CN110286421B - A method for modeling natural fractures in tight sandstone reservoirs - Google Patents
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Abstract
本发明是一种针对致密砂岩储层中天然裂缝的建模方法，包括以下步骤：1)识别单井裂缝与断层破碎带内部结构；2)基于沉积相控与断裂地震解释，采用确定性与多种随机性算法相结合的建模方法，建立岩相‑构造耦合下基质地质模型；3)在岩相‑构造耦合基质地质模型约束下，采用序贯指示模拟结合垂向概率体的趋势建模方法，建立三维非均匀岩石力学模型；4)对三维非均匀岩石力学模型进行有限元数值模拟，获取三维空间应力场模型；5)通过建立应力场与裂缝参数间关系，采用离散裂缝网络建模的方法建立裂缝模型。本发明能够准确的对天然裂缝在地下的空间展布进行很好的描述，可广泛适用于非常规储层裂缝建模领域。
The present invention is a modeling method for natural fractures in tight sandstone reservoirs, comprising the following steps: 1) identifying single well fractures and the internal structure of the fault fracture zone; 2) based on sedimentary facies control and fracture seismic interpretation, using deterministic and A modeling method that combines a variety of random algorithms to establish a matrix geological model under the lithofacies-structure coupling; 3) Under the constraints of the lithofacies-structure coupled matrix geological model, the sequential indication simulation combined with the trend of the vertical probability volume is used to construct the model. 3-D non-uniform rock mechanics model is established by using the model method; 4) Finite element numerical simulation is performed on the 3-D non-uniform rock mechanics model to obtain a 3-D spatial stress field model; 5) By establishing the relationship between the stress field and fracture parameters, the discrete fracture network The model of the crack is established by the method of the model. The invention can accurately describe the underground spatial distribution of natural fractures, and can be widely used in the field of unconventional reservoir fracture modeling.
Description
技术领域technical field
本发明涉及非常规储层表征与建模领域，是一种针对致密砂岩储层中天然裂缝的建模方法。The invention relates to the field of unconventional reservoir characterization and modeling, and is a modeling method for natural fractures in tight sandstone reservoirs.
背景技术Background technique
致密砂岩储层中，孔喉半径小，孔隙空间有限，天然裂缝成为这类非常规储层中油气运移与储集的通道与场所。因此，明确天然裂缝在地下储层中的空间分布是核心内容，对天然裂缝进行建模是最为立体直观展现天然裂缝空间分布的手段，同时，为后期进行油藏数值模拟建立准确的裂缝-基质模型。天然裂缝建模一直是世界上存在的难题，时至今日，通过地球物理手段对地下天然裂缝进行识别与建模是最为直接的方法。但由于裂缝存在多尺度的问题，从地球物理手段进行建模存在局限于单井，同时空间上仅能对较大尺度的裂缝进行识别的问题，本文提出一种基于断层破碎带地质概念模式指导下，从裂缝发育力学成因入手，借助有限元数值模拟的方法，通过模拟应力场的空间分布，建立离散裂缝网络模型的方法。是一种从机理角度，建立真实地层下天然裂缝网络模型的方法。In tight sandstone reservoirs, the pore throat radius is small and the pore space is limited. Natural fractures become the channels and places for oil and gas migration and accumulation in such unconventional reservoirs. Therefore, clarifying the spatial distribution of natural fractures in underground reservoirs is the core content, and modeling natural fractures is the most three-dimensional and intuitive means to display the spatial distribution of natural fractures. Model. Natural fracture modeling has always been a difficult problem in the world. Up to now, it is the most direct method to identify and model underground natural fractures through geophysical means. However, due to the multi-scale problem of fractures, the modeling from geophysical methods is limited to a single well, and only large-scale fractures can be identified in space. This paper proposes a model based on the geological concept of fault fracture zone guidance. Next, starting from the mechanical causes of fracture development, with the help of finite element numerical simulation method, by simulating the spatial distribution of stress field, a method of establishing a discrete fracture network model. It is a method to establish a real subterranean fracture network model from a mechanism point of view.
