非达西渗流拟启动压力梯度推算

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

岩体启动压力梯度的大小是石油开采,工程防渗处理等方面需要考虑的影响因素。非线性渗流现象已被广泛所认同,低渗透性岩渗流存在启动压力梯度,它基本代表了流体产生渗流时的压力梯度大小。试验对试样进水端水压进行长期稳定控制,出水端采用精确测定流出水体积变化量的方法进行流量测定;当渗出端水体积的变化量与时间呈直线关系时,即认为该时段渗透特征符合达西定律。通过对具有不同渗透性的软弱岩系统的试验测试,根据实测渗透系数与压力梯度的正相关关系,提出了拟启动压力梯度推算的"三次平均法",即:首先统一调整实测渗透系数值,然后确定绝对渗透系数值,再根据该值确定V0值,3次取平均推算拟启动压力梯度的方法。分析表明:拟启动压力梯度随围压升高而增大,随岩石的绝对渗透系数降低而增大;在渗透压差增大过程测得的拟启动压力梯度大于降低过程的拟启动压力梯度,这些都与理论分析结果相吻合。实例说明拟启动压力梯度推算"三次平均法"较为合理,易于推广使用。这为通过室内试验确定工程岩体启动渗流压力梯度的大小,提出了一种新的思路。

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.