Effect of sorption intensities on dispersivity and macro-dispersion coefficient in a single fracture with matrix diffusion

Matrix diffusion and sorption are among the key processes impacting the efficiency of natural attenuation in the subsurface. While these processes have been studied extensively in fractured media, limited information exists on the sorption nonlinearity. To address this shortfall, a numerical model has been developed that couples matrix diffusion and nonlinear sorption at the scale of a single fracture using the dual-porosity concept. The study is limited to a constant continuous-solute-source boundary condition. The influence of sorption intensities on dispersivity and macro-dispersion coefficient is investigated using a method of spatial moments. Results suggest that mixing of solutes is significantly lowered by nonlinear sorptive behavior, with respect to the mixing caused by matrix diffusion for linearly sorbing solutes. Also, the magnitude of time dependent dispersivity during the pre-asymptotic regime is lower for nonlinearly sorbing solutes with respect to the linearly sorbing solutes. Reduced mixing is also observed for nonlinearly sorbing solutes under combined mechanisms of matrix diffusion and decay.

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Résumé Les phénomènes de diffusion et de sorption comptent parmi les processus clés qui influent sur l’atténuation naturelle en subsurface. Si ces processus ont été massivement étudiés en milieu fracturé, il existe en revanche peu d’informations sur la non-linéarité de la sorption. Afin de combler cette lacune, un modèle numérique a été développé ; il couple la diffusion dans la matrice et la sorption non-linéaire à l’échelle d’une fracture unitaire en utilisant le concept de double porosité. La seule condition aux limites fixée pour toute l’étude est une source de soluté constante et continue. L’influence de l’intensité de la sorption sur la dispersivité et sur le coefficient de macro-dispersion est étudié via une méthode utilisant les moments spatiaux. Les résultats suggèrent que les mélanges de solutés sont significativement réduits par les phénomènes de sorption non-linéaires, comparativement aux mélanges causés par la diffusion pour des solutés à sorption linéaire. C’est pourquoi la dispersivité, dépendante du temps, est plus faible pour les solutés à sorption non-linéaire que pour les solutés à sorption linéaire, lors du régime pré-asymptotique. Des mélanges réduits ont également été observés pour des solutés à sorption non-linéaire, sous les mécanismes combinés de diffusion dans la matrice et de dégradation.

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Resumen La difusión intersticial y la adsorción se encuentran entre los procesos clave que impactan la eficiencia de la atenuación natural en el subsuelo. Mientras que estos procesos se han estudiado extensamente en medios fracturados existe información limitada acerca de la no linealidad de la adsorción. Para abordar esta situación se ha desarrollado un modelo numérico que acopla la difusión intersticial y la adsorción no lineal a la escala de una sola fractura usando el concepto de porosidad doble. El estudio se restringe a condiciones limitantes de fuente de soluto constante y continuo. Se investiga la influencia de las intensidades de adsorción sobre el coeficiente de dispersión-macro y dispersividad usando un método de momentos espaciales. Los resultados sugieren que la mezcla de solutos disminuye significativamente mediante comportamiento de adsorción no lineal, en relación con la mezcla causada por difusión intersticial para solutos con adsorción lineal. Se observa que la magnitud de la dispersividad dependiente del tiempo durante el régimen pre-asintótico es más baja para solutos con adsorción no lineales en relación a los solutos con adsorción lineal. También se observa mezcla reducida para solutos con adsorción no lineal bajo mecanismos combinados de difusión intersticial y desintegración.

