基于三维数字岩心的碳酸盐岩电阻率数值模拟研究

Archie equations have been widely applied in the evaluation of water saturation of homogeneous clastic. However, they are no more applicable for carbonate formation because of its heterogeneity and anisotropy. Carbonate resistivity experiments are not sufficient to investigate conductive mechanisms because the cores from fracture and vug interval are easily broken. Firstly, 3-D images with low resolution obtained by X-ray CT are used to gather the information of vug and fracture. 3-D structures of matrix pore are reconstructed by multi-points geostatistics (MPS) from 2-D SEM images of rock with high resolution. 3-D digital core with dual pore system is built by multi scale fusion. The effects of pore structure on carbonate resistivity are analyzed qualitatively based on 3-D digital cores and experimental results of resistivity. Secondly, the resistivities of carbonate are calculated by the finite element method and mathematical morphology and the algorithm is thus validated by comparing the measured resistivities with simulated results. Finally, the characteristic information of fractures and vugs are obtained from 3-D digital cores derived from X-ray CT experiments. The 3-D digital cores with dual pore systerm are synthesized by stochastic method, where fracture porosity, vuggy porosity, matrix porosity, fracture dip and wettability can be adjusted according to the research requirments. The effect of those factors on electrical properties are unraveled quantitatively using numerical simulation. And then the coupling style of conductive networks composed of different pore systerm are discussed. In this study we apply the technology of 3-D digital cores to heterogeneous carbonate reservoir to reveal its conductivity mechanism. This will greatly improve the evaluation of water saturation for carbonate.

碳酸盐岩的非均质性和各向异性导致经典的饱和度评价模型-阿尔奇公式不能直接应用。裂缝、孔洞发育段岩心破损严重，仅采用岩电实验难以研究其导电机理。首先，采用X射线CT建立岩心低分辨率三维图像，反映裂缝和孔洞信息。以高分辨率扫描电镜图像为基础，采用多点地质统计学重构基质孔隙的三维结构。通过尺度融合建立双孔隙三维数字岩心，并结合岩电实验结果，分析孔隙结构对岩石电阻率的影响规律。然后，采用数值模拟算法计算岩石电阻率，并与实验结果比较，验证算法的准确性。最后，从X射线CT建立的三维数字岩心中提取裂缝和孔洞的特征信息，随机生成具有不同裂缝孔隙度、孔洞孔隙度、基质孔隙度、裂缝倾角、润湿性等参数的双孔隙三维数字岩心。采用数值模拟方法研究孔隙结构对碳酸盐岩岩电参数的影响规律，分析不同孔隙系统的电阻率耦合关系。本研究将三维数字岩心技术拓展至非均质的碳酸盐岩储层，进而揭示其导电机理，有助于改进储层饱和度评价。

碳酸盐岩储层的孔隙空间具有较强的非均质性，导致岩电参数取值范围广，因此有必要定量、系统地研究碳酸盐岩的电阻率特性。由于裂缝和孔洞发育的层段取芯困难，并且岩电实验中岩石微观孔隙结构、流体分布状态等微观因素无法直接观测和控制，所以仅通过岩电实验难以揭示碳酸盐岩的电阻率变化规律。基于数字岩心的岩石物理数值模拟已发展为岩石物理实验的有益补充。.首先，采用低分辨率X射线CT扫描大尺寸碳酸盐岩样品，建立毫米和微米尺度孔隙三维模型。然后，采用多点统计法在高精度SEM图像基础上重建微孔隙三维模型，并计算其等效电导率。最后，以CT建立的三维数字岩心为基础，设岩石骨架部分电导率等于微孔隙等效电导率，采用有限元方法计算电阻率。由于耦合了不同尺寸孔隙系统的电流传导作用，数值模拟结果与实验结果吻合,提高电阻率数值模拟精度。.碳酸盐孔隙结构复杂，选取孔喉比（孔隙体与喉道半径的比值）作为孔隙特征参数。保持孔隙度不变，孔喉比由1起始逐渐增大，则构建的三维数字岩心孔隙空间由裂缝过渡到孔隙，最后发展为孔洞。电阻率数值模拟结果表明胶结指数m值主要受孔隙结构的影响，由裂缝型的m=1.3逐渐增大到孔洞型的m=3。若增大孔隙度，孔洞数量增多，连通性变好，胶结指数m则递减到2。采用连续随机附加法建立具有不同裂缝特征参数的三维数字岩心，电阻率数值模拟结果表明饱和度指数n值主要受裂缝和润湿性的影响。当裂缝开度小于基质孔隙尺寸时，裂缝对n值影响较小；当裂缝开度较大时，出现明显的非阿尔奇现象，高含水饱和度下，电阻增大率I急剧增大,n值较高，低含水饱和度下，I增速放缓，n值减小。数值模拟结果还表明油湿碳酸盐岩储层n值明显高于水湿储层。本研究明确了阿尔奇参数的分布特点和变化规律，为拓展阿尔奇方程在碳酸盐岩储层的应用提出了可行的思路。.砂砾岩等非均质性强的沉积岩矿物组分复杂， 且X射线CT识别孔隙不连通，无法直接模拟电阻率。首先，结合CT和SEM-EDS测试结果，建立多矿物组分三维数字岩心。然后，采用高精度SEM分析CT未识别微孔隙的分布规律，进而确定不同矿物组分的等效电阻率。最后，采用有限元方法模拟多组分三维数字岩心电阻率，模拟结果与实验结果基本吻合，建立了适用于致密性岩石的电阻率数值模拟方法。电阻率数值模拟结果说明：相比孔隙度和粘土含量宏观参数，孔隙尺寸分布及其连接形式、粘土类型及其分布对m和n值的影响更为显著。

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