The Experimental Investigation Of Shale Aqueous Phase Trapping Auto-removal

Aqueous phase trapping(APT)is an important factor for damaging oil/gas reservoirs in the process of oil/gas development.APT appears in the process of oil/gas development,due to fracturing fluid?completion fluid and working fluid filtrating into the formation and reducing the formation permeability.Horizontal wells combined with large scale multi-stage hydraulic fracturing is an important way to efficiently develop shale gas.After the fracturing,lots of fracturing fluid remains in the formation in the good gas shale,the flow-back fluid of which sometimes is less than 5% when flow-back behavior was finished,thus large number of remaining fluid seriously damage the formation permeability and APT appears.However,this type of APT can be auto-removed after the well shut-in for a period of time in some type of shale formations.In order to research this special phenomenon,the shale samples were used which is obtained from lower Silurian Long-ma-xi formation in Chongqing area and the comparative study by using tight sandstones and volcanic rocks was proceeded.The nuclear magnetic resonance experiment and pulse-decay permeability experiment were performed for researching the liquid distribution and permeability variation during the process of sample spontaneous imbibition.In the process of spontaneous imbibition,there are intervals of permeability increasing and decreasing,and the permeability recovery and increasing with imbibition time are conducive to the shale aqueous phase trapping auto-removal.The results are presented as follows.Sufficient imbibition of shale is a process of continuous imbibition,the large pores and cracks ware filled quickly and the liquid diffused into the matrix subsequently;under the strong capillary force the liquid preferentially enter into the small pores;at the same time the largish pores were filled gradually,which is benefit for gas displacement.In the process of insufficient imbibition,the liquid in the largish pore diffused into the small pores,which reduced the liquid saturation in the largish pores as well as displacing the gas in the small pores,so the relative gas permeability of shale increased,which is an important mechanism of shale aqueous phase trapping auto-removal.During the imbibition process of shale,lots of micro cracks appear in the shale,which is caused by the capillary force and clay expansion.These micro cracks play an important role in the permeability increase of shale rock,which is another important factor for shale aqueous phase trapping auto-removal.Therefore,some shale reservoirs have the capacity of APT auto-removal due to spontaneous imbibition after well shut-in for a period of time when hydraulic fracturing was finished.In a word,the micro fractures expansion and fracturing fluid diffusion contribute to the auto-removal of shale APT.This study is conducive to the understanding of shale APT auto-removal and provides theoretic basis for the field staffs to realize high production of shale gas.