Blocks Of Shale Research Articles

Opening-dependent phase field model of hydraulic fracture evolution in porous medium under seepage-stress coupling

In this paper, a seepage-stress coupling numerical analysis method is developed to simulate complex hydraulic fracturing in porous medium, in which the characteristics of hydraulic fracture evolution in porous medium under seepage-stress coupling are revealed. The classical Biot's poroelasticity theory is applied to couple the stress and fluid fields, and the crack propagation path of hydraulic fracturing is described by an opening-dependent phase field model. Potential energy caused by fluid pressure is also coupled into the governing equation of phase field evolution. The system of fracture opening affecting fluid flow is established. Staggered iterative scheme is applied to solve the above three-field coupled problem based on Newton–Raphson iterative algorithm. Three classical cases are presented to validate the proposed method: the first one is an analytical solution of pressure distribution in a fixed opening crack and the second one is a benchmark case of Sneddon analytical solution for a pressurized pre-crack and the third one is the KGD problem for hydraulic fracture propagation. Moreover, numerical simulations of hydraulic fracturing in shale block, multi-layer and natural fractures are also investigated. The numerical results agree well with the results in published literatures. It is indicated that hydraulic fracture evolution under seepage-stress coupling can be forecasted by the proposed method. The proposed method not only provides a powerful analytical tool for the oil or gas production, but also provides assessment tools for the stability of geotechnical engineering in water rich environments.

Experimental study on brittle-to-ductile transition mechanism of lower Silurian organic-rich shale in south China

Brittle-ductile transition conditions and mechanisms are key scientific issues embedded in the reservoir deformation-related difficulties faced during shale gas exploration and exploitation. In this study, shale deformation experiments under different temperatures, confining pressure and differential stress were conducted using a NaCl solid confining pressure medium, taking the Lower Silurian marine shale from South China as an example. We aimed to clarify the characteristic textures and structures of brittle and ductile deformation in organic-rich shale and then, to reveal the conditions and mechanisms of the brittle-to-ductile transition under the deep in-situ environment of high-temperature, high-confining pressure and high-stiffness surrounding strata. Deformation characteristics of experimentally deformed shale showed significant differences between brittle and ductile deformed shale in terms of fracture development, failure combining form, deformation distribution homogeneity, shear angle and the structures and textures within intensely deformed zones. The micro-deformation structures of brittle deformed shale are characterized by the development of fractures and the simple mechanical crushing of shale blocks and mineral particles. The strong deformation zones of ductile deformed shale mainly show micro-deformation features, such as solid-state rheological deformation and accompanying fragmentation, rounding and directional arrangement of mineral particles. Comparison of control group experiments supported the conclusion that increasing confining pressure significantly improves the elastic modulus and compressive strength of shale and leads to a transition from large-area uniform deformation to narrow-band concentrated deformation. A confining pressure of 75–100 MPa is required for the brittle-to-ductile deformation transition of the target shale at 200 °C. An elevated temperature enhances shale deformation in the yield and creep stages. Finally, we revealed the progressive process and mechanism of brittle-to-ductile transition with the increase of confining pressure; that is, increasing confining pressure leads to a gradual transition of the shale micro-deformation mechanism, from brittle failures to ductile rheological deformation, by suppressing tension fracturing and enhancing friction.

Integrity assessment methodology of casing ovality deformation in shale gas wells

Casing deformation frequently occurs in shale gas wells during hydraulic fracturing. While the failure mechanism of casing ovality deformation is unclear due to the interweaving and compounding reasons from geological and engineering complexity. The multi-arm caliper tool can quantify the severity of casing deformation, and the spatial and temporal occurrence of casing deformations is analyzed. A finite element model of fractured shale squeezing casing is established to simulate the process of casing deformation. The simulated casing deformation is compared with logging data to validate the model with which the deformation mechanism would be revealed. Simulation results indicate that casing deformation in the Luzhou shale gas field belongs to compressive deformation. The length of casing deformation and the diameter shrinkage are local and small, and the deformation points are scattered along the horizontal well trajectory. The fracturing fluid pressure and fractured shale block generate a nonuniform load on the casing, which leads to stress yield and ovality deformation. The fluid pressure, formation degradation and cementing material are the key controlling factors to mitigate casing ovality deformation. This work provides a prompt & accurate assessment method and controlling measures for casing integrity in shale gas wells.