进行有限元数值模拟，准确的非均质岩石力学模型是关键。从沉积-构造同时控制储层形成角度入手，首先建立岩相-构造耦合下的基质地质模型，对非均质岩石力学模型进行约束。致密砂岩储层中，天然裂缝往往发育在断层等构造部位区域，因此，断层破碎带的内部结构制约着天然裂缝的空间发育；同时，在远离断层区域，天然裂缝的发育受控于岩石力学性质的差异性，受岩相控制作用明显。以往的模型中，更多注重将沉积或岩相的空间分带性刻画的较为详细，断层仅作为一个二维的存在。本次发明中，将断层考虑成体，将断层破碎带的分带性与岩相分布的分带性进行耦合，建立岩相-构造耦合下的基质地质模型。For finite element numerical simulation, an accurate heterogeneous rock mechanics model is the key. From the point of view of simultaneously controlling reservoir formation by deposition and structure, the matrix geological model under the coupling of lithofacies and structures is first established to constrain the mechanical model of heterogeneous rock. In tight sandstone reservoirs, natural fractures often develop in structural areas such as faults. Therefore, the internal structure of the fault fracture zone restricts the spatial development of natural fractures; at the same time, in areas far from faults, the development of natural fractures is controlled by the mechanical properties of the rock. The difference is obviously controlled by lithofacies. In previous models, more attention was paid to describing the spatial zoning of sediments or lithofacies in more detail, and faults only existed as a two-dimensional. In this invention, the fault is considered as an adult body, and the zoning of the fault fracture zone and the zoning of the lithofacies distribution are coupled to establish a matrix geological model under the coupling of lithofacies and structures.
在传统连续性属性建模过程中，往往是基于沉积相控下的序贯高斯模拟算法，存在相带边界条件控制范围过大，垂向分辨率较低的问题。在本次发明中，是在岩相-构造耦合下的基质地质模型约束下，将平面模拟效果较好的序贯高斯算法，和垂向模拟效果较好的垂向概率体方法，通过趋势建模的方法进行整合，建立起准确的非均质岩石力学模型。In the traditional continuous attribute modeling process, it is often based on the sequential Gaussian simulation algorithm under the control of depositional facies, and there are problems that the control range of the facies zone boundary conditions is too large and the vertical resolution is low. In this invention, under the constraints of the matrix geological model under the coupling of lithofacies and structures, the sequential Gaussian algorithm with better plane simulation effect and the vertical probability volume method with better vertical simulation effect are constructed through trend construction. The method of modeling is integrated to establish an accurate heterogeneous rock mechanics model.
以往对地质模型进行有限元数值模拟，其地质模型多为均一岩石力学分布，或仅断层处做特殊赋值的方法进行。本文将前期获得的三维非均质岩石力学模型通过网格赋值法进行力学模型的建立，模拟了非均质储层状态下应力场的空间分布，更为真实的反应了地下应力的空间分布状态。In the past, the finite element numerical simulation of the geological model was carried out by the method of homogeneous rock mechanics distribution, or only by the method of special assignment at the fault. In this paper, the three-dimensional heterogeneous rock mechanical model obtained in the previous stage is established by the grid assignment method to simulate the spatial distribution of the stress field under the state of the heterogeneous reservoir, which more truly reflects the spatial distribution of the underground stress. .
将岩石破裂时释放的应变能与岩石破裂时新增的表面积通过能量守恒建立联系，进而对裂缝密度等参数进行基于应力场的空间计算，最后采用离散裂缝网络建模方法建立裂缝模型。该方法真正从裂缝形成力学机理角度出发，对裂缝空间分布进行刻画。The strain energy released when the rock fractures is connected with the newly added surface area when the rock fractures through energy conservation, and then the parameters such as fracture density are calculated based on the stress field, and finally the fracture model is established by using the discrete fracture network modeling method. This method truly characterizes the spatial distribution of cracks from the perspective of the mechanical mechanism of crack formation.
发明内容SUMMARY OF THE INVENTION
本发明的目的是为了建立一种对沉积和构造共同控制下所形成的强非均质储层，进行基于应力场模拟的裂缝建模方法。能够更为准确的描述地下天然裂缝的空间分布，建立起更符合实际地质情况的裂缝模型。The purpose of the present invention is to establish a fracture modeling method based on stress field simulation for the strong heterogeneous reservoir formed under the joint control of deposition and structure. It can more accurately describe the spatial distribution of underground natural fractures, and establish a fracture model that is more in line with actual geological conditions.
