A novel approach to estimate the variations in stresses and fault state due to depletion of reservoirs

Geomechanical changes may occur in reservoirs due to production from reservoirs. Study of these changes has an important role in upcoming operations. Frictional equilibrium is one of the items that should be determined during the depletion as it may vary with respect to time. Pre-existing faults and fractures will slide in regions where there is no frictional equilibrium. Hence, it may be concluded that reduction in pore pressure can initiate the sliding of faults. Whereas, it is also possible that faults will not exist after a certain time as production recovers the equilibrium. Casing shearing or lost circulation may be occurred due to faulting. In reservoirs which depletion leads to frictional equilibrium, decrease of fractures and faults leads to some variations in permeability. Hence, predicting the effect of depletion on frictional equilibrium is required in dealing with casing shearing or lost circulation in drilling of new wells. In addition, permeability modeling will be more precise during the life of reservoirs. Estimation of necessary time to create or vanish faults is vital to be successful in production from wells or drilling new wells. Achieving or loosing of equilibrium mainly depends on in situ stresses and rate of production. Estimation of the in situ stresses at the initiation state of reservoir is the key to study the state of faults. The next step is to predict the effects of depletion on in situ stresses. Different models are suggested in which decrease of horizontal stresses is predicted as function of pore pressure variation. In these models, different assumptions are made such as simplifying the poroelastic theory, ignoring the passing time, and considering the geometry of reservoir. In this article, a new model is proposed based on theory of inclusions and boundary element method. This state-of-the-art model considers the geometry of reservoir. In addition, changes of in situ are obtained as a function of time to reach to a more precise model capable of applying during the reservoir life. Finally, the model is imposed on real cases. The effect of depletion on faults is studied in reservoirs of normal and strike-slip stress regimes, and conventional and proposed models are compared. For this aim, in the first step, mechanical earth models of these two reservoirs are built using logging and coring data. Stress polygon method and poroelastic horizontal strain model are used for strike-slip and normal regimes, respectively. In reservoir 1 which is in a strike-slip stress regime, a fault is distinguished in formation microimaging (FMI) log. For this reservoir, the required time to achieve to frictional equilibrium is calculated. In the reservoir 2, the potential depth of fault sliding is analyzed and required time for faulting in that depth is predicted. The predicted time for satisfaction of frictional equilibrium using the proposed model has a significant difference with the predicted time using the previous methods. In addition, the proposed model predicts that different parts of reservoir 2 are willing for faulting during depletion. The previous model determines only one point that faulting may happen. It is necessary to use this new model to consider different important factors such as geometry and time to gain more accurate predictions.     

Elastic-wave velocities for porous media with power-law distribution of pore sizes

 A system consisting of an elastic matrix with randomly distributed pores of uniform shapes but with sizes governed by a power-law distribution is investigated in a theoretical way. The velocities of elastic waves propagating through such a system are calculated. The obtained results are that the velocities decrease with increasing total porosity and for a given porosity velocities moderately decrease with increasing absolute value of the exponent D in a power-law distribution of pore sizes. Received: 3 July 1995 / Accepted: 2 August 1996     

A structured approach to enhance flood hazard assessment in mountain streams

An evidence-based flood hazard analysis in mountain streams requires the identification and the quantitative characterisation of multiple possible processes. These processes result from specific triggering mechanisms on the hillslopes (i.e. landslides, debris flows), in-channel morphodynamic processes associated with sudden bed changes and stochastic processes taking place at critical stream configurations (e.g. occlusion of bridges, failure of levees). From a hazard assessment perspective, such possible processes are related to considerable uncertainties underlying the hydrological cause-effect chains. Overcoming these uncertainties still remains a major challenge in hazard and risk assessment and represents a necessary condition for a reliable spatial representation of process intensities and the associated probabilities. As a result of an accurate analysis of the conceptual flaws present in the procedures currently employed for hazard mapping in South Tyrol (Italy) and Carinthia (Austria), we propose a structured approach as a means to enhance the integration of hillslope, morphodynamic and stochastic processes into conventional flood hazard prediction for mountain basins. To this aim, a functional distinction is introduced between prevailing one-dimensional and two-dimensional process propagation domains, i.e., between confined and semi- to unconfined stream segments. The former domains are mostly responsible for the generation of water, sediment and wood fluxes, and the latter are where flooding of inactive channel areas (i.e. alluvial fans, floodplains) can occur. For the 1D process propagation domain, we discuss how to carry out a process routing along the stream system and how to integrate numerical models output with expert judgement in order to derive consistent event scenarios, thus providing a consistent quantification of the input variables needed for the associated 2D domains. Within these latter domains, two main types of spatial sub-domains can be identified based on the predictability of their dynamics, i.e., stochastic and quasi-deterministic. Advantages and limitations offered by this methodology are finally discussed with respect to hazard and risk assessment in mountain basins.     

