Numerical simulation study of shale gas reservoir with stress-dependent fracture conductivity using multiscale discrete fracture network model

The exploitation of shale gas has increasingly become the focus of worldwide energy industry. Due to the existence of natural/hydraulic fractures, most of the shale gas reservoirs exhibit high degree of heterogeneity and complexity which leads to the stress-dependent fracture conductivity of shale gas reservoir. Discrete fracture network (DFN) model is adopted in this research since the conventional continuum model cannot meet the numerical simulation requirements of fractured shale gas reservoir. A series of experiments about the fracture properties stress-dependent have been conducted on some shale core samples, the stress-dependent fracture conductivity correlation is selected and incorporated into the mathematical model to characterize the reduction of fracture conductivity potential with the reservoir pressure drop. The DFN model is applied to a shale gas reservoir with fracture network to study the effect of the stress-dependent fracture conductivity on the shale gas well performance. The results show that the effect of fracture conductivity reduction with pressure drop on the shale gas well performance depends on both the initial fracture conductivity and matrix permeability. The complex interactions between the fracture and matrix permeability should be considered when select the appropriate size of proppants for fracturing.     

页岩储层网络压裂技术研究与试验

页岩储层压裂技术是页岩油气高效勘探开发的关键技术和核心技术。与常规低渗油气储层压裂单一长缝改善压裂效果不同,低孔极低渗的页岩压裂主要目标是形成具有有效导流能力的网络裂缝,确保压裂改造体积足够大,且经济有效。提出了页岩网络压裂有效改造体积（ESRV）的概念。在借鉴北美页岩气压裂的经验和前期国内页岩气压裂实践的基础上,针对我国页岩储层的具体特点,在压前进行评价方法、射孔参数优化、诱导复合测试压裂、网络压裂对策和排采技术等方面进行了探索性的研究,初步形成了页岩网络压裂技术,现场试验效果明显,解决了裂缝性脆性页岩压裂易砂堵、成功率低的难题。     

Interactions between the Design and Operation of Shale Gas Networks,Including CO2 Sequestration

As the demand for energy continues to increase, shale gas, as an unconventional source of methane (CH₄), shows great potential for commercialization. However, due to the ultra-low permeability of shale gas reservoirs, special procedures such as horizontal drilling, hydraulic fracturing, periodic well shut-in, and carbon dioxide (CO₂) injection may be required in order to boost gas production, maximize economic benefits, and ensure safe and environmentally sound operation. Although intensive research is devoted to this emerging technology, many researchers have studied shale gas design and operational decisions only in isolation. In fact, these decisions are highly interactive and should be considered simultaneously. Therefore, the research question addressed in this study includes interactions between design and operational decisions. In this paper, we first establish a full-physics model for a shale gas reservoir. Next, we conduct a sensitivity analysis of important design and operational decisions such as well length, well arrangement, number of fractures, fracture distance, CO₂ injection rate, and shut-in scheduling in order to gain in-depth insights into the complex behavior of shale gas networks. The results suggest that the case with the highest shale gas production may not necessarily be the most profitable design; and that drilling, fracturing, and CO₂ injection have great impacts on the economic viability of this technology. In particular, due to the high costs, enhanced gas recovery (EGR) using CO₂ does not appear to be commercially competitive, unless tax abatements or subsidies are available for CO₂ sequestration. It was also found that the interactions between design and operational decisions are significant and that these decisions should be optimized simultaneously.     

Mixed integer simulation optimization for optimal hydraulic fracturing and production of shale gas fields

Optimal development of shale gas fields involves designing a most productive fracturing network for hydraulic stimulation processes and operating wells appropriately throughout the production time. A hydraulic fracturing network design—determining well placement, number of fracturing stages, and fracture lengths—is defined by specifying a set of integer ordered blocks to drill wells and create fractures in a discrete shale gas reservoir model. The well control variables such as bottom hole pressures or production rates for well operations are real valued. Shale gas development problems, therefore, can be mathematically formulated with mixed-integer optimization models. A shale gas reservoir simulator is used to evaluate the production performance for a hydraulic fracturing and well control plan. To find the optimal fracturing design and well operation is challenging because the problem is a mixed integer optimization problem and entails computationally expensive reservoir simulation. A dynamic simplex interpolation-based alternate subspace (DSIAS) search method is applied for mixed integer optimization problems associated with shale gas development projects. The optimization performance is demonstrated with the example case of the development of the Barnett Shale field. The optimization results of DSIAS are compared with those of a pattern search algorithm.     