本发明通过以下步骤实现：The present invention realizes through the following steps:
步骤一，识别裂缝与断层破碎带内部结构。通过岩心和测井对单井裂缝和断裂带内部结构进行定量描述，确定裂缝密度参数和断裂带内部结构样式。获取裂缝密度参数主要依据岩心上的观察与测量；断裂带内部结构的刻画主要依据白云岩化作用的强弱，结合裂缝密度进行判断。Step 1: Identify the internal structure of fractures and fault fracture zones. Quantitatively describe the internal structure of single-well fractures and fault zones through core and logging, and determine the fracture density parameters and internal structural styles of the fault zone. Obtaining the fracture density parameters is mainly based on the observation and measurement on the core; the characterization of the internal structure of the fault zone is mainly based on the strength of dolomitization, combined with the fracture density.
步骤二，构建沉积-构造耦合下的基质地质模型。构造方面，不同于将断层建成面，而是建成体。按照识别的断层破碎带内部结构，建立断层破碎带模型；沉积方面，考虑岩相差异是造成岩石力学空间差异的主要因素，建立沉积相带控制下的岩相模型。该模型的建立一改以往构造模型断层处的处理方式，真正还原了模型的真实状态，将构造引起的分带性与沉积引起的分带性进行耦合，反映真实储层非均质性特征。The second step is to construct a matrix geological model under the coupling of deposition and structure. In terms of structure, it is different from building a fault into a surface, but a built body. According to the internal structure of the identified fault fracture zone, the fault fracture zone model is established; in terms of sedimentation, considering that the lithofacies difference is the main factor causing the spatial difference of rock mechanics, a lithofacies model under the control of the sedimentary facies zone is established. The establishment of this model changes the way of dealing with faults in previous structural models, truly restores the true state of the model, couples the zoning caused by the structure and the zoning caused by the deposition, and reflects the real reservoir heterogeneity.
步骤三，建立三维非均质岩石力学模型。在沉积-构造耦合下的基质地质模型约束下，采用多种随机模拟算法，建立三维非均质岩石力学模型。平面上，采用序贯高斯模拟算法，分不同沉积-构造耦合相带进行约束，取多次模拟后的算术平均，进行平面模型的建立；垂向上，采用概率体的建模方法，对不同岩石力学属性(密度、泊松比和杨氏模量)建立概率分布进行约束，增加模型垂向上的分辨率；采用趋势建模的方法将序贯指示与概率体进行融合，建立能够反映真实地下情况的三维非均质岩石力学模型。The third step is to establish a three-dimensional heterogeneous rock mechanics model. Under the constraints of the matrix geological model under the coupling of deposition and structure, a variety of stochastic simulation algorithms are used to establish a three-dimensional heterogeneous rock mechanics model. On the plane, the sequential Gaussian simulation algorithm is used to constrain different sedimentary-tectonic coupling facies belts, and the arithmetic average after multiple simulations is taken to establish the plane model; vertically, the modeling method of probability volume is used to analyze different rocks. The mechanical properties (density, Poisson's ratio and Young's modulus) are constrained by probability distribution to increase the vertical resolution of the model; the trend modeling method is used to fuse the sequential indication with the probability volume to establish a model that can reflect the real underground conditions. 3D heterogeneous rock mechanics model.
步骤四，对三维非均质岩石力学模型进行有限元数值模拟。在岩石力学模型的基础上，依次经过网格划分、载荷施加、位移约束，最后进行有限元求解，获取空间应力场的分布。In the fourth step, finite element numerical simulation is performed on the three-dimensional heterogeneous rock mechanics model. On the basis of the rock mechanics model, through mesh division, load application, displacement constraint, and finally the finite element solution is carried out to obtain the distribution of the spatial stress field.
步骤五，基于应变能与表面能理论，运用能量守恒的原理，将应力场与裂缝密度建立联系。将求解计算获得的密度作为输入参数，采用离散裂缝网络建模方法建立裂缝模型。Step 5: Based on the theory of strain energy and surface energy, using the principle of energy conservation, establish a relationship between the stress field and the crack density. Taking the density obtained by the solution calculation as the input parameter, the fracture model is established by the discrete fracture network modeling method.
在上述技术方案中，所述步骤一中，断层破碎带的内部结构包括断层核部、两侧的诱导裂缝带以及原状地层。In the above technical solution, in the first step, the internal structure of the fault fracture zone includes the core of the fault, the induced fracture zones on both sides, and the original formation.