A numerical approach to estimate shaft friction of bored piles in sands

A new approach to estimate shaft capacity of bored piles in sandy soils, based on numerical analysis, is presented. The topic is relevant as current design methods often largely underestimate the shaft capacity of piles in sands, thus resulting in an over-conservative design. The proposed approach is based on explicitly modelling the thin cylinder of soil surrounding the pile, where strain localization concentrates (shear band), and on the fundamental mechanic behaviour of sandy soils (e.g. dilatancy, softening). This approach is both simple and easy to apply. Results of a broad parametric study involving axially loaded single piles embedded in different sandy soils are presented, highlighting that relative density and grain size distribution mainly affect the shaft capacity. The capability of the procedure to predict shaft friction is checked against data from a well-documented full-scale axial load test on instrumented pile. Some suggestions for calibration and application of the method are also reported.     

A computerized method to estimate friction coefficient from orientation distribution of meso-scale faults

The friction coefficient controls the brittle strength of the Earth’s crust for deformation recorded by faults. This study proposes a computerized method to determine the friction coefficient of meso-scale faults. The method is based on the analysis of orientation distribution of faults, and the principal stress axes and the stress ratio calculated by a stress tensor inversion technique. The method assumes that faults are activated according to the cohesionless Coulomb’s failure criterion, where the fluctuations of fluid pressure and the magnitude of differential stress are assumed to induce faulting. In this case, the orientation distribution of fault planes is described by a probability density function that is visualized as linear contours on a Mohr diagram. The parametric optimization of the function for an observed fault population yields the friction coefficient. A test using an artificial fault-slip dataset successfully determines the internal friction angle (the arctangent of the friction coefficient) with its confidence interval of several degrees estimated by the bootstrap resampling technique. An application to natural faults cutting a Pleistocene forearc basin fill yields a friction coefficient around 0.7 which is experimentally predicted by the Byerlee’s law.     

不同固结压力下强结构性黏土孔隙分布试验研究

为探讨结构性土变形的微观机制,以湛江地区结构性黏土为研究对象,对原状土和压缩试验后土样进行压汞试验,分析不同固结压力下土的孔隙分布、孔径大小以及孔隙结构特征参数的变化规律,辅以SEM图像进行孔隙形态的定性分析,并从分形理论角度对此解释与验证。结果表明,湛江黏土各孔隙组对外力的敏感度与孔隙体积含量正相关。由于压汞过程存在瓶颈效应以及边-面-角的空间接触形式,其结果可能会夸大真实小孔隙的分布密度而低估大孔隙的分布密度。结构性对压缩过程中孔隙分布影响较大,当固结压力增大至结构屈服压力后,孔径为0.01～0.50 μm的孔隙组变化明显,孔隙的连通性变差,孔径分布向小孔径范围移动,孤立式孔隙增多,大、中孔隙的界限变得不明显。湛江黏土孔隙具有多重分形特征,据此确定微裂隙、粒间孔隙和孤立孔隙孔径的分界点为0.01 μm与0.50 μm,同时给出了湛江黏土的孔径界定标准。     

Particle transport within water-saturated porous media: Effect of pore size on retention kinetics and size selection

The transport and filtration behaviour of fine particles (silt) in columns packed with sand was investigated under saturated conditions by using step-input injections. Three samples of different particle size distributions (coarse medium, fine medium and a mixture of both) were used in order to highlight the influence of the pore size distribution on particle retention and size selection of recovered particles. The main parameters of particle transport and deposition were derived from the adjustment of the experimental breakthrough curves by an analytical model. The higher particle retention occurs in the mixture medium, owing to its large pore size distribution, and the filtration coefficient decreases with increasing flow velocity. Particle size distribution of recovered particles shows a thorough size selection: (i) the first recovered particles are the coarser ones; (ii) the size of the recovered particles increases with increasing flow velocity and enlarger pore distribution of the medium.     

