# Hydraulic Expansion Techniques for Fracture-Cavity Carbonate Rock with Field Applications
## 3 Analysis of Hydraulic Expansion  
To counter the insufficient fluid supply caused by the weakening formation energy of fracture-cavity carbonate reservoirs and the ineffective exploitation of remaining crude oil around or between wells, we devised various hydraulic expansion techniques and confirmed their effectiveness through field applications. To further demonstrate their effectiveness, we compared the proposed techniques with conventional water injection and hydraulic expansion. Expansion well 7 has a depth of 6367.2 m/6147.3 m (inclined/vertical). It was directly put into production without acid fracturing. On 29 November 2013, the natural flow was put into production and was stopped after 35 days. The oil pressure dropped from 23.5 MPa to 4.5 MPa, and the daily oil output dropped from 70.1 t to 10.1 t. In addition, the staged oil production was 913 t, and the water production was 18 m\({}^{3}\). After the natural flow stopped, three rounds of conventional water injection were implemented. The first two rounds of water injection suitably replaced oil. The initial drainage of the well was followed by maintaining a low water cut and stable production. After the third round of water injection to replace oil, the water content of the initial drainage dropped from 100% to 0%, before suddenly increasing to 100% in the later stage. This stage produced 8331 t of oil, 4561 m\({}^{3}\) of water, and 10,167 m\({}^{3}\) of accumulated water. After the water injection pressure was increased, the water injection pressure increased linearly with the water injection volume, and the bottom hole clearly showed a constant volume. After the third round of water injection, the sudden increase in water content was analyzed. At this time, the oil-water interface of the near-well fracture-cavity system was uplifted, rendering conventional water injection ineffective to replace oil. Using high-pressure hydraulic expansion, the first round of high-pressure water injection showed a cumulative water injection of 9361 m\({}^{3}\), which produced 917 t of oil, 1067 m\({}^{3}\) of water, and 56% water content. Thus, the production effect was not ideal. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**No.**} & **Water Injection** & **Injected Water** & **Pump Pressure** & **Sleeve** & \multirow{2}{*}{**Description**} \\  & **Displacement (L/min)** & & **(m\({}^{3}\))** & & **(MPa)** & \\ \hline
1 & 200–300–400 & Positive & 30 & \textless{}50 & \textless{}15 & Monitor amount of injected water \\  & & & & & & If positive injection pressure exceeds \\
2 & 800–1000 & Positive & 4000 & \textless{}50 & \textless{}15 & 3–5 MPa, casing begins to achieve balanced pressure and is gradually repressurized to 5–10-15 MPa \\  & & & & & & Immediately after water injection, \\
3 & Pressure drop & Positive & – & – & – & – \\
4 & Acidification & Positive & Based on acidification design, the fracking team performs this step \\ \hline \hline \end{tabular} Key requirements: (1) If the sealer release is sealed, the pressure is controlled at 24 MPa, and water filling proceeds or the construction is stopped depending on the situation to determine the next step. (2) If the initial pressure is very high (displacement below 30 m\({}^{3}\)/h) and pressure above 30 MPa) and acidic conditions occur, small-scale acidification is considered after water injection or construction is stopped, which should be decided through discussions. (3) According to the situation of on-site adjustment, the whole process pressure should not exceed 30 MPa, and oil pressure should not exceed 50 MPa. If the construction displacement is large, it is necessary to pipe the network and reverse transport to ensure adequate water supply for the processes performed in advance and during preparation. Do not operate the pump during smooth starting and stopping of equipment, maintenance, or other activities.  
\end{table} 
Table 2: Construction results of expansion well 6 using the composite expansion and acidification process.