Step pump injection pressure drop test to determine the morphology of formation fractures after fracturing

The mini-frac tests can be used to obtain reservoir fracturing characteristics and parameters, which can guide the design of construction parameters in formal fracturing process. By optimizing the method for analyzing the parameters in step pump injection pressure drop testing, the testing process was divided into three stages: pump injection period-early pump injection stage, pump injection period-fracture propagation stage and pressure drop period-fracture extension closure stage. In view of the situations where there are few testing steps, unequal step duration and unobvious inflection points, the pressure characteristics in the early pump injection stage and fracture propagation stage were analyzed by using the radial flow and linear flow models of unsteady flow, respectively, which helps to make the fracture propagation timing and fracture propagation pressure clear. Based on the volume balance relationship during dynamic fluid loss of fracturing fluid and other pressure drop testing methods, the fracture propagation length, fracture width and fracture toughness after pumping-off were calculated. The research results show that this method makes up for the inability to obtain effective fracture propagation pressure due to unclear inflection points of the injection rate-pressure curve curve in non-standard tests, and the calculation results of reservoir and fracturing parameters are reliable. In the absence of experiments, reasonable reference values for fracture toughness were obtained, expanding the application scope of the step-rate tests.