A fluorescent approach to the identification of grout injected into fissures and pore spaces

A new fluorescent approach was developed to visualize grouting processes. The evaluation of the effect of grouting is carried out by methods based mainly on physical measurements such as permeability and porosity characteristics. However, these estimates do not necessarily correlate with practical grouting data. In this paper, the distributions of various grout mixtures containing fluorescent substances were clearly visualized under ultraviolet light, using a borehole television system or a microscope. Since this approach enabled direct observation of grout distribution, it is considered that this approach is useful for analysis of grouting mechanisms, designing grout curtains, etc.     

Adsorption characteristics of natural organic matter on activated carbons with different pore size distribution

The adsorption characteristics of natural organic matter (NOM) were investigated on the basis of fluorescence excitation emission matrix (EEM) by using four different coal-based activated carbons (ACs). For each AC, batch adsorption isotherms were analyzed using a modified Freundlich isotherm model on the basis of the fluorescence intensity of three major fluorescence peaks appeared in the fluorescence EEM reported to be reflective of the humic acid-like (P1), fulvic acid-like (P2) and aromatic protein-like (P3) substances, respectively, together with the well-used overall quality indices of dissolved organic carbon (DOC) and ultraviolet absorbance at the wavelength of 260 nm (UV260). It was found that, for all five quality indices, the adsorption capacity differed with the ACs used, and the modified Freundlich isotherm constant K estimated for P1, P2 and P3 was in close correlation with that of the total organic matter evaluated by DOC. Moreover, no matter which AC was concerned, the magnitude of the estimated K and the removal rate over a broader range of AC dose for P3 were apparently smaller than those for P1 and P2, suggesting the adsorbability of aromatic protein-like substances was lower than that of the humic acid-like and fulvic acid-like substances. The dependency of the adsorption capacity of NOM on the volume of pores in some specific size ranges of the ACs was also revealed.     

A 4th order fabric tensor approach applied to granular media

In this paper, we propose a micromechanical approach of the behavior of granular media, which takes into account the anisotropy by means of a fourth order fabric tensor. The proposed approach is implemented in an homogenization scheme based on Voigt and Reuss localization assumption. The fabric tensor-based approach is then combined with a new kinematic localization rule and yields a general homogenization scheme for anisotropic granular media.     

A rapid approach to estimate the rockfall energies and distances at the base of rock cliffs

This paper presents a new methodology for estimating the expected energies and first impact distances at the base of a rock cliff, subject to the geometry and properties of the cliff and the representative block being known. The method is based on a sensitivity analysis, conducted by means of kinematic simulations and carried out for a large range of input parameters and their combinations, taking into account the uncertainty associated with their estimate. The proposed approach is validated by comparing predictions to experimental data and shows great potential for a quick qualitative hazard assessment.     

A novel multiproxy approach to reconstruct the paleoecology of extinct cephalopods

Reconstructing marine paleoenvironments from the skeletal hardparts of nektic organisms is often hampered by their unknown migrational pathways involving different water masses and their corresponding physico-chemical parameters. Despite significant progress over the last years, the reconstruction of migration patterns of extinct ammonoids is difficult because both, vertical and horizontal ones lead to an intricate combination of reconstructed seawater temperatures. Paleonvironmental data retrieved from exceptionally well-preserved lower Albian (Douvilleiceras mammillatum ammonite zone, uppermost CC8a nannofossil zone, 110.5–111.0 Ma) cephalopod shells from Madagascar (Mahajanga Basin, 40–42° southern latitude) are reconstructed based on a novel multi-proxy approach. Here we combine and contrast: (i) nacre tablet-paleobathymetry, (ii) Westermann Morphospace, and (iii) shell isotope geochemistry analysis. Results obtained allow for a robust reconstruction of habitat depth, paleo-seawater temperatures, and paleoecology of extinct cephalopod taxa. The discocone nautiloid Cymatoceras is regarded as vertical migrant with a maximum habitat depth of 250 m and a mean habitat water temperature of 20–21 °C. All three ammonoid taxa (Cleoniceras, Desmoceras, and Eogaudryceras) share a platycone to planorbicone shell shape suggesting a demersal life habit at maximal water depths of 450–500 m and temperatures of 19–21 °C for Cleoniceras and Desmoceras, and the deepest (525 m) and coolest (14 °C) habitat for Eogaudryceras. Circumstantial evidence from benthic mollusk shells from the same stratigraphic interval as well as sedimentological and paleopathological data provide further support for habitat reconstructions. Reconstructed paleotemperatures shown here provide well-constrained evidence for the thermal structure of past oceanic water masses and have significance for Cretaceous paleoceanography in general.     