在上述技术方案中，所述步骤二中，对断层体的建立采用分段建模的方法，中间用面进行边界划分，并对断层体区域网格进行加密处理；非断层区域采用相控的方式，采用沉积微相边界条件去控制岩相的分布。In the above technical solution, in the second step, the segmental modeling method is used for the establishment of the fault body, the boundary is divided by the middle plane, and the grid of the fault body area is refined; the non-fault area adopts the phase control method, The sedimentary microfacies boundary conditions are used to control the distribution of lithofacies.
在上述技术方案中，所述步骤三中，所有随机模拟方法均基于密井网的单井岩石力学解释，首先进行序贯指示算法模拟建模，再进行概率体的建模，最后将两个模型，用趋势建模中的多元线性回归方法进行融合。In the above technical solution, in the third step, all stochastic simulation methods are based on the single well rock mechanics interpretation of the dense well pattern. First, the sequential instruction algorithm simulation modeling is performed, and then the probability volume modeling is performed. Finally, the two The model was fused using the multiple linear regression method in trend modeling.
在上述技术方案中，所述步骤四中，运用的大型有限元软件Ansys中的MechanicalAPDL模块，很好的将地质模型、力学模型与数学模型有机结合在一起，实现了对复杂强非均质地质体的有限元求解。In the above technical solution, in the fourth step, the MechanicalAPDL module in the large-scale finite element software Ansys is used, which organically combines the geological model, the mechanical model and the mathematical model, and realizes the complex and strong heterogeneous geological model. Finite element solution of the volume.
在上述技术方案中，所述步骤五中，由于研究区既存在张裂缝又存在剪裂缝，不同裂缝类型遵循不同的破裂准则，张裂缝遵循格里菲斯破裂准则，剪裂缝遵循库伦-摩尔破裂准则。因此在计算裂缝密度时，分不同的裂缝类型进行计算。In the above technical solution, in the fifth step, since there are both tension cracks and shear cracks in the study area, different types of cracks follow different fracture criteria, tension cracks follow the Griffith fracture rule, and shear cracks follow Coulomb-Moore fracture guidelines. Therefore, when calculating the fracture density, different fracture types are calculated.
本发明致密砂岩储层天然裂缝建模方法，具有以下效益成果：真正将沉积与构造共同控制下的储层非均质性进行了建模，一改以往仅将构造断层简化成面的思想，准确构建了断层破碎带的内部结构，并与非断层区域沉积控制下的岩相模型进行耦合，更加合理的描述了储层的非均质性，为后期进行有限元数值模拟提供了更为准确的岩石力学模型。同时，从裂缝形成力学机理角度入手，运用岩石破裂瞬间释放的能量与裂缝新增表面积之间的关系，计算获取空间裂缝密度分布。从成因入手，更好的表征了各类尺度下裂缝的分布，增加了储层裂缝预测与建模的精度，有助于后期水力压裂与井位部署。The method for modeling natural fractures in tight sandstone reservoirs of the present invention has the following beneficial results: the reservoir heterogeneity under the joint control of deposition and structure is truly modeled, and the previous idea of only simplifying structural faults into planes is changed, The internal structure of the fault fracture zone is accurately constructed, and coupled with the lithofacies model under the control of the non-fault area deposition, the heterogeneity of the reservoir is more reasonably described, and it provides a more accurate finite element numerical simulation in the later stage. rock mechanics model. At the same time, starting from the mechanical mechanism of fracture formation, the relationship between the energy released instantaneously by the rock fracture and the newly added surface area of the fracture is used to calculate the spatial fracture density distribution. Starting from the cause, the distribution of fractures at various scales is better characterized, the accuracy of reservoir fracture prediction and modeling is increased, and it is helpful for later hydraulic fracturing and well location deployment.
