Order of diagenetic events controls evolution of porosity and permeability in carbonates

The properties of carbonate rocks are often the result of multiple, diagenetic events that involve phases of cementation (porosity occlusion) and dissolution (porosity enhancement). This study tests the hypothesis that the order of these events is a major control on final porosity and permeability. A three-dimensional synthetic model of grainstone is used to quantify trends that show the effect of early cementation, non-fabric selective dissolution, and then a second-generation of (post-dissolution) cement. Models are 3 mm³ with a resolution of 10 μm. Six simple paragenetic sequences are modelled from an identical starting sediment (without accounting for compaction) where the same diagenetic events are placed in different sequences, allowing for quantification of relative changes in the resultant porosity and permeability for each diagenetic event, the trajectory through time, as well as for each final rock. All modelled paragenetic sequences result in reductions in porosity and permeability, but the order of diagenetic events controls the trajectory and final rock properties. Differences in the order of early cement precipitation alone produce variable final values, but all follow the porosity–permeability relationship as expressed by the Kozeny-Carman equation. However, final values for the sequences which include a phase of dissolution fall on a new curve, which departs from that predicted by the Kozeny-Carman relationship. This allows an alternative form of porosity–permeability relationship to be proposed: κ = 2280ϕ–30,400, where ϕ is porosity (%) and κ is permeability (mD). Hence while the Kozeny-Carman relationship predicts porosity–permeability changes that occur with cementation, it is unable to capture accurately changes within the pore network as a result of dissolution. Although the results may be dependent on the properties of the initial carbonate sediment and simplified diagenetic scenarios, it is suggested that this new porosity–permeability relationship may capture some generalized behaviour, which can be tested by modelling further sediment types and diagenetic scenarios.     

Two-dimensional reactive transport modeling of the alteration of a fractured limestone by hyperalkaline solutions at Maqarin (Jordan)

Two-dimensional reactive transport modeling of the Maqarin Eastern Springs site, a natural analogue for the alteration of a fractured limestone by high-pH Portland cement waters, has been performed using the CrunchFlow code. These 2D calculations included transport by advection–dispersion–diffusion along a single fracture and diffusion in the wall rock. Solute transport was coupled to mineral dissolution and precipitation. A limited sensitivity analysis evaluated the effect of different values of primary mineral surface areas, flow velocity and sulfate concentration of the inflowing high-pH solution.Major secondary minerals include ettringite–thaumasite, C–S–H/C–A–S–H and calcite. C–S–H/C–A–S–H precipitation is controlled by the dissolution of primary silicates. Ettringite precipitation is controlled by diffusion of sulfate and aluminum from the wall rock to the fracture, with aluminum provided by the dissolution of albite. Calcite precipitation is controlled by diffusion of carbonate from the wall rock. Extents of porosity sealing along the fracture and in the fracture-wall rock interface depend on assumptions regarding flow velocity and composition of the high-pH solution. The multiple episodes of fracture sealing and reactivation evidenced in the fracture infills were not included in the simulations. Results can qualitatively reproduce the reported decrease in porosity in the fractures and in the wall rock next to the fractures. Instances of porosity increase next to fractures caused by carbonate dissolution were not reproduced by the calculations.     

查干凹陷早白垩世碎屑岩储层成岩作用及孔隙演化

在对大量薄片和岩心观察分析基础上,结合扫描电镜和全岩分析、 X衍射粘土分析、 包裹体测温等手段,对查干凹陷碎屑岩储层的成岩作用和孔隙演化进行了研究。结果表明: 研究区碎屑岩储层主要为低成分成熟度、 低结构成熟度的长石岩屑砂岩,成岩作用呈现“强压实、 强胶结、 弱交代、 弱溶蚀“的“两强两弱”特征;成岩阶段主要处于中成岩A期,局部达中成岩B期至晚成岩期。两期胶结主要发生在苏二早期和乌兰苏海时期,中浅层（<2 500 m）较低孔隙度主要由于强压实和早期致密胶结所致;较高孔隙度是由于原生孔隙保存较好,长石及碳酸盐胶结物的溶蚀作用导致孔隙度增大。深层碎屑岩储层（>2 500 m）较低孔隙度除压实作用外,碳酸盐致密胶结是主因;较高孔隙度则为早期酸性溶蚀叠加晚期碱性溶蚀产生次生孔隙为主。晚期碱性溶蚀发生在油气充注之后,为无效溶孔。     