A novel approach to eliminate error induced by cell to node projections

In engineering practices, different numerical methods for fluid flow simulation and solid deformation/stress simulation are adopted to model fluid–structure interaction problems in porous media. Cell‐centered finite volume method is widely used in fluid flow simulation, while the solid deformation/stress simulation is usually accomplished by using the Galerkin vertex‐centered finite element method, which leads to the incompatibility between cell variables with nodal variables. Therefore, the data transfer between cell variables and nodal variables is inevitable. Consequently, this kind of transfer will lead to extra artificial error. Hence, the major concern is how to minimize the error due to cell to node projections. In this paper, a problem of pore pressure diffusion within a one‐dimensional heterogeneous porous medium is investigated. We present a new projection scheme and corresponding error formula, where the error control factor is introduced. The new projection scheme is based on piecewise linear interpolations. Results demonstrate that if the error control factor is chosen properly, the error due to the projection from cell to node can be controlled effectively, and the most desired zero error can be achieved. Finally, we analyze some practical cases in consideration of permeability contrast and mesh uniformity. Copyright © 2015 John Wiley & Sons, Ltd.     

Geochemical approach to estimate the quality of water entering abandoned underground coalmines

Geochemical modeling was employed to estimate the chemistry of water infiltrating into abandoned underground flooded and unflooded coalmines of the Uniontown syncline, Fayette County, Pennsylvania. This was done to help evaluate factors governing the long-term evolution of coalmine discharge water quality in the Uniontown syncline area. The subsurface structure and lithology was delineated using borehole, mine shaft, and stratigraphic information. Hydrogeologic analyses indicated that most of the recharge to mine voids occurs in the zones of shallow overburden cover of less than 20 m in thickness. The water–rock contact period in the recharge areas of the overburden was estimated to be 5 days or greater for the flooded mines, and a day or less for the unflooded mines. Flow-through reaction-path models were applied to the topsoil and shale–sandstone lithological units identified in the recharge areas. The model predicted water entering the flooded mines at a pH of 6.65, alkalinity of 6.92 mequiv. l^–1 and a total sulfate concentration of 7.33 mM, and the unflooded mines at a pH of 6.68, alkalinity of 6.99 mequiv. l^–1 and a total sulfate concentration of 3.08 mM. The model predictions for the flooded and unflooded mines are consistent with groundwater data from the study site, indicating the usefulness of this approach in evaluating the contribution of overburden chemistry to the evolution of mine discharge quality.     

A hydrochemical approach to estimate mountain front recharge in an aquifer system in Tamilnadu,India

Mountain-front recharge (MFR) is a process of recharging an aquifer by infiltration of surface flow from streams and adjacent basins in a mountain block and along a mountain front (MF). This is the first attempt in India to estimate MFR along the foothills of Courtallam using hydrogeochemistry and geostatistical tools. The estimation of MFR has been carried out by collecting groundwater samples along the foothills of Courtallam. Collected water samples were analyzed for major cations and anions using standard procedures. Hydrogeochemical facies show the existence of four water types in this region. Calcium-rich water derived from gneissic rock terrain indicates significant recharge from higher elevation. Log pCO₂ and ionic strength of the samples were also calculated to identify the geochemical process. Majority of the collected samples have sodium-rich water and weak ionic strength, which indicate foothill recharge and low residence time. Silicate and carbonate weathering have an equal interplay along the foothills with a relatively large fraction of Mg from the MF. The spatial diagrams of three factors show that the southern part of the study area is dominated by both weathering and anthropogenic processes, whereas the northern part is dominated by both leaching and weathering processes. Thus, the dominant weathering process represented by the second factor indicates the large recharge process along the foothills.     

不同沉积环境下马兰黄土孔隙分布与土水特征的模式分析

为研究黄土的孔隙分布(PSD)和土 -水特征曲线(SWCC)与黄土沉积环境的关系,在甘肃榆中县和平镇和正宁县及陕西泾阳县等黄土台塬上部采取原状马兰黄土样,先测定其基本物理指标,再用压汞法测得各土样的PSD曲线,同时用滤纸法测得其SWCC。结果表明：和平镇马兰黄土的PSD曲线为单峰形态,泾阳县马兰黄土的PSD曲线为双峰形态,正宁马兰黄土的PSD曲线具有三峰特点。各土样的SWCC具有与PSD曲线相对应的阶梯状形态。SWCC与PSD曲线之间的对应关系,说明黄土的土水特征受孔隙分布的控制。获取3个原状样扫描电镜图像,微结构显示出这种地域性分异现象。对各采样点的气候环境分析表明,黄土PSD曲线之间的差异主要是由成土作用强弱决定的。将降水量与蒸发量的比值定义为蒸渗系数 (CEI),作为刻画成土作用强弱的指标。发现当CEI≤0.3时,PSD曲线为单峰模式;0.3＜CEI≤0.4时,PSD曲线为双峰模式;0.4＜CEI≤0.5时,PSD曲线为三峰模式,相应的SWCC分别为单拐点、双拐点和三拐点模式。     

