Characterization method of pore throat structure in dense sandstone based on NMR and CMP experiments
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摘要:
致密砂岩储层孔隙结构复杂、纳米孔隙发育，需集成多种技术对孔隙结构进行综合表征，以更好地认识储层。在优选6块延长组长71储层代表性岩心基础上，采用场发射扫描电子显微镜（FESEM）、恒速压汞（CMP）和核磁共振（NMR）等方法，研究了岩心样品的孔隙类型及结构特征。采用CMP数据对NMR孔隙分布进行了修正，识别了喉道半径与孔隙半径的分布范围，建立了适用于致密砂岩的孔隙半径分类方法。研究结果表明，目标储层可动水与不可动水孔隙度之比仅为0.14～0.47，渗流能力差。将NMR与CMP数据相结合可精确识别出目标储层喉道半径中值为0.151～0.525 μm，孔隙半径中值为4.38～9.76 μm。孔隙内赋存水类型分为可动水、束缚水和黏土结合水，对应的饱和度平均值分别为23.4%、14.8%和9.4%。微小孔（
T 2<T 2c1）、中孔（T 2c1<T 2<T 2 c2）和大孔（T 2c2<T 2）的平均孔隙度分别为3.12%、3.42%和1.35%。孔喉半径r 2c1可作为储层渗流能力划分的评价指标，r 2c1的降低会导致微小孔（即吸附孔）孔隙度的降低，以及中孔和大孔（即渗流孔）孔隙度的增加。研究成果为优选致密砂岩优质储层，提高致密油采收率提供了参考和借鉴。Abstract:Objective The pore structure of tight sandstone reservoirs is complex, featuring the presence of nanopores, which is essential to integrate multiple technologies for a systematic characterization of the pore structure to enhance the understanding of these reservoirs.
Methods Six representative cores from the Chang 71 Chang Yanchang reservoir were selected for analysis. The pore types and structural characteristics of the core samples were examined using field emission scanning electron microscopy (FESEM), constant rate mercury injection (CMP), and nuclear magnetic resonance (NMR). The NMR pore distribution was adjusted based on CMP data, allowing for the identification of distribution ranges for the throat and pore radii, and a pore size classification method tailored for tight sandstone was developed.
Results Findings indicate that the ratio of the movable water porosity to the immovable water porosity of the target reservoir is only 0.14-0.47, indicating poor seepage capacity. The integration of NMR and CMP data enabled accurate characterization of the reservoir, identifying a median throat radius of 0.151-0.525 μm and a median pore radius of 4.38-9.76 μm. The pore types include mobile water, bound water, and clay-bound water, with average saturation values of 23.4%, 14.8%, and 9.4%, respectively. The average porosities of the small pores (
T 2 <T 2c1), medium pores (T 2c1 <T 2 <T 2c2), and large pores (T 2c2 <T 2) were found to be 3.12%, 3.42%, and 1.35%, respectively. The parameterr 2c1 serves as an evaluation index for the classification of reservoir seepage capacity. A decrease inr 2c1 leads to a decrease in the porosity of small pores (i.e., adsorption pores) and an increase in the porosity of medium and large pores (i.e., seepage pores).Conclusion The research findings offer valuable insights for the selection of high-quality tight sandstone reservoirs and the enhancement of tight oil recovery.
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图 1 实验岩心SEM及薄片分析图像
A. 粒间孔，铸体薄片，1号岩心；B. 长石溶蚀后形成的蜂窝状溶蚀孔，铸体薄片，1号岩心；C. 残余粒间孔和粒内孔，铸体薄片，2号岩心；D. 石英和绿泥石填充的粒间孔，SEM，2号岩心；E. 粒间溶蚀孔，SEM，2号岩心；F. 粒内长石溶孔，SEM，3号岩心；G. 微裂缝，SEM，3号岩心；H. 黏土矿物内的晶间微孔，SEM，5号岩心；I. 绿泥石与晶间微孔，以及树杈形孔喉，铸体薄片，6号岩心
Figure 1. Experimental core SEM and thin section analysis images
图 2 实验岩心在完全饱和水下的T2谱分布
Figure 2. T2 spectrum distribution of experimental core under fully saturated water