伊拉克艾哈代布油田中-下白垩统碳酸盐岩储层孔隙结构及控制因素

伊拉克艾哈代布油田中-下白垩统碳酸盐岩储层孔隙结构类型划分及其差异性的受控因素和成因的认识是制约该类储层分类评价的一个关键问题。本文综合运用了常规薄片、铸体薄片、扫描电镜、电子探针、常规物性、压汞分析、阴极发光、稳定同位素分析等手段及统计分析方法对储层的主要孔隙类型、喉道类型、喉道分布类型进行了研究,并在此基础上确定了高孔中渗细喉型、中高孔低渗细喉型、中高孔特低渗微喉型、低孔特低渗极微喉型四种孔隙结构类型及其划分依据和方案,同时指出了在该类储层中好的孔隙结构具有网络状、偏细型、分选较差的孔喉特征。通过分析明确了孔隙结构差异性的主要控制因素为岩石组构和溶蚀作用,认为该套储层的孔喉空间及结构是以沉积形成的原生孔隙受同生期或准同生期的溶蚀改造为基础,并在后期表生岩溶作用下的改造而成;而先期受岩石组构控制下的溶蚀作用控制了储层孔隙结构的差异性。通过本文研究将对中东地区该类储层的形成及分类评价研究具有较好的指导意义。     

Moisture expansion associated to secondary porosity: an example of the Loseros Tuff of Guanajuato, Mexico

The old mining city of Guanajuato in middle Mexico preserves one of the most important historical legacies in colonial buildings, the UNESCO declared the city World Heritage Site in 1988. Practically all the colonial constructions were built with natural stones from the neighbourhood, of which stands a greenish to reddish vulcanite, called Loseros Tuff. Although the Loseros Tuff is widely used in historical buildings in the city. It shows significant deterioration and weathering effects, principally in the parts where the tuff shows a coarse grain size. The petrographic, petrophysical, mineralogical and geochemical properties of the Loseros Tuff were analysed in order to determine the causes, effects, behaviour and response to deterioration of this volcanic rock. The results of the investigations suggest that in addition to the parameters like the grain size and the porosity properties, the pore radii distribution is decisive for the effectiveness of porosity and the water transport into the rock. It is recognized that once the liquid water invades the rock the dissolution of the matrix occurs, which is accompanied by a sudden moisture expansion favoured by the newly formed secondary porosity and the high content of expandable clay minerals.     

黔东南地区寒武系碳酸盐岩成岩作用分析

黔东南地区寒武系地层是典型的碳酸盐岩地层。分布广泛且有较好的储层。为了更好地了解该地层的岩性、孔隙状况等地质特征,以贵州东部三都-丹寨剖面寒武系地层为例,分析了各组段的岩石学特征,确定了其沉积环境,在此基础上对形成现有岩石孔隙等特征的相应的成岩作用进行了分析。指出白云化作用是该区的主要成岩作用,主要表现在中寒武统都柳江组-在准同生白云化作用下形成大套厚层的白云岩。埋藏溶解作用形成的次生溶孔是该区中上寒武统的典型特征,有利于改善碳酸盐岩的储层物性。重结晶作用、压溶作用等对储层孔隙的形成也起到重要作用,并列出了黔东南地区寒武系地层各层组的成岩演化史。     

Simulations of hydro-fracking in rock mass at meso-scale using fully coupled DEM/CFD approach

The paper deals with two-dimensional (2D) numerical modelling of hydro-fracking (hydraulic fracturing) in rocks at the meso-scale. A numerical model was developed to characterize the properties of fluid-driven fractures in rocks by combining the discrete element method (DEM) with computational fluid dynamics (CFD). The mechanical behaviour of the rock matrix was simulated with DEM and the behaviour of the fracturing fluid flow in newly developed and pre-existing fractures with CFD. The changes in the void geometry in the rock matrix were taken into account. The initial 2D hydro-fracking simulation tests were carried out for a rock segment under biaxial compression with one injection slot in order to validate the numerical model. The qualitative effect of several parameters on the propagation of a hydraulic fracture was studied: initial porosity of the rock matrix, dynamic viscosity of the fracking fluid, rock strength and pre-existing fracture. The characteristic features of a fractured rock mass due to a high-pressure injection of fluid were realistically modelled by the proposed coupled approach.

