Research Institute of Petroleum Exploration & Development, PetroChina, Beijing, 100083, China.
Education Ministry Key Laboratory of Petroleum Engineering, China University of Petroleum, Beijing, 102249, China.
Sci Rep. 2019 Feb 12;9(1):1818. doi: 10.1038/s41598-018-38211-0.
Hydraulic fracturing has become a key technology to economically extract oil and gas from unconventional reservoirs. During hydraulic fracturing, fluid loss and water invasion into formation can cause serious permeability reduction near fracture face. At the same time, field practice also showed that well shut-ins after hydraulic fracturing could significantly increase hydrocarbon outputs, whereas the inner mechanism still remains unknown. In this paper, firstly, we studied permeability reduction after water invasion and permeability enhancement after well shut-ins using a core flooding system. Then, to investigate the inner mechanism, we studied aqueous phase migration during shut-ins using nuclear magnetic resonance (NMR) method. Results showed that fluids invasion reduce matrix permeability while well shut-ins can improve permeability and this improvement depends on the length of shut-ins time. NMR results showed that aqueous phases mainly distribute in macropores and mesopores after water invasion, while in shut-ins period, these invaded aqueous phases redistribute and migrate from larger pore spaces to smaller ones. Aqueous phase redistribution and migration during shut-ins period can remove near fracture water-block, reduce capillary discontinuity and increase the relative permeability of hydrocarbon phase, and this is the reason for permeability enhancement and hydrocarbon output increase after well shut-ins.
水力压裂已成为从非常规储层中经济开采石油和天然气的关键技术。在水力压裂过程中，流体损失和水侵入地层会导致裂缝面附近的渗透率严重降低。同时，现场实践也表明，水力压裂后的关井可以显著提高烃类产量，但其内在机制尚不清楚。本文首先利用岩心驱替系统研究了水侵入后的渗透率降低和关井后的渗透率增强。然后，为了研究内在机制，我们使用核磁共振（NMR）方法研究了关井期间的水相运移。结果表明，流体侵入会降低基质渗透率，而关井可以提高渗透率，这种提高取决于关井时间的长短。NMR 结果表明，水侵入后水相主要分布在大孔和中孔中，而在关井期间，这些侵入的水相会重新分布并从较大的孔隙空间迁移到较小的孔隙空间。关井期间水相的重新分布和迁移可以去除近裂缝水堵塞，减少毛管不连续性，增加烃相的相对渗透率，这是关井后渗透率增强和烃类产量增加的原因。
