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）野外试验场中，共布设三口试验井。其中一口井用于注入高温高压氮气，另一口命名为FK-2，为油气产品生产井；最后一口为命名为M1的监测井，用于实时监测井下温度与进水量。根据压裂现场的微地震监测（microseismic monitoring）结果，我们判定本次压裂作业已在油页岩地层内形成两条裂缝。由于油页岩原位热解工艺需要高温高压注入氮气，因此需对两条裂缝间的地层连通性进行监测，本次连通性监测无需采用高温气体，仅需高压氮气即可完成。首先开展压裂液（fracturing fluid）回流工序，待压裂液回流完成后，即可测定地层连通性与进水量。基于微地震监测数据，我们构建了三维流场模型，并采用有限元软件COMSOL Multiphysics 5.3，对向FK-1井注入高压氮气、FK-2井采气的过程进行了流场模拟。模拟结果表明：当以FK-1井作为高压氮气注入井，并以8.5 MPa的压力持续注入氮气时，FK-2井的出口压力可达到2.8 MPa；部分压裂液会经由FK-2井排出，另有部分流体渗入地层。若向FK-2井注入高压氮气，亦可得到一致的模拟结果。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：在压裂液回流阶段同步监测压裂液体积，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的准确性。此外，我们通过对FK-1井实施主动泄压的方式监测FK-2井的压力变化，结果显示FK-2井的压力同样出现下降，再次证实了两口井的连通状态。