附图说明Description of drawings
图1为一种致密砂岩储层天然裂缝建模方法流程示意图；1 is a schematic flowchart of a method for modeling natural fractures in tight sandstone reservoirs;
图2为本发明致密砂岩储层天然裂缝建模方法中断层破碎带地质概念模型；Fig. 2 is the geological conceptual model of the fracture fracture zone of the natural fracture modeling method of the tight sandstone reservoir according to the present invention;
图3为本发明致密砂岩储层天然裂缝建模方法中构建的沉积-构造耦合基质地质模型；3 is a sedimentary-structural coupled matrix geological model constructed in the method for modeling natural fractures in tight sandstone reservoirs of the present invention;
图4为本发明致密砂岩储层天然裂缝建模方法中在图3约束下建立的三维非均质岩石力学模型；Fig. 4 is a three-dimensional heterogeneous rock mechanics model established under the constraints of Fig. 3 in the method for modeling natural fractures of tight sandstone reservoirs according to the present invention;
图5为本发明致密砂岩储层天然裂缝建模方法中对图4进行有限元数值模拟获取应力云图；Fig. 5 is a finite element numerical simulation of Fig. 4 to obtain a stress cloud map in the method for modeling natural fractures of tight sandstone reservoirs according to the present invention;
图6为本发明致密砂岩储层天然裂缝建模方法中基于图5模拟结果，通过建立应力场与裂缝密度间关系获取的裂缝密度模型；6 is a fracture density model obtained by establishing the relationship between stress field and fracture density based on the simulation result of FIG. 5 in the method for modeling natural fractures in tight sandstone reservoirs of the present invention;
图7本发明致密砂岩储层天然裂缝建模方法中建立的离散裂缝网络模型；7 is a discrete fracture network model established in the method for modeling natural fractures in tight sandstone reservoirs of the present invention;
具体实施方式Detailed ways
下面结合附图及本发明所应用的实作进一步的详细描述，但该实例不应作为对本次发明的限制。The following is a further detailed description in conjunction with the accompanying drawings and the implementation of the present invention, but this example should not be used as a limitation to the present invention.
图1为一种致密砂岩储层天然裂缝建模方法流程示意图，如图1所示，该方法包括如下步骤：Figure 1 is a schematic flow chart of a method for modeling natural fractures in tight sandstone reservoirs. As shown in Figure 1, the method includes the following steps:
步骤一，识别裂缝与断层破碎带内部结构；Step 1: Identify the internal structure of fractures and fault fracture zones;
步骤二，构建沉积-构造耦合基质地质模型；Step 2, build a sedimentary-structural coupled matrix geological model;
步骤三，建立三维非均质岩石力学模型；Step 3, establish a three-dimensional heterogeneous rock mechanics model;
步骤四，进行有限元数值模拟；Step 4, carry out finite element numerical simulation;
步骤五，建立应力场与裂缝密度间关系；Step 5: Establish the relationship between stress field and crack density;
步骤六，进行离散裂缝网络建模；Step 6, carry out discrete fracture network modeling;
接下来结合具体实例，对上述每一步骤进行解释说明。Next, each of the above steps will be explained with reference to specific examples.
步骤一，根据实地野外露头考察，取心井岩心观测，测井与地震响应特征的研究，断层破碎带内部结构如图2所示；划分出了典型的“二元结构”，包括位于中间的断层核部，两侧的诱导裂缝带，诱导裂缝带外侧为原状地层区域。结合露头、取心井与测井裂缝解释资料，在断层核部区域，岩层破碎程度较大，裂缝发育密度中等；在诱导裂缝带区域，岩层未发生较明显破碎，裂缝发育密度较大；在原状地层部位，根据距离断层滑移面的距离，裂缝密度成反比关系，同时在不同岩性下，裂缝的发育程度也不同，呈现岩石颗粒粒度越细，裂缝发育程度越好的现象，且砂岩发育裂缝情况整体好于泥岩。Step 1: According to field outcrop inspection, core observation of coring wells, and research on logging and seismic response characteristics, the internal structure of the fault fracture zone is shown in Figure 2; a typical "binary structure" is divided, including The core of the fault, the induced fracture zone on both sides, and the outer side of the induced fracture zone is the undisturbed stratigraphic area. Combining the data of outcrop, coring well and logging fracture interpretation, in the core area of the fault, the rock formation is more fragmented, and the fracture density is moderate; in the induced fracture zone, the rock formation is not significantly fractured, and the fracture density is high; In the original stratum, according to the distance from the fault slip plane, the fracture density is inversely proportional. At the same time, under different lithologies, the development degree of fractures is also different. The finer the particle size of the rock, the better the fracture development degree. The development of fractures is generally better than that of mudstone.
