Study On The Micro-fracture Formation Rule During Shut-in Time In Tight Reservoir

The hydraulic fracturing technology has been widely used in tight reservoir,while hydraulic fracturing needs to inject a large amount of fracturing fluid to increase productivity,but if shut-in for a period of time after fracturing,which will make the reservoir productivity for a large increase.This is because the capillary pressure can push the fracturing fluid to the reservoir which can exchange the oil through hole,so as to increase the reservoir productivity.The formation of micro-fractures during the shut-in can promote the imbibition,but currently there is no detailed description of the specific expansion of the newly formed micro-fractures during shut-in,either in terms of theoretical model or numerical simulation.As the imbibition is the main factor affecting the reservoir productivity,in order to study the formation law of micro-fractures during shut-in time,the influence law of imbibition on reservoir transformation was first explored.Therefore,this paper analyzed the mechanism of permeability and carried out the laboratory experimental research on the influence of core size and matrix permeability on imbibition.Followed by further study on micro-fractures formation,this paper based on the principle of conservation of energy and volume conservation equation,analyzing the input energy and output energy,deducing the theoretical model of the microfractures and describing the micro-fractures propagation length curve along with the change of shut-in time,and further revealing the influence of specific surface energy,elastic modulus,poisson's ratio,fracture height and fracture pressure.Finally,this paper studied the influence of the formation of micro-fractures on reservoir stimulation effect,and analyzed the theoretical model by using the actual production data of the oil field,under the two conditions of considering microfracture formation or not during the shut-in time,the increasing of reservoir productivity was compared,so as to analyze the effect of micro-fracture formation on improving reservoir productivity.The theoretical model established in this paper can calculate the cumulative length of micro-fractures.At the same time under the conditions,it was concluded that the formation of the micro-fractures can improve the productivity,which could be beneficial to the production of oil field.