Control Method Of Water Trap Damage In Fractured Tight Sandstone Gas Reservoir

Tight sandstone gas is the most realistic kind of the unconventional gas. Its economic exploitation means much as it can decrease the huge amount of energy consuming and high emissions which exist in our energy structure because of relying too much on coal. Due to low porosity and permeability, narrow throats, diverse clays, high effective stress, ultra-low initial water saturation and large amount of natural fractures in tight gas reservoir, the strong capillary force in the tight gas reservoir makes the filtrate hard to return back to the ground, resulting in the decline of gas production. How to make the water phase trapping damage under control have become the key factors of the exploitation of tight sandstone gas.The samples in this paper are from the deep fractured tight sandstone gas reservoir of Anpeng Oilfield. Through the geological and laboratory measurements, we have identified the potential factors and damage mechanisms of the water phase trapping damage and systematically measured the damage degree in this area. The systematical method to control the water phase trapping damage for the fractured tight sandstone gas formation was constructed and the prescription of the shielding temporarily plugging drilling-completion fluid was developed. This kind of drilling-completion fluid take the dynamic fractures width under considered and is suitable for this area. To minimize the water phase trapping damage, the impact of a surfactant which is based on the interface modification is evaluated, and the application effect of the new theory and technology was also track analyzed.To understand the potential factors and damage mechanism of the water phase trapping damage, the capillary imbibition experiments and the water backflow experiments with gas displacement in tight sandstone reservoir was carried out. The degree of the water phase trapping damage in this area was evaluated. We predicted that the value of damage degree in this area is strong, and the fractured reservoir’s value is weak.The systematical method to control the water phase trapping damage for the fractured tight sandstone gas reservoir was constructed. Considering the technologic level, the control effect and economic value, we developed a systematical method to minimize the water phase trapping damage in tight sandstone gas reservoir after investigating a number of literatures. The first course of this method is to use shielding temporary plugging drilling fluid to avoid the invasion of the filtrate in fractured reservoir. The second one is to use interface modification technology to reduce the interfacial tension and change the wettability of reservoir to stimulate the backflow of filtrate in matrices reservoir.The new prescription of the shielding temporarily plugging drilling-completion fluid was developed, which is suitable for this area and take the dynamic fractures width under considered.We found the static width of the fractures within the formation under in-situ stress was about50μm. By using the finite elemental simulation, we found that the dynamic width of the fractures in the formation could stretch to more than90μm under the3.5MPa pressure. The new prescription contains initial drilling completion fluid+2%LF-2+1%EP-2. This system can from a tight mud cake in less than5minutes and the successful rate reach to99.99%. The amount of the filtrate is very low and its backflow recovery rate of permeability can reach to80%. The last, the mud cake’s pressure capacity can reach to15MPa.The impact of a surfactant which is based on the interface modification was evaluated. The results show that it can significantly reduce the interfacial tension and make the wettability is neutrally or slightly water wet. The rate and capacity of spontaneous imbibition declined between30percent and40percent. The backflow rate and backflow recovery rate of permeability can reach to85%and the best backflow pressure is2.5MPa. It can effectively control the water phase trapping damage of matrices reservoir.The active application effect of this method in HF-1well shows that it can effectively control the water phase trapping damage and lay the foundation for finding, evaluating and economically developing the fractured tight sandstone gas.