图 3 1号（a）和6号（b）岩心离心前后T2谱分布及T2截止值确定
Figure 3. T2 spectrum distribution and T2 cutoff determination before and after centrifugation of Core 1 (a) and Core 6 (b)
图 4 1号（a）和6号（b）岩心CMP毛管压力曲线
Figure 4. CMP capillary pressure curves for Core 1 (a) and Core 6 (b)
图 5 CMP获得实验岩心孔径（a）及孔喉比（b）分布曲线
Figure 5. Distribution curves of pore diameter (a) and pore throat ratio (b) for the experimental core were obtained through CMP
图 6 CMP孔径分布与T2谱分布的重合关系
Figure 6. Coincidence between CMP pore size distribution and T2 spectral distribution
图 7 CMP修正NMR数据后的孔隙与喉道半径分布（A、B、C点为拟合直线与NMR计算结果的交点）
Figure 7. Pore and throat radius distribution following CMP correction of NMR data
图 8 1号（a）和6号（b）岩心孔喉半径分布（O为黏土结合水与束缚水区域分界点）
Figure 8. Distribution of pore size and throat size in Core 1 (a) and Core 6 (b)
图 9 致密砂岩孔隙孔径分类方法（以1号岩心为例）
Figure 9. Classification method for pore size of dense sandstone (taking core 1 as an example)
图 10 不同类型孔隙孔隙度与渗透率的关系
Figure 10. Relationship between porosity and permeability of different types of pores
图 11 r2c1值与吸附孔和渗流孔孔隙度的关系
Figure 11. Relationship between the r2c1 value and the porosity of adsorption pores and seepage pores
表 1 实验岩心物性参数及矿物组成
Table 1. Experimental core physical parameters and mineral composition
岩心编号 物性参数 主要矿物类型及质量分数wB/% 孔隙度/% 渗透率/10−3μm2 石英 钾长石 斜长石 方解石 白云石 黏土矿物 其他矿物 1 7.89 0.0630 48.9 14.2 21.9 4.8 3.2 4.8 2.2 2 7.97 0.0580 40.3 17.4 21.2 8.2 2.2 7.3 3.4 3 9.32 0.0440 36.1 11.6 29.3 2.3 7.4 8.6 4.7 4 6.53 0.0260 26.2 21.1 18.4 8.8 11.1 10.8 3.6 5 8.58 0.0100 32.5 3.7 26.7 11.6 4.4 16.7 4.4 6 5.73 0.0036 34.0 9.1 27.3 0 6.3 23.3 0 均值 7.67 0.0340 36.3 12.9 24.1 6.0 5.8 11.9 3.1 注：孔隙度为氦气法测量；渗透率为脉冲衰减法测定；其他矿物包括黄铁矿、菱铁矿和金红石等 
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表 2 实验岩心NMR参数
Table 2. Experimental cores NMR parameters
岩心
编号T2截止值/
ms转换系数/
（μm·ms−1）可动水
孔隙度/%不可动水
孔隙度/%T2c1/
msT2c2/
ms孔隙度/% 贡饱和度/% 微小孔 中孔 大孔 可动水 束缚水 黏土
结合水1 13.2 0.083 1.94 5.28 0.86 684.52 2.24 4.04 1.61 33.5 11.2 13.4 2 17.2 0.072 2.26 4.86 1.32 232.18 2.61 3.57 1.71 29.3 19.1 7.7 3 9.4 0.088 2.42 5.81 1.15 368.35 2.42 4.13 1.34 24.4 15.7 10.3 4 20.6 0.059 1.37 4.46 2.68 164.43 3.48 2.87 1.54 18.8 20.2 7.7 5 24.4 0.036 1.28 5.49 5.12 89.63 4.14 2.73 1.02 18.4 13.5 5.8 6 32.8 0.052 0.62 4.32 4.67 113.23 3.84 3.18 0.87 15.7 8.8 11.6 均值 19.6 0.065 1.65 5.04 2.63 275.39 3.12 3.42 1.35 23.4 14.8 9.4 注：T2c1. 划分微小孔与中孔的弛豫时间；T2c2. 划分中孔与大孔的弛豫时间；下同
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表 3 实验岩心CMP孔隙结构参数及结合NMR修正后的参数
Table 3. CMP pore structure parameters and NMR-corrected parameters of experimental cores
岩心编号 CMP孔隙结构参数 NMR修正后的参数 阈压/MPa 总进汞饱和度/% 孔隙进汞饱和度/% 喉道进汞饱和度/% 孔隙半径中值/μm 喉道半径中值/μm 孔喉比 孔隙半径中值/μm 孔喉比 1 1.37 54.89 19.20 35.68 156.22 0.525 312.7 9.76 18.61 2 1.14 54.84 22.83 32.00 141.18 0.380 389.8 7.43 19.54 3 1.45 46.81 15.25 31.57 128.61 0.370 365.1 6.43 17.39 4 1.63 42.29 13.86 28.43 120.84 0.317 399.9 6.04 19.04 5 2.17 35.79 11.86 23.93 115.65 0.226 537.8 4.63 20.49 6 2.43 34.62 7.01 27.61 109.61 0.151 764.2 4.38 29.11 均值 1.70 44.87 15.00 29.87 128.69 0.328 461.6 6.45 20.70
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