# The Principle and Downhole Testing of Water Injection Exploration in Depleted Reservoirs
## 1 Introduction
### Geophysical Situation During Water Injection  
During the water injection testing process, geophysical monitoring work was conducted simultaneously, specifically transient electromagnetic detection. The transient electromagnetic results from the left side of borehole 151 were selected for illustration (see Figure 4). During the water injection testing of the borehole, transient electromagnetic surveys were conducted along the azimuth and inclination directions of the borehole to characterize the electromagnetic field responses of the surrounding rock layers. By comparing and analyzing the contour profiles of the resistivity values of the intact rock layers before water injection and the contour profiles of the resistivity values after water injection at pressures of 4.5 MPa and 5.5 MPa, it was evident that the resistivity of the rock layers surrounding the borehole changed significantly after water injection.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Distance Between Water Injection** & **Number of Water Injections** & \multicolumn{2}{c}{**Flow Rate V (L/min) P (MPa)**} & \multicolumn{1}{c}{**Water Pressure P (MPa) P (MPa)**} \\ \hline \multirow{3}{*}{7.8–9.9} & \multirow{3}{*}{28} & Min & 9.97 & 4.98 & 0.00549 \\  & & Max & 20.9 & 6.64 & 0.00872 \\  & & Average & 16.13 & 5.68 & 0.00747 \\ \hline \multirow{3}{*}{5.7–7.8} & \multirow{3}{*}{28} & Min & 12.35 & 4.99 & 0.00651 \\  & & Max & 21.37 & 6.24 & 0.00912 \\  & & Average & 16.705 & 5.6 & 0.00787 \\ \hline \multirow{3}{*}{5–5.7} & \multirow{3}{*}{29} & Min & 11.4 & 4.58 & 0.00662 \\  & & Max & 22.08 & 5.8 & 0.01016 \\ \cline{1-1}  & & Average & 16.32 & 5.215 & 0.00822 \\ \hline \hline \end{tabular}
\end{table}
Table 4: Hole 156 data statistics.  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Distance Between Water Injection** & **Number of Water Injections** & \multicolumn{2}{c}{**Flow Rate V (L/min) P (MPa) P (MPa) P (MPa)**} \\ \hline \multirow{3}{*}{34–38} & \multirow{3}{*}{14} & Min & 8.07 & 9.45 & 0.00146 \\  & & Max & 11.16 & 10.01 & 0.00191 \\  & & Average & 9.88 & 9.73 & 0.00173 \\ \hline \multirow{3}{*}{30–34} & \multirow{3}{*}{32} & Min & 9.02 & 4.95 & 0.00284 \\  & & Max & 26.6 & 5.85 & 0.00881 \\  & & Average & 15.93 & 5.33 & 0.00518 \\ \hline \multirow{3}{*}{26–30} & \multirow{3}{*}{15} & Min & 2.37 & 8 & 0.00055 \\  & & Max & 2.85 & 9.73 & 0.00093 \\  & & Average & 2.58 & 8.89 & 0.00065 \\ \hline \multirow{3}{*}{22–26} & \multirow{3}{*}{41} & Min & 13.54 & 8.4 & 0.00275 \\  & & Max & 22.32 & 9.8 & 0.00424 \\ \cline{1-1}  & & Average & 18.62 & 9.3 & 0.00343 \\ \hline \hline \end{tabular}
\end{table}
Table 3: Hole 153 data statistics.  
From Figure 4, the arrow indicates the direction of drilling, it can be observed that when the pressure reached 4.5 MPa, the top plate region of borehole 151 (left side) within the range of 10-90 m and the area in front of the borehole within the range of 20-80 m became low-resistivity zones. When the pressure reached 5.5 MPa, the top plate region of borehole 151 (left side) within the range of 0-90 m, the area in front of the borehole within the range of 20-80 m, and the range of 100-110 m turned into low-resistivity zones. Based on this, it can be inferred that high-pressure water has undergone a certain distance of seepage, leading to a decrease in resistivity in the original high-resistivity zone.