Multiple experimental studies of pore structure and mineral grain sizes of the Woodford shale in southern Oklahoma, USA

Pore structure study is an important part of unconventional shale reservoir characterization, since the pore system provides the primary petroleum storage space and fluid flow pathways. Previous studies have suggested that the pore structure is related to the total organic carbon (TOC) content, mineral compositions, and the maturity of the organic matter (OM). However, few studies have focused on the mineral grains, the primary grains being deposited but before cementation, which are the building blocks of shale. Eight Woodford Shale outcrop samples from southern Oklahoma were chosen to study the effects of mineral grain size on the pore structure characterization, using multiple and complementary experimental approaches, including laser diffraction, mineralogy, TOC, pyrolysis, liquid immersion porosimetry, mercury intrusion porosimetry, gas physisorption, (ultra) small angle X-ray scattering, scanning electron microscopy, and spontaneous imbibition. The results from different experiments of eight samples show that the Woodford Shale has the mean mineral grain diameters at 3–6 μm, a wide range of porosity at 3–40% and pore diameters at 50–1,000 nm, and various pore connectivity. Grain size variation was probably caused by the sea-level fluctuation during its deposition, which affect the porosity, pore size distribution, and pore connectivity. With decreasing mineral grain sizes, the porosity tends to increase while the pore connectivity worsens. The results also indicate that OM and carbonates in this low-maturity Woodford Shale could block the pores and decrease the porosity. Coupling with the grain size analyses, the control of depositional environment on grain sizes and subsequent effects on pore structure is identified. The pore structure characteristics over a wide pore-diameter range provided by multiple experiments could improve the understanding of storage space and fluid flow in the Woodford Shale to further increase its petroleum production.

Damage and fracture characteristics of rocks with different structures under high-velocity water jet impact

This paper aims to investigate the damage and fracture characteristics of rocks with different structures under water jet impact. Three types rocks including heterogeneous coal, homogeneous sandstone, and transversely isotropic shale, were selected as the impact target rock. Water-jet impingement experiments with jet velocities varying from 447 m/s to 774 m/s were conducted. The rock macro-breakage characteristics were obtained by describing the breakage patterns, measuring the erosion depth and area, and counting the cracks number. A 3D reconstruction method based on the computed tomography and digital image processing technology was proposed to visualize the internal breakage characteristics and quantitatively analyze the damage field distribution. Combined with fracture morphology from scanning electron microscope, the failure mechanisms of rocks with three structures under water jet impact were revealed. The results indicate that coal, sandstone and shale will experience different breakage patterns with the increasing jet velocity: layered transverse cracks → “十”-type crack network → splitting crack, only one spindle-shape erosion pit, surface debris spalling → layered transverse cracks → “T”-type crack network. The effects of jet velocity and rock structure on the evaluation indexes (such as erosion depth and damage area, the number and angle of transverse cracks) were investigated thoroughly. In addition, the relationships between total damage degree based on breakage volume and jet velocity, and between damage variable based on breakage area and erosion depth, were compared and discussed. The failure mechanisms of three rocks impacted by water jets are as follows: (i) the reflection and interference of stress wave combined with the pressured water wedge effect cause the layered breakage and longitudinal splitting breakage of heterogeneous coal; (ii) the grinding effect of back flow mainly accounts for the formation of spindle-shape erosion pit in homogeneous sandstone; (iii) the shock stress wave effect and bedding structure lead to the spalling of large shale block and layered breakage, and the reflected wave tensile accompanied with pressured water wedge effect cause the tensile splitting breakage.

