In the process of injecting water into low and ultra-low permeability reservoirs，serious damage to reservoirs often occurs，such as hydration swelling，water lock，and Jamin effect，increasing flow resistance and water injection pressure loss，and giving rise to high water injection pressure，rapid increasing in water injection pressure，low water absorption index，and even inability to get water injected into reservoir. As a result，reservoirs subjected to water injection have insufficient water injected into them for a long time，and the effectiveness of plugging removal measures cannot last long in such reservoirs，though they are effective. Consequently，beneficiary wells have low production and oil and gas recover percent. In order to solve this problem，it is a common practice to inject anti-swelling agent and surfactant into the injection water for a long time without interruption to mitigate the damage to reservoirs caused by hydration swelling and capillary effect. However，due to the demand for long-term injection of chemicals，production cost is high，and the effect is not so satisfactory. In this study，small cationic trapezoidal anti-swelling protectant A and trapezoidal wetting protectant B were developed and screened out based on the understanding that the hydration swelling of reservoirs is not only related to the TDS of foreign fluid，but also has a close relationship with the decrease rate of TDS. Both of them were injected into the injection water at a certain time interval and a concentration gradient from high level to low level until reservoirs were adapted to the properties of the injection water and demanded no more injection of protectants. This means that，at this point，hydration swelling and capillary effect of the reservoirs subjected to water injection were controlled. With the two
protectants，a lot of chemicals and costs have been saved，and cost-effective replenishment of reservoir energy and significant increase in the efficiency of water flooding have been achieved. This technology is referred to as trapezoidal protection technology for reservoirs subjected to water injection. Field application shows that，under the same water injection regime，the daily water injection volume of a single well was significantly increased at a lower cost，and the rise rate of injection pressure was significantly slowed down，which met the requirements of injection allocation，providing a key technology to replenish energy for low and ultra-low permeability reservoirs subjected to water injection.