步骤二，用两个边界对断层破碎带的内部分带结构进行划分，同时与沉积相的边界进行耦合，采用确定性建模方法，建立起完整的沉积-构造耦合下组合模型。非断层区域，分五种沉积相类型(河道、辫状河道间、河道前缘砂坝、朵叶体、半深湖相)，采用序贯指示模拟算法，进行沉积相控下的岩相模型的建立，主要划分了六种岩相类型，分别是：泥岩、粉砂岩、细砂岩、中细砂岩、中粗砂岩和砂砾岩，最终建立沉积-构造耦合基质地质模型图3。In step 2, two boundaries are used to divide the internal zoning structure of the fault fracture zone, and at the same time, it is coupled with the boundary of the sedimentary facies, and a deterministic modeling method is used to establish a complete combination model of sedimentary-tectonic coupling. The non-fault area is divided into five types of sedimentary facies (channel, braided channel, channel front sand bar, lobe, semi-deep lacustrine facies), and the lithofacies model under the control of sedimentary facies is carried out by using sequential indication simulation algorithm The establishment of lithofacies is mainly divided into six types of lithofacies, namely: mudstone, siltstone, fine sandstone, medium-fine sandstone, medium-coarse sandstone and glutenite, and finally the sedimentary-structural coupling matrix geological model is established (Fig. 3).
步骤三，采用地质约束结合多种随机建模方法的形式建立岩石力学模型。在沉积-构造耦合基质地质模型的约束下，依次进行序贯高斯随机模拟，概率体建模和趋势建模。序贯高斯随机模拟采用多次模拟结果取算术平均的方法；概率体建模突出垂向分辨率高的优势建立垂向概率体；通过趋势建模的线性拟合，将序贯高斯随机模拟结果与概率体建模结果进行融合，建立平面、垂向精度更高的三维非均质岩石力学模型图4。The third step is to establish a rock mechanics model in the form of geological constraints combined with a variety of stochastic modeling methods. Under the constraints of the sedimentary-tectonic coupled matrix geological model, sequential Gaussian stochastic simulation, probabilistic volume modeling, and trend modeling are performed in sequence. The sequential Gaussian stochastic simulation adopts the method of taking the arithmetic mean of the results of multiple simulations; the probability volume modeling highlights the advantage of high vertical resolution to establish a vertical probability volume; through the linear fitting of the trend modeling, the sequential Gaussian stochastic simulation results are combined. Combined with the results of the probability volume modeling, a three-dimensional heterogeneous rock mechanics model with higher plane and vertical accuracy is established (Fig. 4).
步骤四，对建立的非均质岩石力学模型进行有限元数值模拟，获取应力场模型图5。首先进行网格划分，单元类型选择Soild187，对断层破碎带和破碎带与原状地层交界处进行小步长的精细划分，对其他区域选择较大步长的划分，以缩短整体运算时间；之后进行边界条件的施加，分为两部分，分别为载荷施加和位移约束；为模拟主要造缝期所受力状态，载荷施加上，对模型左上和右下边界施加32MPa力，左下和右上边界施加68MPa力；同时为了模拟所受到的右旋剪切应力，对模型的左上角和右下角施加一组垂直于最大主应力方向的剪切力9MPa；同时施加自身重力，重力加速度取9.8m/s2；位移约束上，为防止模型施加载荷后发生刚体位移，对模型的底面施加Z方向的位移约束，并对模型的左角与右角施加X，Y方向的位移约束。Step 4, perform finite element numerical simulation on the established heterogeneous rock mechanics model, and obtain the stress field model as shown in Figure 5. First perform grid division, select Soild187 for the unit type, finely divide the fault broken zone and the junction between the broken zone and the undisturbed stratum with small steps, and select larger step size for other areas to shorten the overall operation time; The application of boundary conditions is divided into two parts, namely load application and displacement constraint; in order to simulate the stress state during the main cracking period, when the load is applied, 32MPa is applied to the upper left and lower right boundaries of the model, and 68MPa is applied to the lower left and upper right boundaries. At the same time, in order to simulate the right-handed shear stress, a set of shear forces of 9MPa perpendicular to the direction of the maximum principal stress are applied to the upper left and lower right corners of the model; at the same time, self-gravity is applied, and the acceleration of gravity is 9.8m/s 2 ; On the displacement constraint, in order to prevent rigid body displacement after the model is loaded, a displacement constraint in the Z direction is applied to the bottom surface of the model, and a displacement constraint in the X and Y directions is applied to the left and right corners of the model.