A physical model study of shale seismic responses and anisotropic inversion

The seismic responses of the shale properties are critical for shale gas reservoir evaluation and production. It has been widely reported that the clay minerals have substantial influences on the seismic wave anisotropy and brittleness. Hence, knowing the seismic responses of the clay-rich shales and estimation of shale elastic properties are significant for the shale gas industry. A physical model containing two groups of shale blocks as the target formations is constructed in laboratory. The group S contains six shale blocks with different clay contents, and the group N contains six shale blocks with different porosity. The acquired 2D seismic data is used to analyze the seismic responses of two corresponding seismic lines. An anisotropic three-term inversion method is applied to one of the seismic inline to estimate the elastic properties the target shale blocks. The inversed attributes can be used to reveal the effects of shale clay contents. This study shows the substantial significance of using a physical model to observe the seismic responses of shale properties. The inversion results indicate that the anisotropic three-term inversion method could provide accurate results of elastic properties as well as the P- wave anisotropy parameter for shale formations.

Shear testing and analysis of the response of confined masonry walls with centered openings made with innovative sintered insulation shale blocks

The innovative sintered insulation shale (SIS) blocks could use as thermal insulation masonry units to construct the green architecture applying the adaptive construction measures. Above all, the seismic behavior of the wall with an opening should clarify in order to apply in earthquake region. Besides the comparative solid wall, the behavior of typical window-type and door-type walls analyzed under the cyclic lateral loads incorporating with the constant vertical load to achieve the purpose. The main variable of the specimens was the size of the opening. Hence, all the walls were with the same innovative construction measure and geometry parameter. The failure mechanism, energy dissipation, damping ratio, lateral stiffness, deformation and strength capacity conducted according to the record data. Based on the results, the opening caused the weaker masonry parts showing serious damage in wall piers and masonry under the opening, which mainly induced for the lack of restrained condition around the opening. In other words, it changed the failure mechanism. Furthermore, the contribution of RC level bonds and tie-columns to the specimen increased as the opening showing the greater strain than yield strain after the maximum strength, which need to take into account their participation in the lateral resistance calculation. The impact of opening still presented in the decreased lateral stiffness and energy dissipation, smaller ductility and shear strength. The reduced portion would result in the opening wall acting as the weaker part in the SIS masonry structure. In addition, the theoretical equations acquired through Strut-and-Tie method proposed considering the failure mechanism and opening impact. The reduction factors for decreased stiffness and angle of masonry strut, relative stiffness between the RC components and masonry parts applied to conduct the calculation of the maximum strength. Compared with the experimental results, the proposed equations showed enough accurate prediction on the maximum shear resistance.

Investigation on Nonuniform Extension of Hydraulic Fracture in Shale Gas Formation

Hydraulic fracturing with multiple clusters has been a significant way to improve fracture complexity and achieve high utilization of shale formation. This technology has been widely applied in the main shale area of North America. In Changning shale block of China, it, as a promising treatment technology, is being used in horizontal well now. Due to the anisotropy of mechanical property and the stress shadowing effect between multiclusters, fractures would extend nonuniformly and even some clusters are invalid, leading to a poor treatment performance. In this work, based on the geology and engineering characteristics of Changning shale block, different cluster number, cluster spacing, perforation distribution, and flow rate were discussed by the numerical simulation method to clarify multifracture propagation. It is implied that with the reduction of cluster number and the growth of cluster spacing and flow rate, the length and average width of interior fractures are inclined to increase due to the mitigation of stress shadowing effect, contributing to the lower standard deviation (SD) of fracture length, but too small cluster number or too large cluster spacing is not recommended. Besides, the perforation distribution with more perforations in interior fractures can get larger length and average width of interior fractures compared with another two perforation distributions because of more fractional flow rates obtained, which results in more even fracture propagations. In Changning shale block, multicluster hydraulic fracturing with 4-6 clusters in a stage has been employed in 300-400 m well spacing, and diversion technology, limited-entry perforation (36-48 perforations per stage), high flow rate (16 m3/min), and small-sized ceramic proppant (100 mesh) are used to get better shale gas production. To promote the even propagation of fractures further, nonuniform perforation distribution should be introduced in the target shale area.

