Data for: Modelling and field testing of back-flow fracturing fluid after acid fracturing in Oil Shale reservoirs

In the field experiment site of the in-situ pyrolysis of oil shale, there are three wells. One well is used to inject high temperature and high-pressure nitrogen gas. The other one, named FK-2, is a well that is used to produce oil and gas products. The last well is a monitoring well named M1 for the real-time monitoring of the underground temperature and water inflow. According to the microseismic monitoring at the fracturing site, we have determined that the fracturing may have formed two fractures in the oil shale formation. Because the in situ pyrolysis process of oil shale requires nitrogen injection at a high temperature and pressure, we must monitor the connectivity between the two fractures. There is no need to use high temperature gas to determine the formation connectivity, so we only use high-pressure nitrogen gas. First, the backflow of the fracturing fluid is conducted. After the fracturing fluid backflow, we can determine the connectivity and water inflow of the formation. Based on the microseismic monitoring data, we built a three-dimensional model. The flow field of the high-pressure nitrogen injection into well FK-1 and the gas production from well FK-2 were simulated using the finite element software COMSOL Multiphysics 5.3. The results show that the outlet pressure at the FK-2 well can reach 2.8 MPa when well FK-1 is used as a high-pressure nitrogen injection well and gas is continuously injected at 8.5 MPa. Part of the fracturing fluid flows out of well FK-2, but some of fluid can flow into the formation. When high-pressure nitrogen is injected into the FK-2 well, the result is also the same. To verify the accuracy of the simulation results, experiments were carried out involving high-pressure nitrogen injection into well FK-1. The fracturing fluid volume was monitored while the fracturing fluid flowed back, and the pressure change in well FK-2 was monitored in real time. Finally, the pressure in well FK-2 could reach 2.8 MPa, which proved the connectivity between the two wells. It also confirmed the accuracy of the numerical simulation. In addition, we monitored the pressure changes in the FK-2 well by means of active pressure relief in FK-1 well. The results show that the pressure in the FK-2 well would also decrease, which once again confirms the connectivity status of the two wells.在油页岩原位热解（in-situ pyrolysis）现场试验场地中，共布设三口试验井。其中一口为高温高压氮气注入井；另一口命名为FK-2，用于产出油气产物；最后一口为监测井M1，用于实时监测井下温度与产水量。根据压裂（fracturing）现场的微震监测（microseismic monitoring）结果，我们判定本次压裂可能在油页岩地层中形成了两条裂缝。由于油页岩原位热解工艺需采用高温高压注入氮气，因此必须对两条裂缝间的连通性进行监测。无需使用高温气体来判定地层连通性，因此本次监测仅采用高压氮气。首先开展压裂液（fracturing fluid）返排作业，待压裂液返排完成后，即可确定地层连通性与产水量。基于微震监测数据，我们构建了三维数值模型，并采用有限元软件（finite element software）COMSOL Multiphysics 5.3，对FK-1井注入高压氮气、FK-2井产气的流场进行了模拟。模拟结果表明，当以FK-1井作为高压氮气注入井、以8.5 MPa的压力持续注入氮气时，FK-2井的出口压力可达2.8 MPa。部分压裂液会经FK-2井排出，另有部分流体渗入地层。若将FK-2井作为高压氮气注入井，所得结果与上述一致。为验证模拟结果的准确性，我们开展了向FK-1井注入高压氮气的试验：在压裂液返排过程中监测压裂液体积，并实时监测FK-2井的压力变化。最终FK-2井的压力可达到2.8 MPa，证实了两口井间的连通性，同时验证了数值模拟（numerical simulation）的准确性。此外，我们通过对FK-1井主动泄压的方式，监测了FK-2井的压力变化。结果显示FK-2井的压力同样会出现下降，这再次证实了两口井间的连通状态。