     

油气储层裂缝定量描述及其地质意义

在裂缝型油气藏勘探开发过程中,裂缝发育区带、裂缝密度以及连通性是影响生产的关键参数。文章在详细综述储层裂缝定量描述的必要性、地质意义、分维描述、人工智能模糊描述及微观描述等定量描述法的基础上,介绍了裂缝系统各参数之间的关系及裂缝预测方法,进而介绍了定量描述法在裂缝预测中应用的几个实例。     

龙滩水电站蠕变体B区折断面及折断错滑面成因分 析

通过对广西龙滩水电站左岸边坡蠕变体B区折断面和折断错滑面地质特征,变形特征,滑带充填物特征以及力学参数研究,结合监测,岩体风化以及构造特征、表层岩土体滑移-牵引等特征分析,认为折断面和折断错滑面的形成主要是由于边坡岩性以及河水侵蚀等作用下,岩体的风化差异所造成的,而且在目前有效的工程处理下,由监测资料可知,边坡变形波动较大,变形量值小,没有明显的滑移趋势,稳定性较可靠。     

利用方位角道集处理方法预测煤层裂隙

煤层瓦斯含量受多种因素影响,而煤层中的节理裂隙密集带则容易导致瓦斯聚积。在煤田地质勘探中,依据振幅随炮点和检波点入射角的变化可以预测裂隙的发育方向和发育密度。在对淮南地区张集西三煤矿的三维地震数据资料处理时,按不同的方位角增量抽取方位角道集,并对其方位角道集进行速度分析、NMO校正、叠加和偏移,得到6个方位偏移数据体。根据Malick等人的理论模型对其进行计算,获得d、A、B值,并据此预测裂缝发育方向与发育密度。将其预测成果与常规地震解释成果对比,发现两者之间非常吻合,可见利用方位角道集处理方法预测煤层裂隙具有可行性。     

复杂岩性油藏精细描述研究进展

复杂岩性油藏作为目前油田开发中十分重要的油藏类型之一,其精细描述一直受到研究者的关注和重视。为了给有效开发和调整部署提供依据,系统梳理了目前复杂岩性油藏精细描述中存在的主要问题、主要研究内容和技术发展方向,并总结复杂岩性油藏研究进展。从国内复杂岩性油藏精细描述研究现状入手,总结了该项研究存在的6项主要问题,主要包括岩性岩相识别与分类、储层地质成因分析难度大、地层精细划分与对比具有特殊性、裂缝表征难度很大、测井精细二次解释精度低、地质建模井间储层预测准确率低等。基于文献调研和综述,结合科研实践,认为复杂岩性油藏精细描述核心内容包括岩性岩相识别与分类、储层地质成因机制分析、储集空间识别和描述、储层物性精细测井解释和储层地质建模等5个方面。在此基础上,指出了该项研究的发展趋势,主要包括地层精细划分与对比、微观孔隙结构表征、储层裂缝表征、储层综合定量评价和流体识别等。     

The role of bedding in the evolution of meso- and microstructural fabrics in fault zones

To investigate the role of bedding in the evolution of meso- and microstructural fabrics in fault zones, detailed microscopic, mineralogical, and geochemical analyses were conducted on bedding-oblique and bedding-parallel faults that cut a folded Neogene siliceous mudstone that contains opal-CT, smectite, and illite. An analysis of asymmetric structures in the fault gouges indicates that the secondary fractures associated with each fault exhibit contrasting characteristics: those of the bedding-oblique fault are R₁ shears, whereas those of the bedding-parallel fault are reactivated S foliation. The bedding-oblique fault shows the pervasive development of S foliation, lacks opal-CT, and has low SiO₂/TiO₂ ratios only in gouge, whereas the bedding-parallel fault exhibits these characteristics in both gouge and wall rocks. The development of S foliation and the lack of silica can result from local ductile deformation involving the sliding of phyllosilicates, coupled with pressure solution of opal-CT. Although such deformation can occur in gouge, the above results indicate that it may occur preferentially along bedding planes, preceding the formation of a gouge/slip surface. Thus, in sedimentary rocks that contain phyllosilicates and soluble minerals, bedding can influence the rheological evolution of meso- and microstructural fabrics in fault zones.     