3D modelling of soil-rock mixtures considering the morphology and fracture characteristics of breakable blocks

Failure mechanisms of a soil-rock mixture (S-RM) can be efficiently investigated by the discrete element method (DEM). This paper proposes a stochastic approach for 3D DEM modelling of S-RM samples accounting for morphological features and internal fractures of blocks and their potential breakage. The research refers to the case-history of an artificial S-RM filling slope, mainly containing highly-weathered shale blocks, constructed at the ±500 kV electronic converter station in the Funing County, Yunnan Province, China. The 3D morphological features of the blocks and the characteristics of their internal fractures were obtained by CT technology and image processing. A stochastic method based on harmonic series was developed starting from the real blocks, allowing the generation of random block clusters with characteristics similar to the original ones. Another stochastic approach was implemented for the creation of the internal fractures, simulated as voids in the block clusters, following the characteristic of the real fractures. Finally, the procedure was applied for the definition of a 3D S-RM DEM model with 40% block proportion, whose meso-parameters were determined for simulating direct shear tests. These latters were also useful to explore the mechanical response of the sample at the meso-scale, including the formation and development of the localization band. The numerical results showed that the generated random S-RM DEM model well reproduced the experimental behaviour of S-RM samples with breakable blocks. Also, they highlighted the importance of modelling the block breakage and internal fractures; in fact, companion simulations with unbreakable blocks and breakable blocks without fractures were all characterised by increased shear strength with higher friction angle but reduced cohesion.

Seismic behavior of unreinforced and confined masonry walls using innovative sintered insulation shale blocks under cyclic in-plane loading

An innovative masonry unit, sintered insulation shale block, is produced with excellent thermal insulation performance and high compressive strength. In order to provide comprehensive understanding of the seismic behavior of sintered insulation shale masonry wall and promote the application in the severe cold region, seven prototype single leaf wall specimens test under the cyclic in-plane loading corresponding with the constant vertical load, including the unreinforced masonry and confined masonry walls. Previous research has indicated the tie-columns could enhance the seismic behavior of common masonry walls, and we further set out thin reinforced concrete bonds to connect the end tie-columns and hinder the brittle destruction of masonry. The main investigated parameters are axial load ratio, comprehensive effect of construction measures and aspect ratio. Thereafter, the failure mechanism, hysteretic behavior, deformation and strength capacity comparatively conducted between the tested specimens. The axial load ratio significantly impacts the failure modes. Moreover, the higher axial load would result in severe brittle destruction presenting crushed webs and shells for shear effect, which also causes the relatively fast decrease of strength after the maximum strength. The tie-columns corresponding to reinforced concrete bonds play a crucial role to improve the seismic behavior, in particular the maximum strength and ultimate deformation. Nevertheless, the effective constraint would not reverse the brittle destruction failure induced by the SIS masonry. Finally, the formulas propose to predict the critical stress and drift in the skeleton hysteresis curves. For example, the reinforcement yield strength and ductility use as the main parameters to determine the maximum stress and ultimate drift, respectively. Moreover, the formula results are in agreement with the experimental results.

Discovery of Cosmic Spherules from the Mesoproterozoic Strata and its Significance—in Case of the Ming Tombs Area, Beijing

AbstractThis study covers cosmic spherules derived from the Mesoproterozoic Dahongyu Formation in the Ming Tombs area, Beijing. The cosmic spherules include iron oxide cosmic spherules, carbonaceous chondrites, and atomic iron “steely bead”‐shaped cosmic spherules. The mineral assemblage of silicon carbide, forsterite, zircon, and glass spherules and fragments were picked from melt‐silicified carbonate of the Mesoproterozoic Dahongyu Formation (ca. 1625 Ma). Cosmic spherule assemblages are solely discovered from sedimentary rocks in China. Platinum group elements (PGE) were determined for the first time in cosmic spherules and associated minerals. PGE comparative observation between meteorite and cosmic spherules is presented in this study. It is recognized that an extraterrestrial meteorite impact event might have occurred in the Dahongyu Stage. The main evidence is a large number of iron cosmic spherules in silicified oncolitic limestone, and associated cosmic silicon carbide, glass spherules, and fragments, as well as the presence of forsterite. The impact‐volcanic crater is characteristic of a big black shale block dropped into the bended silicified limestone.