     

Numerical modelling of the hydro‐chemo‐mechanical behaviour of geomaterials in the context of CO2 injection

Safety assessment of geosequestration of CO₂ into deep saline aquifers requires a precise understanding of the study of hydro‐chemo‐mechanical couplings occurring in the rocks and the cement well. To this aim, a coupled chemo‐poromechanical model has been developed and implemented into a research code well‐suited to the resolution of fully coupled problems. This code is based on the finite volume methods. In a 1D axisymmetrical configuration, this study aims to simulate the chemo‐poromechanical behaviour of a system composed by the cement well and the caprock during CO₂ injection. Major chemical reactions of carbonation occurring into cement paste and rocks are considered in order to evaluate the consequences of the presence of CO₂ on the amount of dissolved matrix and precipitated calcium carbonates. The dissolution of the solid matrix is taken into account through the use of a chemical porosity. Matrix leaching and carbonation lead, as expected, to important variations of porosity, permeability and to alterations of transport properties and mechanical stiffness. These results justify the importance of considering a coupled analysis accounting for the main chemical reactions. It is worth noting that the modelling framework proposed in the present study could be extended to model the chemo‐poromechanical behaviour of the reservoir rock and the caprock when subjected to the presence of an acidic pore fluid (CO₂‐rich brine). Copyright © 2013 John Wiley & Sons, Ltd.     

惠州凹陷东部珠海组储层碱性成岩作用及孔隙演化

根据铸体薄片、扫描电镜、粒度分析、X衍射等资料,对惠州凹陷东部珠海组储层的岩石学特征、成岩作用和孔隙演化过程进行分析研究。研究认为,惠州凹陷东部珠海组储层的岩石类型以岩屑砂岩、长石质岩屑砂岩、岩屑质石英砂岩和岩屑质长石砂岩为主,结构成熟度与成分成熟度较低。砂岩埋藏过程中经历了明显的碱性成岩作用:石英的溶蚀与交代、碳酸盐矿物胶结、伊利石和绿泥石的沉淀以及钠长石化等。碱性成岩作用对孔隙的影响包括:石英溶蚀形成次生溶孔、碳酸盐胶结物沉淀损失粒间孔隙、黏土矿物沉淀形成晶间微孔。研究区珠海组主要发育粒内溶孔和粒间溶孔,原生孔隙较少。储层现今处于中成岩阶段A期,早期经历了强烈的压实作用,使其孔隙度由原始孔隙度32.1%降低至8.8%。早成岩阶段为碱性成岩环境,石英溶蚀增孔约0.5%;碳酸盐、硫酸盐、伊利石等胶结物沉淀减孔约2.3%。中成岩阶段A期为酸性成岩环境,硅质、高岭石等胶结物沉淀减孔约1.2%;长石、岩屑等溶蚀增孔约4.3%。最终,储层演化至现今孔隙度10.1%。     

松辽盆地北部中——深层砂岩的次生孔隙研究

松辽盆地北部中——深层砂岩的次生孔隙主要是由不稳定的骨架颗粒组分溶解而成,按结构系列,次生孔隙可划分为八种类型。斜长石的溶解是造成次生孔隙的主要原因,它可以在埋藏浅处开始溶解,在1000-1500m 深处溶解变得集中和剧烈。钾长石的溶解一般发生在埋藏较深处。钾长石溶解和浊沸石溶解大大丰富了深层砂岩的孔、渗条件。次生孔隙的形成不仅取决于酸性介质环境,而偏碱性的孔隙溶液对长石的溶解亦具有重要意义。     

海拉尔盆地贝尔凹陷铜钵庙组成岩作用及其对孔隙演化的影响

海拉尔盆地贝尔凹陷铜钵庙组发育一套火山碎屑岩类、火山-沉积岩类和少量的陆源碎屑岩岩石组合。其中,火山碎屑岩类、火山-沉积岩类成岩作用受火山碎屑物质控制作用明显。典型的成岩作用类型包括压实作用,胶结作用和溶蚀、溶解作用等。成岩阶段属中成岩A期。通过孔隙演化研究发现,压实作用是该区原始孔隙损失的最主要因素;其次为胶结作用。早期压实作用和胶结作用共损失83%以上的原生孔隙,晚期岩屑溶蚀和长石溶蚀作用增加6.53%的孔隙度,有效地改善了储层的物性特征。此外,不同岩石类型成岩作用特征及其对孔隙的影响差异较大。     