步骤五，建立应力场与裂缝密度间关系，计算获取裂缝密度模型图6。有限元数值模拟后可以获得整个模型的应力、应变和能量的空间分布。利用岩石在破裂前在体内会积聚能量，破裂时会释放应变能用于抵消新增裂缝表面积之间的关系，进而进行裂缝密度的计算。其计算过程主要分为两步，首先计算积聚在岩石内部的应变能密度：Step 5, establish the relationship between the stress field and the crack density, and calculate and obtain the crack density model as shown in Figure 6. After the finite element numerical simulation, the spatial distribution of stress, strain and energy of the whole model can be obtained. The fracture density is calculated by utilizing the energy accumulated in the body of the rock before fracture, and the strain energy released when fractured is used to offset the relationship between the surface areas of the newly added fractures. The calculation process is mainly divided into two steps. First, the strain energy density accumulated in the rock is calculated:
式中，
μ为泊松比，无量纲；μ is Poisson’s ratio, dimensionless;
σ1,σ2,σ3分别为最大、中间和最小主应力，MPa；σ 1 , σ 2 , σ 3 are the maximum, intermediate and minimum principal stresses, respectively, in MPa;
岩石发生脆性破裂的时候会释放应变能，用于抵消新增裂缝表面能和弹性波能的逸散，忽略微小的弹性波能，运用能量守恒定律，裂缝密度为：When the rock is brittle fracture, the strain energy will be released to offset the new fracture surface energy and the dissipation of elastic wave energy, ignoring the tiny elastic wave energy, using the law of conservation of energy, the fracture density is:
式中，Dvf为裂缝体密度，m2/m3；where D vf is the fracture density, m 2 /m 3 ;
J为单位面积裂缝产生所需要能量，即裂缝表面能，J/m2；J is the energy required to generate cracks per unit area, namely the crack surface energy, J/m 2 ;
Sf为新形成裂缝表面积，m2；S f is the newly formed fracture surface area, m 2 ;
a,b为相关系数。a and b are the correlation coefficients.
步骤六，进行离散裂缝网络建模图7。将计算获取的裂缝密度模型作为输入参数，裂缝形态设置为边数为4，延伸率2的长方形面片状，其中裂缝长度服从Power law；裂缝产状设置，方位服从应力场模拟水平最大主应力方向，倾角服从于Kent模型，基于岩心统计，均值倾角设为78°，集中度设为40；基于岩心和薄片裂缝开度数据，设置裂缝开度服从Powerlaw，裂缝渗透率设置与开度相关；裂缝空间分布服从裂缝密度模型；随机种子点设置17861，建立离散裂缝网络模型。Step 6, conduct discrete fracture network modeling (Fig. 7). The fracture density model obtained by calculation is used as the input parameter, and the fracture shape is set as a rectangular facet with 4 sides and an elongation rate of 2, in which the fracture length obeys the Power law; the fracture occurrence is set, and the orientation obeys the maximum principal stress of the stress field simulation level. The direction and dip angle obey the Kent model. Based on core statistics, the mean dip angle is set to 78°, and the concentration degree is set to 40; based on the fracture opening data of cores and thin slices, the fracture opening is set to obey Powerlaw, and the fracture permeability setting is related to the opening; The spatial distribution of fractures obeys the fracture density model; the random seed point is set to 17861 to establish a discrete fracture network model.
本发明所建立的天然裂缝模型，能够更为准确的表征强非均质储层中天然裂缝的空间分布。并且，从裂缝力学形成机理角度入手，很好体现了沉积与构造的耦合作用对天然裂缝空间分布的影响。从而为后期水力压裂，井位部署，提供准确的天然裂缝模型。The natural fracture model established by the present invention can more accurately characterize the spatial distribution of natural fractures in strongly heterogeneous reservoirs. Moreover, from the perspective of fracture mechanics formation mechanism, the influence of the coupling of deposition and structure on the spatial distribution of natural fractures is well reflected. In this way, an accurate natural fracture model is provided for later hydraulic fracturing and well location deployment.
说明书中所述内容及具体事例，相关领域人员可以对本发明进行任何改动与变型，并不用限定本发明的限定范围。未脱离本发明精神和范围，进行的修改、变型等，则本发明也包含这些修改与变型在内。The content and specific examples described in the specification, those in the relevant art can make any changes and modifications to the present invention, which are not intended to limit the limited scope of the present invention. Modifications, variations, etc. made without departing from the spirit and scope of the present invention are also included in the present invention.
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