Effect of Clay and Organic Matter Content on the Shear Slip Properties of Shale

AbstractShale gas reservoirs contain a large number of natural fractures. When shear slip occurs during hydraulic fracturing, it will increase reservoir permeability and improve the reservoir stimulation performance. Because shale contains a large amount of clay and organic matter, its shear slip behavior differs from that of other rocks. In this study, the shear behavior of different shale blocks is experimentally studied, and the proportion of clay and organic matter in the shale dominates the shear slip behavior. Specifically, the friction coefficient and shear strength decrease with the increasing content of clay and organic matter. A microscopic support model was developed to describe the different shear slip behaviors for different contents of clay and organic matter. When the clay and organic matter content is lower than a critical threshold (~35%), shale is microscopically grain supported and brittle shear occurs and the material is prone to shear dilation. When this content is greater than a critical content (~43%), the microscopic structure is a matrix‐supported fabric and plastic shear occurs, which is prone to shear contraction. Shale with contents between these two critical thresholds has a mixed‐support fabric, which is a transition type between brittle and plastic shear. If the proportion of clay and organic matter is less than ~35%, the natural fracture tends to dilate and retain a certain degree of opening after shearing, improving stimulation performance. When the proportion is more than ~43%, shear contraction occurs on the natural fracture surface, which is not conductive and cannot increase reservoir permeability.

Coiled tubing friction reduction of plug milling in long horizontal well with vibratory tool

Abstract With the rapid increase in the exploitation of shale reservoirs in the Sichuan province of China, the demand for long horizontal wells for contacting reservoirs has also significantly expanded. With increasing horizontal sections, the coiled tubing (CT) penetration depth and efficiency of the surface weight transferred to the weight-on-bit (WOB) downhole are limited by the friction between the CT and wellbore. Plug milling with CT has become less efficient, and the final bridge plugs close to the bottom cannot be removed. Therefore, an experiment is conducted to study the effects of the pump rate on the vibration amplitude of the axial and radial directions, vibration frequency, and pressure difference. An improved tubing force analysis model is used to study the effect of a vibrator with different pump rates and friction coefficients on the CT surface weight, maximum set-down force, maximum pickup force, CT lockup depth, and maximum WOB of different plugs in well wx202 of the Wei Yuan shale block. The calculated data exhibit strong agreement with the real-time data collected from previous CT operations. The simulated results are used to solve the problems faced when milling the final four plugs at the end of a long horizontal section in well wx202. The results indicate that the amplitude variation in the radial vibration at both ends of the hydraulic oscillator is greater than that in the axial direction under the same pump rate. The optimized flow rate for the vibrator is 8.33 L/s. The use of a vibrator can greatly reduce the friction and transfer that are sufficient for a WOB downhole.

Organic Geochemical Characteristics of Source Rocks of Hongshuizhuang Formation in the Mengjiawopu Section of Lingyuan-Ningcheng Basin in North China

Through the regional geological survey, outcrop section measurement and other field geological work, a block of black shale is found at the outcrop of Mengjiawopu Section of Hongshuizhuang Formation in Jixian County, and organic geochemical source rock analysis and evaluation methods are used to study the source rocks in Hongshuizhuang Formation. The set of source rocks is mainly gray-black silty mudstone and gray-black shale, with a cumulative thickness of 78 m; the total organic matter (TOC) is generally high, with an average content of 1.35%, of which good and rich source rock samples account for 86%. A combined analysis of the stable carbon isotope of kerogen, the correlation between Pr/nC17 and Ph/nC18, and the ratio of saturated hydrocarbon to aromatic hydrocarbon identifies that the organic matter is mainly Type Ⅱ kerogen; the equivalent vitrinite reflectance Ro of organic matter is from 1.68% to 1.94%. Rc, the correlation of the methylphenanthrene index (MPI) against the vitrinite reflectance is from 1.63% to 1.81%, with an average of 1.71%, and the Odd/Even Predominance (OEP) value is between 0.87 and 1.11, both indicating that the source rocks of the Hongshuizhuang Formation are at the stage of high to over maturation. In summary, the set of source rocks in Hongshuizhuang Formation is characterized by high organic matter content, good kerogen types, and high to over maturation stage.

