.