L ABORATORY TESTS FOR DETERMINATION OF THRESHOLD PRESSURE GRADIENT FOR NON-DARCY FLOW IN LOW PERMEABLE ROCK MASS

The threshold pressure gradient is defined as the initiation condition of occurring seepage flow in low permeable rock mass.This value is an important factor in oil production and engineering against leakage,because of the existence of nonlinear infiltration phenomenon in the rock mass.This paper presents a laboratory test method and associated test results to determine the threshold pressure gradient value.During the test,the inlet water pressure of the test sample is stably controlled for long time.The outlet water volume change quantity is exactly measured.When the quantity of seeped water volume change is linear with respect to the time,the corresponding infiltration features are satisfied the Darcy's law.On the base of the new permeability test method,the different permeability soft rocks have been systemically tested.According to the positive correlation of the measured permeability coefficient and the pressure gradient,the Averaged Three Times Method has been put forward to calculate quasi-threshold pressure gradient.At first,the measured permeability coefficient values are adjusted uniformly.Then the absolute permeability coefficient value is determined.Thirdly,the V0 value is specified by taking the three times if the average value.Subsequently,the value of quasi-threshold pressure gradient is determined.On the basis of test data analysis,the quasi-threshold pressure gradient increases as the confining pressure increases.It increases as the rock absolute permeability coefficient decreases.Furthermore,the quasi-threshold pressure gradient is larger during the infiltration pressure increase process than that during the infiltration pressure decrease process.These results are similar to the theory,which showed that the Averaged Three Times Method is better for the quasi-threshold pressure gradient.This method put forward a new thinking about determination of the infiltration threshold pressure gradient of rock mass in Laboratory.