Monitoring hydraulically-induced fractures in the laboratory using acoustic emissions and the fluorescent method

We investigated the relation between seismic events and fractures induced by hydraulic fracturing in laboratory experiments under uniaxial loading conditions using granite and shale blocks by monitoring acoustic emissions (AEs). We used a thermosetting acrylic resin mixed with a fluorescent compound as the fracturing fluid and fixed it within the blocks by heating immediately after fracturing. This allowed observation of fluid penetration regions and fracturing patterns on cross-sectional planes under ultraviolet light irradiation. The obtained AE hypocenters and resin penetration regions observed after the fracturing extended on both wings along the loading axis from the fracturing hole for all the samples. This was as expected theoretically, although the AEs were concentrated primarily on only one side for some samples. Patterns of wellbore pressurization histories, AE activities, and resin penetration regions were significantly different between the two rock types. In the experiments using granite samples, wellbore pressure increased linearly until 87.2–97.4% of the peak pressure, followed by a gradual decrease of the rate and sudden drop of pressure (breakdown). AEs started to occur at 49.6–93.2% of the peak pressure, significantly before the breakdown. Resin penetration regions have a width of 10–30 mm and such wide penetration regions resulted in indistinguishable main fractures. For the shale samples, both the nonlinearity of the wellbore pressure–time curve and the AE activity were initiated immediately before breakdown. The number of detected AEs was much smaller than for the granites. Resin in the rock samples showed thin traces with widths of < 1 mm, corresponding to the main fracture. Such dissimilarities between the two rock types likely resulted from differences in their permeability, grain size, and/or mineralogy. We also found aseismic regions that lacked AEs despite evidence of fluid penetration. Although such regions were affected by the fracturing operations, they cannot be revealed by the microseismic observations used in shale gas/oil fields and enhanced geothermal systems.

Estimation of uniaxial compressive strength of shale using indentation testing

A number of methods have been proposed to indirectly assess the uniaxial compressive strength (UCS) of intact rock in the drilling of oil wells and underground drilling. Indentation testing is a method in which an indentor ‎of a specific diameter penetrates a particle of rock and the force-displacement curve is plotted to determine the critical transition force (CTF). In the present study, 10 shale block samples were collected from a cretaceous shale formation in Iran from which standard cores were prepared and subjected to UCS testing. Cubic particles 4, 5 and 7mm3 in size were cut and entrenched in disks containing resin and a total of 300 indentation tests were conducted on them. Empirical relations for the relation between UCS and CTF were developed for each size. The highest correlation coefficient was recorded for the 7mm3 particles and the lowest for the 4mm3 particles. A simple method is proposed to determine the empirical relationship independent of particle dimensions between UCS and CTF that has a correlation coefficient of 0.78. Verification of the proposed equations show that they predicted UCS with 85% accuracy. A comparison of the proposed relationships and those from previous studies indicates that the empirical relationship between these two variables is influenced by variation in the uniaxial compressive strength and lithology of the different samples.

Evolution of organic matter and nanometer-scale pores in an artificially matured shale undergoing two distinct types of pyrolysis: A study of the Yanchang Shale with Type II kerogen

Two distinct pyrolysis experiments were performed to investigate variations in the content and composition of organic matter (OM) and their relation with the growth of nanometer-scale pores during the thermal maturation of shale. One was conducted in glass tubes under vacuum (closed system), and the other was performed in a semi-closed system in which generated oil can flow out of the system under an 80MPa pressure compressing the sample. The maturation of OM, especially the secondary cracking of residual bitumen, occurs much slower in a semi-closed than in a closed system. However, greater carbon loss (up to 6%) was shown by pyrolyzed samples in the semi-closed system relative to those in the closed system (< 3%), which is related to differences in hydrocarbon expulsion and consolidation of retained bitumen. During the main oil generation stage, declining pore volume in both systems is mainly attributed to bitumen infilling that masked some primary and secondary pores. A significant increase in total pore volume by 2.5cm3/100g was observed in pyrolyzed samples from the semi-closed system after the oil peak. In contrast, a much smaller pore growth of 1.1cm3/100g was detected in samples within the closed system. A relatively high pore pressure in a block of compressed shale, resulting from a large amount of retained bitumen, can generate, in addition to a large carbon loss, substantial pore growth in pyrolyzed samples within the semi-closed system. Current results suggest a close, but not straightforward, relationship between the transformations of OM and pore development.

