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.

在油页岩原位热解的现场实验站中，设有三口井。一口井用于注入高温高压氮气。另一口井，命名为FK-2，用于产出油气产品。最后一口井为M1监测井，用于实时监测地下温度和含水层涌入。根据裂缝地点的微地震监测，我们确定在油页岩层中可能形成了两条裂缝。鉴于油页岩原位热解工艺需在高温高压下注入氮气，因此必须监测两条裂缝之间的连通性。无需使用高温气体来确定连通性，因此仅使用高压氮气。首先，进行裂缝流体的反流操作。在裂缝流体反流之后，我们可以确定地层连通性及含水层涌入情况。基于微地震监测数据，我们构建了三维模型。利用有限元软件COMSOL Multiphysics 5.3，对FK-1井中高压氮气注入和FK-2井中气体产出进行了模拟。结果显示，当FK-1井作为高压氮气注入井且持续以8.5 MPa的压力注入气体时，FK-2井的出口压力可达2.8 MPa。部分裂缝流体从FK-2井流出，但部分流体能够流入地层。当向FK-2井注入高压氮气时，结果亦然。为验证模拟结果的准确性，进行了涉及FK-1井高压氮气注入的实验。在裂缝流体反流过程中，监测了裂缝流体体积，并在实时监测FK-2井压力变化的同时进行了压力变化监测。最终，FK-2井的压力可达2.8 MPa，这证明了两口井之间的连通性。同时，也验证了数值模拟的准确性。此外，通过FK-1井的主动压力释放，我们监测了FK-2井的压力变化。结果显示，FK-2井的压力也会下降，这再次证实了两口井的连通状态。