盾构隧道施工引起的土体初始超孔隙水压力分布研究

盾构施工会对周围土体产生扰动,形成超孔隙水压力,引起工后固结沉降。运用应力释放理论推导与衬砌相邻的土体初始超孔隙水压力计算公式。假定扰动范围边界呈圆弧状,确定初始超孔隙水压力的分布范围;同时运用应力传递理论,推导分布范围内任一点土体的初始超孔隙水压力计算公式。通过对实测资料的分析可知,计算值与实测值吻合较好。算例分析表明,与衬砌相邻的土体初始超孔隙水压力呈近似圆形（顶部小、底部大）;随着到衬砌的径向距离增加,土体初始超孔隙水压力呈凹曲线形状;隧道底部的等值线最密,即变化最快;隧道顶部上方土体、不同深度处土体初始超孔隙水压力,以隧道轴线处为最大,呈现类似Peck曲线形状。     

A hybrid finite element-spectral boundary integral approach: Applications to dynamic rupture modeling in unbounded domains

The finite element method (FEM) and the spectral boundary integral method (SBI) have both been widely used in the study of dynamic rupture simulations along a weak interface. In this paper, we present a hybrid method that combines FEM and SBI through the consistent exchange of displacement and traction boundary conditions, thereby benefiting from the flexibility of FEM in handling problems with nonlinearities or small-scale heterogeneities and from the superior performance and accuracy of SBI. We validate the hybrid method using a benchmark problem from the Southern California Earthquake Center's dynamic rupture simulation validation exercises.We further demonstrate the capability and computational efficiency of the hybrid scheme for resolving off-fault heterogeneities by studying a 2D in-plane shear crack in two different settings: one where the crack is embedded in a high-velocity zone and another where it is embedded in a low-velocity zone. Finally, we discuss the potential of the hybrid method for addressing a wide range of problems in geophysics and engineering.     

The tethyan oceanic gates : A tectonic approach to major sedimentary changes within Tethys

Abstract

— Accepting that the opening or closure of seaways has consequences on the oceanic circulation which in turn influences sedimentation, major changes in Tethyan geometry are checked against major changes in sedimentation. Time relationships can be demonstrated between the two group of phenomena and causal links are discussed. As the causes for major sedimentary changes are numerous and their respective roles controversial, it is speculated if the observed change in geometry could have had a positive effect on the associated change in sediments.

Ever since the birth of the Mesozoic Tethys which formed from the break-up of Pangea till its death through multiple collisions, this ocean played a major role in the world ocean circulation, being the sub-equatorial seaway which permitted a circumterrestrial circulation at low latitudes. Five successive steps, separated by four major changes, are recognized : i) during the Trias and the Jurassic, the young Tethys was a triangular cul-de-sac expanding westward through distensive tectonics and subsidence, and influenced along its southern margin by a westward current and associated upwelling; radiolarites take a growing part in its sedimentation; ii) during the Tithonian, radiolarites were abruptly replaced by pelagic limestones (Majolica) in many places and the corresponding CCD drop is tentatively correlated with the full opening of the Tethyan seaway which reached the Pacific Ocean through the Caribbean and with the corresponding reorganization of currents; iii) during the Early Cretaceous, Apulia began its collision with the precursor elements of the European margin, thus hindering deep water exchange; this pre collision favoured the mid Cretaceous anoxia with affected sedimentation in numerous basins; iv) during the Late Cretaceous, repeated distension and subsidence (since the Aptian) between Apulia and Africa gave a growing importance to the south Apulian seaway; this new opportunity of water exchange favoured the end of anoxia and the deposition of new pelagic limestones (scaglia); as a matter of fact, the location of the Late Cretaceous to Early Eocene phosphorites testify to a well-established westward current along this south-Apulian seaway; v) the late Eocene to Oligocene general collision was responsable for the death of this current and the corresponding breakdown of the Tethyan seaway into independent groups of basins.