Low Permeability Gas Reservoir Water Locking Damage Prevention Technology Research

In the21st century, low permeability sandstone reservoirs is one of the most important and most promising energy reality for our country. In China, how to exploit the low permeability sandstone reservoirs is critical significance to sustainable development of oil and gas reservoirs.The loss of productivity due to water-block in gas wells is a common problem, low permeability sandstone reservoirs. The sources for excess water in the formation could be water based fluids used in drilling, completion, matrix or fracture treatment; cross flow of water to dry gas zones or water imbibition into tight zones. Water-blocked low-permeability gas formations with drawdown pressures comparable to capillary entry pressures can take a very long time to clean up.The study quantifies the effects of factors such as permeability, wettability, surface tension/volatility on gas relative permeability. This work aims to find strategies for cleanup of water blocks created by various operations such as drilling, acidizing, and fracturing, as well as making recommendations for the use of surfactants or solvents for well treatments to remove water blocks.An experimental study conducted to evaluate the feasibility of water block prevention in gas wells by chemical treatment is presented. The treatment is aimed at reducing the capillary pressure of the formation near the well bore or the fracture face for a long period of time by altering the wettability to intermediate gas-wet conditions. Three types of experiments namely contact angle, imbibition and core flow tests are conducted in this study to test the wettability alteration produced by the chemicals. Imbibition data shows that one of the main concerns in such treatments is the formation damage caused by the addition of agents C、B、E that can increase the capillary pressure reducing the effectiveness of the treatment. Six different chemical systems, A-F, are tested for their applicability in a preventive water block treatment. The agents B、E show good wettability alteration, the contact angle was greater than60" showing that the wettability is altered to intermediate gas wetting conditions. Spontaneous capillary imbibition force increased as gas permeability of cores increased. Values of spontaneous capillary imbibition force in low permeability core were much larger than in medium permeability core. Spontaneous capillary imbibition force in core could significantly be reduced by adding agents B、C、E. Decreased values of spontaneous capillary imbibition force in super-low permeability and low permeability core were more evident than in medium permeability core. The core flow tests show that the utilization of agent E can significantly enhance the permeability.The chemical system alters the formation wettability to intermediate gas wet conditions thereby decreasing the capillary forces and enhancing the cleanup of trapped water at low drawdown pressures. Six different agents (A-F) are evaluated in this study for their ability to prevent water block formation. Contact angle and air-brine imbibition tests are conducted to determine wettability alteration before and after treatment with the chemicals. The results show that agent E gives the best wettability alteration with minimum formation damage.It is necessary to use agent E in Xujiahe gas field. Core flow experiments of Xujiahe gas field revealed that relative gas permeability of cores were increased by adding water block clean-up agents in the same drilling fluids under the same displacement pressure, and increased values of gas permeability attained85%。It is significant for the results to improve the development of the reservoir of Xujiahe gas field and protect the reservoir. The results also provide a new idea for water block prevention in gas wells by chemical treatment.