松辽盆地北部深层碎屑岩浊沸石成因、演化及与油气关系研究

浊沸石作为松辽盆地深层最重要的一种成岩自生矿物，分布广泛，后期受成岩改造强烈，形成了高产的次生孔隙气藏。通过X射线衍射分析、X射线粉晶分析、红外光谱分析和显微热台等分析，确定了松辽盆地深层碎屑岩内浊沸石含量及矿物学特征，探讨了浊沸石的成因；利用扫描电镜和图像分析仪系统研究了浊沸石溶孔特征和深层有机酸与浊沸石次生孔隙的成因关系，指出浊沸石溶孔在纵向上呈发育到较发育到不发育的旋回性变化，溶孔发育段厚度一般2～3m，平面上升平-汪家屯和昌德地区浊沸石溶孔可能连片；浊沸石次生孔隙形成期与天然气主运聚期具较好的匹配关系，并使被浊沸石胶结的砂岩表现出双重性：早期封盖，晚期储集．表明次生孔隙发育带最有可能成为高产油气层。     

The effects of strain rate and saturation on a micro-cracked marble

It is well known rock masses contain several types of weakness planes varying from micro-fissure to fault in size. The fracture frequency, degree of saturation and time are the basic rock parameters affecting its behaviour. However, in most cases, it is practically difficult to test heavily fractured rock in laboratory environment. In this study, the effects of micro cracks, strain rate and water saturation on strength are discussed using a small-scale physical experiments. It is attempted to detach the grain boundaries of coarse-grained rock specimens of the Mu?la marble by thermal treatment that would serve as a small-scale physical simulation of fractures in rock masses and enable a discussion of the variations in time dependent mechanical behaviour. An experimental study was conducted on marble specimens induced thermally by micro-cracks in different frequencies. The thermal treatment periods being 24 h in inert atmospheric condition were varied for each test specimen except categories A and G. Effective porosity increased up to 2.4% after the last thermal cycle of category F. Then conventional compression tests with different strain rates were carried out using a servo-controlled testing machine on both dry and saturated specimens. It is concluded that both lowering strain rates and increasing porosity related with thermally induced micro cracks have important effects on strength and failure path. The progressive failure entirely occurs along the grain boundaries fissured by cyclical treatment and inter-granular deformation depends upon degree of thermal influence. The small-scaled laboratory model enabled to demonstrate that parameters such as joint frequency, time and saturation have a significant effect on mechanical behaviour of rock masses.     

库车凹陷克拉2气田深层优质储层成因及成岩作用模式

基于成岩作用的影响因素及库车盆地的实际,深入研究了克拉2气田次生孔隙发育段储层岩石成分特征及早期古气候条件下胶结物对于成岩作用及次生孔隙的影响,分析了地层压力系统中超高压带、构造裂缝、自生矿物分带的成因、作用、特点及其与储层物性、成岩环境的相关关系,评价了储层中长石、方解石的溶解,白云石对方解石的交代,压实、胶结等主要成岩作用类型对次生孔隙的具体影响和贡献,认为该区早期碱性成岩环境下方解石胶结物的溶解是形成该区次生孔隙发育带的主导因素,并根据该区自生矿物流体包裹体及埋藏史特征,建立了该区成岩作用及孔隙演化的模式。     

Simulations of a coupled hydro-chemo-mechanical system in rocks

A coupled hydro-chemo-mechanical numerical model is developed for these coupled phenomena in many engineering fields. The model has been applied to predicting the response of a stressed rockmass column to an injected reactive fluid (reagent) flow. The response includes evolutions of porosity, permeability, reagent and mineral concentrations during dissolution. In the model, the progress of dissolution is defined by the change in porosity ratio and the porosity increases with dissolution assuming there is no precipitation. The numerical evolutions of porosity, permeability, reagent and mineral concentrations during dissolution are validated against steady state solutions. The model results show that these evolutions are regulated to a certain extent by the applied external loadings: an applied extensional stress enhances the progress of the dissolution process while an applied compression stress slows the progress of the dissolution process. This revised version was published online in July 2006 with corrections to the Cover Date.     