Engineering Philosophy-based Reflections on Efficient Development of Shale Gas in China

The Shale Gas Revolution began to have a significant impact on global supply and demand of natural gas; also the price trend of natural gas has been greatly affected. This phenomenon raised concerns among natural gas producers and consumers. China has abundant shale gas resources, but the development scale is small while facing problems including weak exploration and evaluation basis, overlapping royalties, complex terrestrial environment, lack of technical practices and accumulation, inadequate management and regulatory mechanisms, etc. To overcome these challenges for large-scale shale gas development, we believe that shale gas development is not only a single technical problem, but a systematic engineering demanding multidisciplinary research that will be bound to spread to humans, nature, and society. Therefore, to ensure the healthy development of China’s shale gas, it is required to coordinate global and local relationships, engineering and community relations, as well as to break up the conflicts between engineering and nature, engineering and economy, along with engineering and society.     

美国Eagle Ford页岩气开发对我国页岩气勘探开发的启示

据估计,我国的页岩气可采储量高达20万亿～36万亿m3,虽然我国页岩气储量评估处于初级阶段,但仍然可以肯定我国是页岩气资源丰富的国家之一,如果都能开发,按当前的天然气年产量计算,可以开发200多年。目前国内外多家企业聚焦中国,逐鹿页岩气,页岩气有望成为我国常规能源替代品,页岩气的勘探开发必将对我国乃至世界能源格局产生重大影响。但是由于页岩气储层致密,渗透率低至纳达西级,若不采取特殊的增产措施难以实现商业开发。美国是目前唯一实现页岩气商业开采的国家,技术和商业模式都相当成熟,中海石油斥资10多亿美元收购了美国页岩气主产区Eagle Ford 33.3%的股份,该地区页岩气地质储量高达6 000亿m3,开发前景广阔。为提升我国页岩气总体水平,笔者率领技术专家深入考察了美国Eagle Ford页岩气项目,增进了对水平钻井与分段压裂等关键技术的了解,同时分析了相应扶持政策,对中国页岩气的勘探开发具有重要的参考价值。     

Strengthening in and fracture behaviour of CNT and carbon-fibre-reinforced epoxy–matrix hybrid composite

Advanced materials such as continuous fibre-reinforced polymer matrix composites offer significant enhancements in strength and fracture resistance properties as compared with their bulk, monolithic counterparts. In the present work, mode-I (tensile) fracture behaviour of the neat epoxy (without nano- or hybrid reinforcements), nanocomposite (with amino-functionalized multi-walled carbon nanotube (MWCNT) reinforcement to neat epoxy) and hybrid composite (with amino MWCNT and carbon fibre reinforcements to neat epoxy) along with their flexural strength and interlaminar shear strength has been reported and discussed. Limited topological studies have also been conducted to understand the nature of material fracture and its dependence on the notch orientation. The results thus obtained are analysed and discussed in detail to elucidate: (i) alignment of fibre and its influence on the anisotropy in strength and fracture resistance, (ii) dependence of notch root radii on the apparent fracture toughness and concurrence to strain-controlled fracture and (iii) finally, the nature of J–R curves. The results thus obtained have revealed that the resistance to fracture is significantly increased with the addition of amino-functionalized MWCNTs and carbon fibres. In the hybrid composite, fracture resistance is greater in the longitudinal orientation of fibres than in the transverse orientation and it exhibits a significantly higher strength–fracture toughness combination.     

Unconventional shale gas development: challenges for environmental policy and EA practice

Abstract

The growth of unconventional shale gas development has been accompanied by controversy over its environmental and social impacts. This paper reviews recent literature to clarify what is known and not known about the physical, chemical and toxicological properties of the process chemicals and wastewaters generated in hydraulic fracturing, the mechanisms and pathways by which they enter surface water and groundwater aquifers and the risks posed to human and ecosystem health. Assessing the impacts of unconventional shale gas development is clearly constrained by a lack of baseline information, complex hydrogeological histories for natural migration of hydrocarbons, lack of tracers to monitor and verify the source, timing and mechanism of contaminant migration into water resources. This is compounded by lack of transparency and accountability in policy decisions. The paper argues that managing the social and environmental risks of unconventional shale gas development calls for a new generation of impact assessment, one that marries the ideals of strategic environmental assessment, cumulative effects assessment, backcasting and deliberative and inclusive processes of community engagement towards collective risk management.     

