Analysis of rock structure stability in coal mines

In this paper, the theory of limit point instability is used to analyse the stability of rock structures in coal mines. A general method of analysing stability of rock structures is put forward and a uniform instability condition of rock structures is set up. Some instability phenomena, such as rock bursts in circular roadways, pillars and long walls, and the outburst of coal and gas from circular roadways, are discussed analytically. At a later stage, the critical point of rock structure instability is determined. The influence of relative parameters (such as the mechanical properties of rock, coal, and the geometric sizes) on the stability of the rock structures is carefully analysed. Copyright © 2005 John Wiley & Sons, Ltd.     

Localization of mining-induced horizontal fractures along rock layer interfaces in overburden: field measurements and prediction

The locations of mining-induced horizontal fractures along rock interfaces in the overburden of Donetsk Coal Basin were identified using an original experimental device. The device traps methane from horizontal fracture zone (100–fold coal seam thickness) over an active longwall mining excavation. Presence or absence of horizontal fractures along rock layer interfaces is correlated with physical characteristics of the overburden, such as thickness, uniaxial compressive strength of overburden rock layers, location of rock layer interfaces and thickness of extracted coal seams. As a result, a combined criterion based on these physical characteristics is proposed to predict the presence of overburden horizontal fracturing in coal mine operations.     

Material selection for the gas containment system of a compressed natural gas carrier fleet

In this work, the possible exploitation of fiber-reinforced composites in the context of maritime transportation of compressed natural gas (CNG) is investigated. In addition to a more conventional steel configuration, two different fiber materials, carbon and glass, are considered as construction materials for pressure vessels (PVs) to be stored on board ships, with thickness optimized by FEM analysis.The considered scenario is represented by the transportation of CNG from an offshore well to a terminal on shore. Fleets of ships carrying CNG in pressure vessels manufactured with the investigated materials are generated by means of a ship synthesis model (SSM) software and compared on the basis of technical and economical indicators.The choice of the construction material influences considerably the weight of the PVs, which represent a major item of total ship weight and reflects directly on the general transport performances in terms of resistance, seakeeping and reliability in the service. On the other hand, capital as well as operating expenditures are considerably affected by the choice. When exploring the design space, the ship synthesis model is able, at a preliminary stage of the design, to account for the various technical and economical aspects, their implications and relationships. Results are presented of computations carried out in a specific case, identified by the annual gas production and other characteristics of the well terminal and a cruising route for the ships. The comparison is carried out on the basis of the cost per transported unit of gas and of the percentage of success in the transportation process. The computations show that the choice of the PV material has a key influence on the results in terms of optimal number, dimensions and speed of the ships.     

Large‐scale poroelastic fractured reservoirs modeling using the fast multipole displacement discontinuity method

An effective approach to modeling the geomechanical behavior of the network and its permeability variation is to use a poroelastic displacement discontinuity method (DDM). However, the approach becomes rather computationally intensive for an extensive system of cracks, particularly when considering coupled diffusion/deformation processes. This is because of additional unknowns and the need for time‐marching schemes for the numerical integration. The Fast Multipole Method (FMM) is a technique that can accelerate the solution of large fracture problems with linear complexity with the number of unknowns both in memory and CPU time. Previous works combining DDM and FMM for large‐scale problems have accounted only for elastic rocks, neglecting the fluid leak‐off from the fractures into the matrix and its influence on pore pressure and stress field. In this work we develop an efficient geomechanical model for large‐scale natural fracture networks in poroelastic reservoirs with fracture flow in response to injection and production operations. Accuracy and computational performance of the proposed method with those of conventional poroelastic DDM are compared through several case studies involving up to several tens of thousands of boundary elements. The results show the effectiveness of the FMM approach to successfully evaluate field‐scale problems for the design of exploitation strategies in unconventional geothermal and petroleum reservoirs. An example considering faults reveals the impact of reservoir compartmentalization because of sealing faults for both geomechanical and flow variables under elastic and poroelastic rocks. Copyright © 2015 John Wiley & Sons, Ltd.     