Properties of Mortar with Red Gypsum as Cement Replacement Material by Using Industrial Approach Method

Utilization of Red Gypsum, a common industrial waste, in the manufacturing of cement brick can mitigate the deterioration of environment as it can preserveland piling of RG waste. X-Ray fluorescence analyzer was utilized to determine the chemical composition of Red Gypsum (RG) while sieving method was used to determine its particle size distribution. From the analyses, it can be concluded that RG properties suit the purpose of replacing cement for production of low strength mortar. A range of physical and mechanical tests were conducted on various percentage of cement replacement in RG cement bricks. This paper focused on verifying formulation and water ratio to meet the compressive strength tests needed as required for low strength mortar and the effects on the quality. From the results of this study, it can be concluded that RG cement bricks complied with the Malaysian Standard 76:1972 (Specifications for bricks and blocks of brick earth, clay or shale) for non-load bearing walls. The results also indicated that a maximum range of 60-70% replacement can be done without compromising the minimum quality of the produced bricks.

Study on the Thermal Properties of Hollow Shale Blocks as Self-Insulating Wall Materials

To reduce energy consumption and protect the environment, a type of hollow shale block with 29 rows of holes was designed and produced. This paper investigated the thermal properties of hollow shale blocks and walls. First, the guarding heat-box method was used to obtain the heat transfer coefficient of the hollow shale block walls. The experimental heat transfer coefficient is 0.726 W/m2·K, which would save energy compared to traditional wall materials. Then, the theoretical value of the heat transfer coefficient was calculated to be 0.546 W/m2·K. Furthermore, the one-dimensional steady heat conduction process for the block and walls was simulated using the finite element analysis software ANSYS. The predicted heat transfer coefficient for the walls was 0.671 W/m2·K, which was in good agreement with the test results. With the outstanding self-insulation properties, this type of hollow shale block could be used as a wall material without any additional insulation measures in masonry structures.

Investigation of casing deformation during hydraulic fracturing in high geo-stress shale gas play

A total of 101 wells in the Changning-Weiyuan shale gas play (Sichuan Basin of China) had been drilled and stimulated by the end of March 2016, including 11 vertical wells and 90 horizontal wells. Casing deformation is one of the more difficult problems that occur during the reservoir hydraulic stimulating process. Especially for horizontal wells, the casing deformation rate was up to 30% (30 wells of 90). Based on the characteristics of the cementing process in shale reservoir horizontal wells and combined with the APB (Annular Pressure Buildup) effect in conventional wells, both physical models and finite element models were established. Many factors, such as the void angles, temperature change, magnitude of the inner pressure and in-situ stress were investigated. The results show that (1) in the process of high-flow-rate hydraulic fracturing, the retention fluid in the cement voids contracted due to sudden temperature decrease inside the casing. Consequently, the pressure dropped inside the cement sheath voids. In particular, tight shale reservoirs have an extremely low permeability and cannot supplement the pore pressure within a short time. Finally, the casing was placed in a non-uniform external supporting situation. On the other hand, the results show that (2) a high pumping pressure was needed in a high geo-stress shale block to fracture the formation, inducing a higher internal pressure inside the casing. Lastly, (3) under the combined effect of high internal pressure and non-uniform external support, the downhole casing has a high risk of deformation. The cement voids pressure decline (CVPD) effect might be the main reason behind the casing deformation in this high geo-stress shale gas play. Rotating the casing string during the cementing process to avoid the cement voids forming or using warm fracturing fluids to minimize the pressure drop inside the voids are possible innovative strategies to solve these difficult problems.