中国页岩气资源潜力及其在天然气 未来发展中的地位

中国页岩气勘探已在四川、鄂尔多斯等盆地及重庆、云南、贵州、湖南等地区获得发现,证实具有良好资源前景。依据最新资料,落实了稳定区面积、集中段厚度、可采资源丰度、含气量等资源评价关键参数,采用3种方法预测了我国页岩气可采资源量及分布。借鉴美国典型页岩气区发展经验,采用多种方法预测了我国页岩气未来达到的产量规模和发展路线图,明确提出了页岩气在我国未来天然气发展中的地位和作用。     

Influence of heat treatment and particle shape on mechanical properties of infiltrated Al2O3 particle reinforced Al-2 wt-%Cu

Abstract

Al-2 wt-%Cu composites were produced by gas pressure infiltration of powder beds with a high volume fraction (45 to 60 vol.-%) of angular or polygonal alumina particles. The tensile behaviour and fracture toughness of the composites were characterised in as cast, solutionised and peak aged (T6) conditions. It was shown that coarse intermetallics that are formed during solidification and located preferentially at the particle/matrix interface lead to lower toughness compared with the same composites in solutionised and T6 conditions. The particle nature and shape exert a strong influence on the properties of the composites: polygonal particles are intrinsically stronger than angular particles and yield stronger, tougher, and more ductile composites. Composite toughness variations are explained in terms of fracture micromechanisms.     

Modelling,simulation, and graphical user interface for industrial gas carburising process

Abstract

A mathematical model has been developed for the gas carburising (diffusion) process using finite volume method. The computer simulation has been carried out for an industrial gas carburising process. The model's predictions are in good agreement with industrial experimental data and with data collected from the literature. A study of various mass transfer and diffusion coefficients has been carried out in order to suggest which correlations should be used for the gas carburising process. The model has been interfaced in a Windows environment using a graphical user interface. In this way, the model is extremely user friendly. The sensitivity analysis of various parameters such as initial carbon concentration in the specimen, carbon potential of the atmosphere, temperature of the process, etc. has been carried out using the model.     

Mechanisms governing cyclic fracture in an Al–Cu–Li alloy

Abstract

The fracture behaviour of a peak-aged, partially recrystallized Al–4·5Cu–1·21Li–0·51Mn–0·20Cd alloy has been investigated as a function of strain amplitude, stress intensity, and environment. It was found that the failure was predominantly intergranular separation, regardless of the environment, stress intensity, or strain amplitude, and that the fracture behaviour was influenced mostly by intrinsic microstructural features, rather than the nature of the environment. The shearable nature of matrix strengthening precipitates, large recrystallized grains, and precipitate-free zones along the high-angle grain boundaries aid in localizing the deformation, resulting in low-energy intergranular fracture. The iron- and silicon-rich intermetallic precipitates in the alloy promote void nucleation following fracture of the particle. A model is proposed which suggests the need for high stresses and strains for the initiation and spontaneous growth and coalescence of microvoids. The mechanisms of fracture behaviour of the alloy are discussed in terms of several concurrent processes involving strength of the material, intrinsic microstructural effects, deformation behaviour, state of stress, and strain.

MST/497     

Pore pressure cohesive zone modeling of hydraulic fracture in quasi-brittle rocks