Prediction of rock alteration patterns: A potential tool in mineral exploration

Hydrothermal alteration of a quartz‐K‐feldspar rock is simulated numerically by coupling fluid flow and chemical reactions. Introduction of CO₂ gas generates an acidic fluid and produces secondary quartz, muscovite and/or pyrophyllite at constant temperature and pressure of 300°C and 200 MPa. The precipitation and/or dissolution of the secondary minerals is controlled by either mass‐action relations or rate laws. In our simulations the mass of the primary elements are conserved and the mass‐balance equations are solved sequentially using an implicit scheme in a finite‐element code. The pore‐fluid velocity is assumed to be constant. The change of rock volume due to the dissolution or precipitation of the minerals, which is directly related to their molar volume, is taken into account. Feedback into the rock porosity and the reaction rates is included in the model. The model produces zones of pyrophyllite quartz and muscovite due to the dissolution of K‐feldspar. Our model simulates, in a simplified way, the acid‐induced alteration assemblages observed in various guises in many significant mineral deposits. The particular aluminosilicate minerals produced in these experiments are associated with the gold deposits of the Witwatersrand Basin.     

准噶尔盆地陆1井砂岩成岩作用及其对储层的影响

本文探讨了陆1井岩屑砂岩的成岩作用及其对砂岩储层的影响.研究表明,压实作用使岩屑砂岩的孔隙度随埋深迅速减小,成岩早期的胶结物使大部分原生孔隙被充填。水化作用在成岩早期及有机质半成熟和成熟期成岩作用过程中,制约着各个阶段的成岩作用特点。在水化作用和有机质热演化作用的共同影响下,使得方解石胶结物发生选择性溶蚀形成了深部(上乌尔禾组)次生孔隙发育带。     

Coupled wellbore erosion and stability analysis

This paper extends earlier work on sand erosion and presents an attempt to couple sand erosion to mechanical damage of rock around a wellbore. Porosity which evolves in time and space as surface erosion progresses, is chosen as the coupling parameter. Both rock elasticity and strength (cohesion) are assumed to depend on porosity in such a way that the material becomes weaker with increasing porosity. The mathematical model, consists of erosion equations, mixture flow equations and stress equilibrium equations, is solved numerically by Galerkin finite element method. Numerical results suggest that erosion, resulting in sand production, is high close to the free surface. Erosion is accompained by changes in porosity and a significant permeability increase. Erosion in the vicinity of the wellbore induces alterations in the mechanical behaviour of the medium. Weakening of rock stiffness leads to severe alteration of both effective stresses and pore pressure near the cavity. Since cohesion decreases with increasing porosity, one can also identify the time instant at which rock mechanical failure starts. © 1998 John Wiley & Sons, Ltd.     

Theoretical analyses of acidization dissolution front instability in fluid‐saturated carbonate rocks

This paper presents an instability theory that can be used to understand the fundamental behavior of an acidization dissolution front when it propagates in fluid‐saturated carbonate rocks. The proposed theory includes two fundamental concepts, namely the intrinsic time and length of an acidization dissolution system, and a theoretical criterion that involves the comparison of the Zhao number and its critical value of the acidization dissolution system. The intrinsic time is used to determine the time scale at which the acidization dissolution front is formed, while the intrinsic length is used to determine the length scale at which the instability of the acidization dissolution front can be initiated. Under the assumption that the acidization dissolution reaction is a fast process, the critical Zhao number, which is used to assess the instability likelihood of an acidization dissolution front propagating in fluid‐saturated carbonate rocks, has been derived in a strictly mathematical manner. Based on the proposed instability theory of a propagating acidization dissolution front, it has been theoretically recognized that: (i) the increase of the mineral dissolution ratio can stabilize the acidization dissolution front in fluid‐saturated carbonate rocks; (ii) the increase of the final porosity of the carbonate rock can destabilize the acidization dissolution front, while the increase of the initial porosity can stabilize the acidization dissolution front in fluid‐saturated carbonate rocks; (iii) the increase of the mineral dissolution ratio can cause an increase in the dimensionless propagation speed of the acidization dissolution front; (iv) the increase of the initial porosity can enable the acidization dissolution front to propagate faster, while the increase of the final porosity can enable the acidization dissolution front to propagate slower in the acidization dissolution system. Copyright © 2012 John Wiley & Sons, Ltd.     

鄂尔多斯盆地北部二叠系下石盒子组致密砂岩成岩作用及孔隙成因

鄂尔多斯盆地上古生界分布有大面积致密砂岩储层.鄂北地区杭锦旗探区和大牛地气田储层的埋藏—成岩作用具较大差异性.两者的对比性研究有助于深刻理解致密砂岩的成储过程.运用铸体薄片和扫描电镜观察等储层测试方法明确了两者储层成岩作用和孔隙发育特征.研究表明:埋深差异、断裂作用和后期抬升作用导致两者储层的成岩阶段分别为中成岩A1期...