An original semi‐discrete approach to assess gas conductivity of concrete structures

For civil engineering structures with a tightness role, structural permeability is a key issue. In this context, this paper presents a new proposition of a numerical modelling of leakage rate through a cracked concrete structure undergoing mode I cracking. The mechanical state of the material, considered in the framework of continuum mechanics based on finite element modelling, is described by means of the stress‐based nonlocal damage model which takes into account the stress state and provides realistic local mechanical fields. A semi‐discrete method based on the strong discontinuity approach to estimate crack opening is then considered in the post‐treatment phase. Using a Poiseuille's like relation, the coupling between the mechanical state of the material and its dry gas conductivity is performed. For validation purposes, an original experimental campaign is conducted on a dry concrete disc loaded in a splitting setup. During the loading, gas conductivity and digital image correlation analysis are performed. The comparison with the 3D experimental mechanical global response highlights the performance of the mechanical model. The comparison between crack openings measured by digital image correlation and estimated by the strong discontinuity method shows a good agreement. Finally, the results of the semi‐discrete approach coupled with the gas conductivity compared with experimental data show a good estimation of the structural conductivity. Consequently, if the mechanical problem is well modelled at the global scale, then the proposed approach provides good estimation of gas conductivity. Copyright © 2016 John Wiley & Sons, Ltd.     

DEM simulations of methane hydrate exploitation by thermal recovery and depressurization methods

Methane hydrate (MH, also called fiery ice) exists in forms of pore filling, cementing and load-bearing skeleton in the methane hydrate bearing sediment (MHBS) and affects its mechanical behavior greatly. To study the changes of macro-scale and micro-scale mechanical behaviors of MHBS during exploitation by thermal recovery and depressurization methods, a novel 2D thermo-hydro-mechanical bonded contact model was proposed and implemented into a platform of distinct element method (DEM), PFC2D. MHBS samples were first biaxially compressed to different deviator stress levels to model different in-situ stress conditions. With the deviator stress maintained at constant, the temperature was then raised to simulate the thermal recovery process or the pore water pressure (i.e. confining pressure for MH bond) was decreased to simulate the depressurization process. DEM simulation results showed that: during exploitation, the axial strain increased with the increase of temperature (in the thermal recovery method) or decrease of pore water pressure (in the depressurization method); sample collapsed during MH dissociation if the deviator stress applied was larger than the compression strength of a pure host sand sample; sample experienced volume contraction but its void ratio was slightly larger than the pure host sand sample at the same axial strain throughout the test. By comparison with the laboratory test results, the new model was validated to be capable of reproducing the exploitation process by thermal recovery and depressurization methods. In addition, some micro-scale parameters, such as contact distribution, bond distribution, and averaged pure rotation rate, were also analyzed to investigate their relationships with the macroscopic responses.     

塔里木盆地西部阿克莫木气藏地层压力特征

阿克莫木气田目前已有多口井完钻,各井在钻揭白垩系砂岩储层前,对地层压力纵向上的变化规律认识不清,在什么层位及深度下7″套管意见仍不统一。本文根据目前研究现状和生产面临问题,对白垩系各组地层分布规律、压力特征进行了详细研究,认为白垩系克孜勒苏群、库克拜组分布稳定,压力窗口相近,白垩系东巴组与上覆古近系阿尔塔什组压力窗口相近。建议今后该区钻探7″套管应下至库克拜组顶部-东巴组底部,减少地层漏失和油气勘探风险。     

无人直升机搭载大气颗粒物采样装置的前期地面评估

与搭载气溶胶观测和分析仪器测量相比,利用无人直升机搭载气囊采集高空大气颗粒物是一种经济、便捷和安全的采集方式。为了解无人直升机采样装置的采样效果,选择2014年10月至2015年3月的4次重污染天气过程,由无人直升机搭载气囊采集大气颗粒物,然后利用气溶胶采样器采集气囊内的大气颗粒物样品并计算采样时间,再利用气溶胶采样器采集相同时间段内的地面大气颗粒物(气囊外),并将两者进行对比。结果表明:在稳定的污染天气条件下,采集气囊内的大气颗粒物,与相同时间段内利用气溶胶采样器直接采集气囊外的大气颗粒物相比,两种方式采集到的大气颗粒物质量最大差值29%,最小差值11%,平均为21%;利用无人直升机的大气采样装置采集大气颗粒物对碳气溶胶组分影响很小,气囊内与气囊外OC/TC最大相差2.0%,最小相差0.5%。上述结果表明,无人直升机的大气采样装置有比较好的采集效率,利用气囊这种经济、便捷、安全的方式,搭载在无人机平台上,采集高空大气是一种获取气溶胶垂直廓线的有效方法,有望在未来的气溶胶气候和环境研究中提供宝贵数据。     