Hydraulic fracturing technology has been widely applied in the petroleum industry for both waste injection and unconventional gas production wells. The prevailing analytical solutions for hydraulic fracture mainly depend on linear elastic fracture mechanics. These methods can give reasonable prediction for hard rock, but are ineffective in predicting hydraulic fractures in quasi-brittle materials, such as ductile shale and sandstone. One of the reasons is that the fracture process zone ahead of the crack tip and the softening effect should not be neglected for quasi-brittle materials. In the current work, a set of chevron-notch three point bending tests were performed on sandstone samples from an oil field in Ordos Basin, Shaanxi province, China, and the results were compared with the cohesive zone method based on finite element analysis. The numerical results fit the experimental data well and it shows that the cohesive zone model and the Traction-Separation law used in the model are effective in modeling fracture nucleation and propagation in sandstone without considering the porous effect. A 3D pore pressure cohesive zone model was developed to predict nucleation and propagation of a penny-shaped fluid-driven fracture. The predictions were compared with the analytical asymptotic solutions and a field minifrac test from the literature; it shows that the proposed method can not only predict the length and aperture of hydraulic fracture well, but also predict the bottomhole pressure with reasonable accuracy. Based on analytical asymptotic and computational solutions, parametric studies were conducted to investigate the effects of different parameters on the fracture aperture and fracture length, fracture process zone and bottomhole pressure.     

Release and permeation studies of propranolol hydrochloride from hydrophilic polymeric matrices

Abstract

In-vitro release of propranolol hydrochloride, from various hydrophilic polymeric bases was studied. These included: methocel®, avicel® CL-611/ methylcellulose, polyvinyl alcohol/gelatin based systems. Several additives, such as, ethyl alcohol, dimethylsulfoxide (DMSO) and polyethylene glycol-400 were included in the formulations for possible enhancement of the drug release. The release studies were carried out using the cellulose membrane and the hairless mouse skin as the diffusion barriers. The general rank order for the drug release through these membranes was observed to be: the methocel® matrix > the avicel® CL-611 matrix > the polyvinyl alcohol/gelatin matrix > and the emulsion base. The additives in the formulations had little or no effect in enhancing the drug release. However, when the hairless mouse skin was soaked in (DMSO) for one hour prior to its use in the diffusion studies, the drug release was found to increase by 40% from the methocel® matrix formulation.

The drug release data were treated with various kinetic principles to assess the relevant parameters, such as the diffusion, partition and permeability coefficients. Using these information, the formulations were screened for their suitability to deliver propranolol hydrochloride via the diadermatic dosage form.     

Microstructure-related fracture behaviour of injection moulded short fibre reinforced polyarylamide in dry and wet states

The fracture behaviour of injection moulded polyarylamide (PAR) composites containing 30, 50 and 60 wt% glass and 30 wt% carbon fibres has been investigated in both dry and wet states. Kinetics of moisture absorption study revealed that PAR and its composites exhibit Fickian behaviour. The incorporation of short fibres into a PAR matrix has resulted in the reduction of both maximum moisture content (M _m ) and diffusion coefficient (D). The fracture mechanical characterization of the various materials was evaluated by using notched compact tension (CT) specimens. Testing was performed as a function of temperature (T = –40, 20 and 80°C) and crosshead speeds (v = 1 and 1000 mm min^–1) on as received (AR) specimens. The influence of water uptake due to the hygrothermal ageing (HA) process on residual fracture performance was also studied. The combined action of moisture-induced plasticization of the PAR matrix and interfacial degradation has been concluded to play a significant role in controlling the fracture behaviour of the (HA) composites. The residual fracture properties of both neat PAR and its composites are almost fully recovered in the case of redrying (RD). Failure mechanisms of both the matrix and the composites, assessed by fractographic studies in a scanning electron microscope (SEM) are discussed.     

As received Ti–6AI–4V/σ-SiC fibre composite

Abstract

A Ti–6Al–4V/σ (SM 1240) composite prepared by diffusion bonding has been studied in the as received condition, using Auger electron spectroscopy, transmission electron energy loss spectroscopy, and scanning electron microscopy. The SiC based σ fibre has a tungsten core, and a duplex coating of carbon (adjacent to the SiC deposit) and TiB_x. It is shown that boron from the TiB_x layer diffused into the matrix and formed TiB needles. Carbon was detected in the TiB_x layer and was present in elemental free form. A continuous SiO₂+ carbon layer was detected at the SiC/carbon layer interface. Analysis of in situ fracture composite surfaces in an Auger spectrometer has shown that the tensile failure was initiated within the carbon layer or at the TiB_x/matrix interface. An oxide layer detected at the TiB_x/matrix interface influenced the fracture behaviour of the composite.