Simulation of mixed-mode fracture using SPH particles with an embedded fracture process zone

This paper focuses on the modelling of mixed-mode fracture using the conventional smoothed particle hydrodynamics (SPH) method and a mixed-mode cohesive fracture law embedded in the particles. The combination of conventional SPH and a mixed-mode cohesive model allows capturing fracture and separation under various loading conditions efficiently. The key advantage of this framework is its capability to represent complex fracture geometries by a set of cracked SPH particles, each of which can possess its own mixed-mode cohesive fracture with arbitrary orientations. Therefore, this can naturally capture complex fracture patterns without any predefined fracture topologies. Because a characteristic length scale related to the size of the fracture process zone is incorporated in the constitutive formulation, the proposed approach is independent from the spatial discretisation of the computational domain (or mesh independent). Furthermore, the anisotropic fracture responses of materials can be naturally captured thanks to the orientation of the fracture process zone embedded at the particle level. The performance of the proposed approach demonstrates its potentials in modelling mixed-mode fracture of rocks and similar quasi-brittle materials.     

Thermo-hydro-mechanical modeling of fracturing porous media with two-phase fluid flow using X-FEM technique

In this paper, a fully coupled thermo-hydro-mechanical model is presented for two-phase fluid flow and heat transfer in fractured/fracturing porous media using the extended finite element method. In the fractured porous medium, the traction, heat, and mass transfer between the fracture space and the surrounding media are coupled. The wetting and nonwetting fluid phases are water and gas, which are assumed to be immiscible, and no phase-change is considered. The system of coupled equations consists of the linear momentum balance of solid phase, wetting and nonwetting fluid continuities, and thermal energy conservation. The main variables used to solve the system of equations are solid phase displacement, wetting fluid pressure, capillary pressure, and temperature. The fracture is assumed to impose the strong discontinuity in the displacement field and weak discontinuities in the fluid pressure, capillary pressure, and temperature fields. The mode I fracture propagation is employed using a cohesive fracture model. Finally, several numerical examples are solved to illustrate the capability of the proposed computational algorithm. It is shown that the effect of thermal expansion on the effective stress can influence the rate of fracture propagation and the injection pressure in hydraulic fracturing process. Moreover, the effect of thermal loading is investigated properly on fracture opening and fluids flow in unsaturated porous media, and the convective heat transfer within the fracture is captured successfully. It is shown how the proposed computational model is capable of modeling the fully coupled thermal fracture propagation in unsaturated porous media.     

Fast multipole displacement discontinuity method (FM‐DDM) for geomechanics reservoir simulations

The displacement discontinuity method (DDM) is frequently used in geothermal and petroleum applications for modeling the behavior of fractures in linear‐elastic rocks. The DDM requires O(N²) memory and O(N³) floating point operations (where N is the number of unknowns) to construct the coefficient matrix and solve the linear system of equations by direct methods. Therefore, the conventional implementation of the DDM is not computationally efficient for very large systems of cracks, often limiting its application to small‐scale problems. This work presents an approach for solving large‐scale fracture problems using the fast multipole method (FMM). The approach uses both the DDM and a kernel‐independent version of the FMM along with a preconditioned generalized minimal residual algorithm to accelerate the solution of linear systems of equations using desktop computers. Using the fundamental solutions for constant displacement discontinuity in a two‐dimensional elastic medium, several numerical examples involving fracture networks representing fractured reservoirs are treated. Numerical results show good agreement with analytical solutions and demonstrate the efficiency of the FMM implementation of the DDM for large‐scale simulations. Copyright © 2015 John Wiley & Sons, Ltd.     

Full 3-D numerical modeling of borehole electric image logging and the evaluation model of fracture

A full 3-D finite element method numerical modeling program is written based on the principle and technical specification of borehole electric image well logging tool. The response of well logging is computed in the formation media model with a single fracture. The effect of changing fracture aperture and resistivity ratio to the logging response is discussed. The identification ability for two parallel fractures is also present. A quantitative evaluation formula of fracture aperture from borehole electric image logging data is set up. A case study of the model well is done to verify the accuracy of the for-mula. The result indicates that the formula is more accurate than the foreign one.