Microscopic characteristics of ultra-low permeability reservoirs in the Shigang Oilfield of the Subei Basin and strategies for enhancing oil recovery
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摘要: 苏北盆地石港油田属低孔、特低渗砂岩油藏，油井自然产能低，采用水力压裂后注水开发可提高产油能力，但油田开发中呈现低采油速度、低采出程度和开发效果差的特点，开发矛盾加剧。因此，需明确低效开发原因，探究提高采收率对策，为提高石港油田开发效果提供理论依据。通过全岩矿物成分分析、气测岩心孔渗参数、岩心敏感性评价等方法，从岩石矿物组成、孔喉结构和岩石敏感性等方面分析了其储层的微观特性；通过油藏数值模拟以及室内岩心实验研究了压裂水驱后剩余油的分布特征；通过核磁共振在线驱替实验探究了提高采收率的对策。结果显示，储层岩心呈现出典型的低孔、特低渗特征，且在开发过程中具有一定的速度敏感性和水敏感性。压裂水驱后，岩心中剩余油主要分布在0.01~1 μm中小孔径的孔道中，使用表面活性剂驱及二次水驱将岩心中剩余油采收率提高了14.81%。储层渗透率特低、微孔隙和微裂缝发育、速敏、水敏等是其低效开发的主要原因。注水开发会导致岩石矿物膨胀、运移，增大流动阻力，所以区块经压裂水驱仅明显提高主流线上剩余油的采出程度，整体动用程度不高，剩余油仍有较多富集。建议采用化学驱及多轮次驱替以增强中小孔道中原油动用程度，进一步提升油田开发效果。Abstract: The Shigang Oilfield in the Subei Basin is characterized by low-porosity and ultra-low permeability sandstone reservoirs. The natural productivity of the oil wells is low, but hydraulic fracturing followed by water injection can improve oil production capacity. However, during the development of the oilfield, problems such as low oil recovery rates, low recovery factors, and poor development effectiveness have become apparent, exacerbating development challenges. Therefore, it is necessary to identify the reasons for inefficient development and explore strategies to enhance recovery, providing a theoretical basis for improving the development effectiveness of the Shigang Oilfield. Using methods such as whole-rock mineral composition analysis, gas-measured core porosity and permeability parameters, and core sensitivity evaluation, the micro characteristics of the reservoir were analyzed in terms of rock mineral composition, pore-throat structure, and rock sensitivity. Numerical reservoir simulations and laboratory core experiments were conducted to study the distribution characteristics of residual oil after fracturing water flooding. Strategies to enhance oil recovery were investigated through nuclear magnetic resonance online displacement experiments. The results showed that the reservoir cores exhibited typical low-porosity and ultra-low permeability characteristics, with certain velocity sensitivity and water sensitivity during development. After fracturing water flooding, residual oil in the cores was mainly distributed in pore channels with diameters ranging from 0.01 to 1 μm. The use of surfactant flooding and secondary water flooding increased the recovery rate of residual oil in the cores by 14.81%. The main reasons for inefficient development included ultra-low reservoir permeability, the development of micro pores and micro fractures, and sensitivity to velocity and water. Water injection development could cause rock mineral swelling and migration, increasing flow resistance. Therefore, after fracturing water flooding, only the recovery of residual oil along the main flow paths was significantly improved, with the overall utilization degree remaining low, and a considerable amount of residual oil still concentrated in the reservoir. It is recommended to use chemical flooding and multiple rounds of displacement to enhance the utilization of oil in medium and small pore channels, further improving the development effectiveness of the oilfield.