MST/2027     

Molecular level investigation into selective permeability of silicon based membranes with potential use in gas separation processes

Abstract

The effect of the modification of the molecular structure on the permeability coefficients of typical rubbery and glassy silane and siloxane polymers at different temperatures was experimentally investigated. It was shown that carbon dioxide had higher permeability coefficients than those of nitrogen and oxygen due to the higher affinity of the various polymers toward the gas molecules. In order to provide a detailed understanding into the effect of the molecular structure on the gas diffusion behaviour in polymers, molecular modelling of carbon dioxide diffusion in silicon based membranes was used. The polymer molecules were shown to have lower self-diffusion coefficients than the gas ones related to the small size of the gas molecules as compared to the large size of the polymeric segments, thus allowing the gas molecules to jump from one unoccupied site to another through a series of connected pores or channels within the polymeric matrix. Increasing the temperature was shown to have a proportional effect on the mean square displacement, possibly due to the increase in the kinetic energy available to the systems. At high temperatures, the glassy siloxane molecules had similar values for the mean square displacement to those of the gas molecules since the polymer in this case is in close proximity to its glass transition temperature. The presence of the alternating oxygen atoms in the main backbone of the polymeric chains led to higher values for the selfdiffusion coefficients for the siloxane polymers as compared to those of the silane polymers as a result of the change in the bond angle about the oxygen atom (~ 144°) as compared to the tetrahedral angle (~ 110°) about the silicon atoms.     

体积约束的非局部扩散问题基于新的技巧的有限元方法

体积约束的非局部扩散问题在复合材料的断裂、多晶体的断裂、纳米纤维网络、裂缝的不稳定、图像处理等领域有重要应用,现存的数值方法精度不高。因此,设计一种高阶的有限元方法来求解二维体积约束的非局部扩散问题是十分必要的,但需克服维数增加带来的自由度骤增的困难。为此,采用了一种新技巧计算线性元的刚度矩阵,该数值方法的刚度矩阵是从一个新的矩阵$B$中提取的,该矩阵易于计算,并给出了单元的编码原理和数值计算节点的编码表达式,并通过数值算例验证了该方法对二维体积约束的非局部扩散问题具有几乎最优收敛阶。值得一提的是,求解二维体积约束的非局部扩散问题并不是平凡的。     

页岩气主要富集因素与核心区选择及评价

影响页岩气规模开发的因素很多,最核心的因素是首先要通过建立地质评价标准,选出页岩气核心区。核心区的确定关系到在页岩气勘探初期,是否能找准页岩气最富集的目标,选择最有利地区进行勘探,突破出气关,进而实现大规模经济开发。从页岩气成藏特征、页岩气富集因素分析入手,重点探讨我国富有机质页岩的丰度、成熟度、脆性矿物含量、含气性以及盖层等关键参数及其作用。借鉴北美页岩气勘探开发和研究成果,结合我国页岩气勘探开发和研究新进展,初步建立我国页岩气核心区地质评价标准,并优选出我国南方海相页岩气的核心区,指出四川盆地南部是我国最现实的页岩气开发区。     

Creep ductility considerations for high energy components manufactured from creep strength enhanced steels

Abstract

It is well established that the tendency for low ductility ‘creep brittle’ fracture behaviour in tempered martensitic steels is linked to the formation and growth of micro voids or ‘cavities’. Details of the contributions of all factors affecting damage development are still under investigation. However, it is known that for tempered martensitic steels voids often initiate over most of the creep life. Nucleation has been recorded on both prior austenite grain boundaries and at other micro structural features such as lath boundaries. The number of voids formed, and the fracture behaviour observed, depend on the type of creep strength enhanced ferritic (CSEF) steel and specific details of fabrication and heat treatment. In Grade 91 steel, void nucleation is sensitive to metallurgical factors such as composition and steel making practices. Key indicators of susceptibility to creep cavitation also include the levels of trace elements present and the presence of hard non-metallic inclusions. In Grade 92 steel, creep void formation has been linked to boron nitrides and other inclusions. These inclusions are present when there has been insufficient control of composition and heat treatment. Metallurgical factors linked to whether a particle will nucleate a void include the nature of the inclusion/matrix interface, the shape and size and the location of the inclusions within the microstructure. This paper describes the results of critical uniaxial and multiaxial testing for CSEF steels and compares data from nominally the same steels which have different metallurgical susceptibilities to void formation.