Experimental Evaluation Of Water-phase Trap Damage In Tight Sandstone Reservoirs

After shale gas, tight oil has become a new bright spot in the unconventional oil and gas resources exploration and development. The global tight oil is abundant and with high potential to be exploited. Horizontal well technology and large scale hydraulic fracturing technology provide a way to the successful exploitation of the tight oil, also bring non-negligible reservoir damages. Among which the water phase trapping damage has become one of the main type of reservoir damages with restriction to efficient exploitation of tight oil. Objective evaluation of the water phase trapping is the key to prevent and remove damage of tight oil reservoir. However, due to the difficulties of tight core fluid seepage, coupled with the lack of understanding of tight oil reservoir in-situ conditions, it is difficult to carry out the water phase trapping damage evaluation experiment. In this paper we selected a typical tight sandstone reservoir as the research object, according to the indoor experiment and literature data, aiming to build up the evaluation of indoor method of tight oil reservoir water phase trapping damage.The tight oil formation's geological features are confirmed as the main control factors of water phase trapping damage. The tight sandstone oil formation is characteristic of low permeability, high development of micro/nanopore, large specific surface and strong absorption potential, which make the fluid easily get inside the formation but hardly flow out, leading to the serious water phase trapping damage. Formation wettability, clay mineral type and content, the natural fracture development situation, the initial water saturation condition also directly affect the water phase trapping damage.The liquid seepage experiments of tight core with back pressure were carried out to discuss the influence of back pressure on liquid permeability of tight cores.By choosing three different permeability levels of tight cores, we assembled the experimental set which mainly concluded the 0.001 mL/min constant pressure pump and back pressure valve. Then the tight core liquid filtration experiment at back pressure conditions was carried out to simulate the down-hole production conditions of tight sandstone reservoir seepage conditions. Under the effect of back pressure, more fine pore throats participate in seepage process, reducing the liquid flow resistance and Jiamin effect, so that the core's liquid seepage ability is improved.The formation process of ultra-low water saturation in tight sandstone oil reservoir is simulated. According to geological process, after saturating the tight cores with simulated formation water, the oil displace water experiment to form irreducible water saturation at different back pressure conditions were carried out. The experimental results showed that under the effect of back pressure, it was able to get a lower core irreducible water saturation which is around 33% on average, which was much closer to the in-situ reservoir initial water saturation distribution.The influence of back pressure on water phase trapping damage of tight sandstone oil reservoir was evaluated. The difference between the tight sandstone reservoir water phase trapping damage of the simulated well production conditions of bottom hole pressure and the evaluation result of conventional method was analyzed. Compared to the evaluation result without back pressure on the tight core outlet, the water phase trapping damage degree under 3 MPa back pressure reduced average at 15%; the conventional evaluation result of water phase trapping damage is higher than the experimental result with back pressure.The indoor experimental method to evaluate the tight sandstone reservoir water phase trapping damage is established to simulate the water absorption and flowing back processes, based on the characteristics of the water phase trap damage process. Coupled with the formation geological features and the seepage environment of the well production condition, two key technologies were formed to evaluate the tight sandstone reservoir water phase trapping damage: ?establish the water saturation range which can reflect the formation initial reservoir water saturation distribution;?simulate the oil reservoir production condition by implementing one specific back pressure on the tight core outlet. The water phase trapping damage index is evaluated by the permeability damage rate.