College of Resources and Earth Sciences, China University of Mining and Technology
General Prospecting Institute of China National Administration of Coal Geology

致密砂岩中气液渗流的启动压力梯度(TPG)随岩石有效应力变化而动态变化，即动态启动压力梯度效应，其动态变化特征受孔喉结构控制。为了认识煤系致密砂岩多层合注CO2开发过程中储层启动压力梯度动态规律，通过对不同物性岩心进行不同有效应力下TPG的测试，基于分形方法定量研究岩石孔喉结构及渗透率对动态TPG的影响规律，建立动态TPG的计算方法，并计算储层注CO2动态和常规固定启动压力及造成的产能损耗。研究结果表明：致密岩石有效应力由1 MPa增加到29 MPa，TPG随有效应力呈现对数上升增加趋势，增加幅度为 31.2%～120.1%。低应力下TPG取决于岩石渗透率，随渗透率整体呈乘幂下降趋势，孔喉结构分形维数控制动态TPG应力敏感性，岩石孔喉结构分型维数与TPG敏感性系数呈乘幂上升趋势；较大的注入压力和较小的注采井间距将减小动态TPG效应造成的额外附加压力消耗，减弱启动压力引发的层间干扰，根据固定TPG计算的储层最小有效注入压力偏小32.8%～69.6%，储层孔喉结构越复杂，估算值的偏差越大，注入压力增加到29 MPa时动态启动压力下降3.4～7.4 MPa，动态启动压力导致的额外附加压力损耗为1.0～5.8 MPa，相同注入压力下动态与固定TPG影响下的注入CO2波及范围差异为40.5%～51.3%；最大注入压力下动态TPG造成的储层产能损耗为24.6%～65.3%，比固定TPG大4.1%～21.1%，储层岩石孔喉结构越复杂，注入压力越低，动态TPG造成的额外附加产能损耗也越大。煤系致密砂岩多层合注CO2开发时，应避免储层压力衰减至较低值，采用较大的注入压力和较小的注采井间距，减小动态启动压力梯度效应造成的附加额外产能损耗，同时减缓层间干扰，提高多层合注CO2开发效果。

The threshold pressure gradient (TPG) of gas-liquid seepage in tight sandstone varies with the rock effective stress, that is, the dynamic threshold pressure gradient effect, its dynamic variation characteristics are controlled by the pore-throat structure. In order to explore the characteristics of dynamic TPG of reservoirs in the process of multi-layer co-injection CO2 development in coal-measure tight sandstone reservoirs, the TPG tests were conducted on cores with different physical properties at different effective stress. The influence of rock pore-throat structure and permeability on the dynamic TPG was quantitatively studied based on the fractal analysis method, the calculation method of dynamic TPG was established. The dynamic and fixed threshold pressures and the corresponding gas productivity loss in reservoirs during CO2 injection were calculated. The experimental results show that the effective stress of tight rocks increased from 1 MPa to 29 MPa, and TPG showed a logarithmic increasing trend with effective stress, with an increase range of 31.2%−120.1%. The TPG value depended on the rock permeability at low stress and showed a power-decreasing trend with permeability. The fractal dimension of the pore-throat structure controlled the dynamic TPG stress sensitivity. The fractal dimension of the rock pore-throat structure and the TPG sensitivity coefficient showed a power-increasing trend. The larger injection pressure and smaller spacing between injection and production wells could reduce the additional pressure consumption caused by dynamic threshold pressure, and reduce the interlayer interference during CO2 co-injection. The calculated minimum effective injection pressure of the reservoir was 32.8%−69.6% lower according to the fixed TPG. The more complex the pore-throat structure of the reservoir, the greater the deviation of the estimated value. When the injection pressure increased to 29 MPa, the dynamic threshold pressure decreased by 3.4−7.4 MPa, and the additional pressure loss caused by the dynamic threshold pressure was 1.0−5.8 MPa. The differences in the sweep range of injected CO2 under the influence of dynamic and fixed TPG under the same injection pressure were 40.5%−51.3%. The reservoir productivity loss caused by TPG at maximum injection pressure was 24.6%−65.3%, which was 4.1%−21.1% higher than the fixed TPG. The more complex the pore-throat structure of reservoir rock was, and the lower the injection pressure, the greater the difference in productivity loss caused by dynamic threshold pressure. In the development of multi-layer CO2 co-injection in the coal-measure tight sandstone reservoir, the reservoir pressure should be prevented from weakening to a lower reservoir pressure value. A larger injection pressure and a smaller injection-production well spacing should be adopted to reduce the additional production capacity loss caused by the dynamic TPG effect, at the same time, the interference between layers should be mitigated to improve the multi-layer CO2 co-injection development effect.

国家自然科学基金资助项目(41872171)；山西省揭榜招标资助项目(20201101003)

王千，申建，赵岳，等. 煤系致密砂岩储层注CO2启动压力梯度动态规律[J]. 煤炭学报，2023，48(8)：3172−3181.

WANG Qian，SHEN Jian，ZHAO Yue，et al. Dynamic threshold pressure gradient characteristics of co2 injection in coal-measure tight sandstone reservoirs[J]. Journal of China Coal Society，2023，48(8)：3172−3181.