准噶尔盆地油气源、油气分布与油气系统

准噶尔盆地是中国西部典型的多旋回叠合盆地,发育有石炭系、二叠系、三叠系、侏罗系、白垩系和古近系6套烃源岩,同时存在6大类原油和3大类天然气,广泛分布于盆地不同地区。西北缘原油总体相似,碳同位素组成轻(δ~(13)C-29‰),胡萝卜烷、类异戊二烯烷烃、三环萜烷、伽马蜡烷丰富,甾烷以C_(28)、C_(29)为主,基本没有重排甾烷,为第二类原油,来源于二叠系湖相烃源岩。腹部绝大多数原油与西北缘原油相似,但胡萝卜烷、类异戊二烯烷烃、伽马蜡烷等有差异,来源于不同凹陷的二叠系湖相烃源岩;少量原油碳同位素组成重(δ~(13)C-28‰~-26‰),Pr/Ph大于2.5,三环萜烷以C_(19)、C_(20)为主,藿烷丰富而伽马蜡烷极低,以C_(29)规则甾烷及重排甾烷为主,为第四类原油,来源于侏罗系煤系烃源岩。东部存在5种类型原油,第一类原油碳同位素组成特别重(δ~(13)C-26‰),来源于石炭系烃源岩;第二类原油与腹部地区绝大多数原油十分相似,来源于二叠系湖相烃源岩;第三类原油碳同位索组成轻,重排甾烷、Ts、C_(29)Ts及C_(30)重排藿烷异常丰富,来源于中上三叠统湖相烃源岩;第四类原油源于侏罗系煤系烃源岩;混合类原油为二叠系、三叠系、侏罗系原油的混合,各自贡献平均分别为20%、15%和65%。南缘存在4类典型原油,为第二、第四、第五和第六类原油,其中第二、第四类分别源于二叠系和侏罗系;第五类原油碳同位素组成轻(δ~(13)C-29‰)、Pr/Ph1.0、伽马蜡烷丰富且有两个异构体、Ts、C_(29)Ts、C_(30)重排藿烷、C_(27)~C_(29)异胆甾烷及C_(30)甲基甾烷丰富,来源于白垩系湖相烃源岩;第六类原油主要为中低成熟原油,碳同位素组成δ~(13)C~28‰~-26‰,C_(27)、C_(28)、C_(29)甾烷呈"V"型分布,甲藻甾烷异常丰富,来源于古近系湖相烃源岩。准噶尔盆地天然气有油型气、混合气和煤型气,前两类主要来源于二叠系湖相烃源岩和石炭系海相烃源岩,煤型气主要来源于石炭系和侏罗系煤系烃源岩。不同类型油气分布与不同时代烃源灶具有良好对应关系:石炭系油气主要分布于陆东-五彩湾;二叠系油气主要分布于西北缘、腹部与东部;三叠系原油仅分布于东部;侏罗系原油主要分布于东部与南部;白垩系原油仅分布于南缘中部;古近系原油仅分布于南缘西部。按照盆地构造特征及不同时代烃源灶与油气关系,将准噶尔盆地划分为西部、中部、东部、南部及乌伦古5个油气系统及15个子油气系统。     

鄂尔多斯盆地北部直罗组有机质特征及与铀成矿关系

本文研究了鄂尔多斯盆地典型铀矿床的有机质与铀成矿的关系。探讨了煤屑的赋存状态、煤质特征及成熟度,认为煤屑的演化程度均较低,处于低成熟阶段,有利于对铀的吸附。总体上看煤质有机质和铀矿化关系密切,铀含量与煤屑含量呈正相关关系,对铀矿物为吸附态形式。通过对该区油气包裹体的研究认为,该区存在多期次的成岩方解石胶结过程,并伴随多期次的油气包裹体发育,尤以晚期的胶结方解石矿物中存在丰富的晚期油气包裹体。研究认为晚期可能发生过油、气的运移和聚集,铀矿床的形成与赋矿砂岩中有机流体或还原性气体的充注和络合作用密切相关,因而这些丰富的有机流体和还原性气体对于铀的富集沉淀具有重要意义。