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图 1 苏北盆地石港油田石5井古新统阜宁组2段浅灰色粉砂岩心SEM图
a.石5-1岩心，片状裂隙发育；b.石5-2岩心，微孔发育，具有片状长石及不同粒径的石英；c.石5-4岩心，具有大孔和微裂隙；d.石5-5岩心，裂缝、微裂隙发育。
Figure 1. SEM images of shallow gray siltstone core in E1f2 layer of well Shi 5, Shigang Oilfield, Subei Basin
图 2 苏北盆地石港油田岩心压汞和T2曲线
Figure 2. Core mercury injection histogram and T2 curve of Shigang Oilfield, Subei Basin
图 3 苏北盆地石港油田岩心敏感性评价曲线
Figure 3. Core sensitivity evaluation curves of Shigang Oilfield, Subei Basin
图 4 苏北盆地石港油田石5断块剩余油分布
Figure 4. Overall remaining oil distribution of block Shi 5, Shigang Oilfield, Subei Basin
图 5 苏北盆地石港油田岩心饱和油及压裂水驱后的T2曲线
Figure 5. T2 curves of core about saturated oil and water flooding after fracturing of Shigang Oilfield, Subei Basin
图 6 苏北盆地石港油田岩心水驱后剩余油在各类孔径中的分布
Figure 6. Distribution of remaining oil in various types of pores after water flooding in cores from Shigang Oilfield, Subei Basin
图 7 低场核磁共振在线驱替实验流程示意
1.ISO泵; 2.蒸馏水; 3.管线; 4.六通阀; 5.模拟地层水; 6.THSB表面活性剂体系; 7.中间容器; 8.压力表; 9.低场核磁共振设备; 10.量筒
Figure 7. Flow diagram of low field NMR online displacement experiment
图 8 驱替模拟实验中岩心表面活性剂驱油曲线
Figure 8. Surfactant displacement curves of cores in displacement experiment
图 9 驱替模拟实验过程中T2曲线变化
Figure 9. Change of T2 curves in cores during displacement experiment
图 10 表面活性剂驱过程中岩心饱和度变化
Figure 10. Changes of saturation in cores during surfactant flooding
图 11 驱替实验不同阶段不同孔径中采收率变化
Figure 11. Recovery rate changes in different pore sizes at different stages during displacement experiment
表 1 苏北盆地石港油田石5井浅灰色粉砂岩岩心矿物成分
Table 1. Rock mineral composition of shallow gray siltstone cores in well Shi 5, Shigang Oilfield, Subei Basin
单位: % 岩心编号 石英 钾长石 斜长石 方解石 白云石 铁白云石 黏土 石盐 石5-1 37.9 5.8 22.2 6.8 3.7 16.3 6.9 0.4 石5-2 31.7 4.4 25.8 5.9 5.2 12.8 14.3 0.0 石5-3 42.8 5.7 22.1 7.1 7.5 8.0 6.8 0.0 石5-4 39.1 5.9 18.8 6.6 3.9 18.1 7.3 0.3 石5-5 40.2 3.8 19.6 5.4 2.3 22.4 6.1 0.2 
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表 2 苏北盆地石港油田石4井浅灰色粉砂岩岩心孔隙度和渗透率
Table 2. Core porosity and permeability results of shallow gray siltstone cores of well Shi 4, Shigang Oilfield, Subei Basin
岩心编号 孔隙度/% 渗透率/10-3 μm2 石4-1 7.51 0.110 石4-2 11.57 0.257 石4-3 9.42 0.165 石4-4 13.59 2.540 石4-5 14.83 4.566
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表 3 苏北盆地石港油田石5断块油藏数值模拟参数
Table 3. Reservoir numerical simulation parameters of block Shi 5, Shigang Oilfield, Subei Basin
数值模拟参数 参数值 原始地层压力/MPa 22.6 饱和压力/MPa 2.9 平均渗透率/10-3 μm2 4.2 平均孔隙度/% 13.4 地下原油黏度/(mPa·s) 7 地面原油密度/(g/cm3) 0.877 原油体积系数 1.05 原始溶解油气比/(m3/t) 1.13 原油压缩系数/(1/MPa) 4.8×10-4 岩石压缩系数/(1/MPa) 5.6×10-4 地层水压缩系数/(1/MPa) 4.2×10-4
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表 4 驱替实验不同阶段采收率情况
Table 4. Recovery rates at different stages of displacement experiment
驱替阶段 采收率/% 小孔(＜0.1 μm) 中孔(0.1~10 μm) 大孔(>10 μm) 综合 水驱 8.91 4.88 13.04 6.77 表面活性剂驱 15.50 19.67 16.50 18.12 二次水驱 19.86 22.73 19.43 21.58
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