Study and practice of cyclic water injection in Mazhong tight oil reservoir in the Santanghu Basin

After the horizontal well in Mazhong tight oil reservoir of Malang sag, the Santanghu Basin is put into production, the formation pressure drops fast and the natural decline is great. The natural decline in the first year after the commissioning is over 50%, revealing the significant contradict of low recovery factor of primary commissioning, so it is in urgent need to carry out EOR field tests. The reservoir physical properties, oil bearing property and rock wettability of Mazhong tight oil reservoir were comparatively analyzed. It is shown that the relative wettability index of the reservoir is 0.18~0.48, and the characteristics of weak water wetting-water wetting are presented, so cyclic water injection is suitable in Mazhong tight oil reservoir. Then, the mechanisms and influential factors of cyclic water injection were analyzed deeply to provide the effective guidance for field practice. The pilot test of cyclic water injection was conducted at 7 wells and their average daily oil increment per well and average cumulative oil increment per well are 10.3 t and 805 t, respectively. So far, cyclic water injection has been a new effective development technology for Mazhong tight oil reservoir. In conclusion, the effect of cyclic water injection is in positive correlation with the water throughput and it is better when the injection pressure reaches the super fracture pressure. In addition, the effect of the cyclic water injection in the wells whose horizontal sections are located in the middle and upper parts of the oil layer is better than that in those whose horizontal sections are located in the lower part of the oil layer. As the cycle of the cyclic water injection increases, however, the effect diminishes. Therefore, it is necessary to study the EOR technologies after multiple cycles' cyclic water injection in the following step.