Gas Injection Wells in the Ras Budran Offshore Oil Field (Gulf of Suez)
The Suez Oil Company sought to develop the offshore oil field Ras Budran located in the Gulf of Suez. Several gas lift wells were drilled but the wells were only producing at 25 percent of design capacity. EnSys Yocum was contracted to assist with field performance improvement.
- Reservoir and Well Characteristics:
- API 32.1˚gravity and GOR=525 scf/stb
- Water cut of 60 percent
- Reservoir location depth 12,500 feet
- Well type: Annular gas lift wells with 7 gas injection valves located along the tubing
- Injection gas source: high pressure first stage separator located on the adjacent shoreline
A gas compressor plant was situated to increase the gas injection pressure for the injection gas flowing to a manifold located on an offshore platform. The gas injection flow rate was controlled for each well by a control valve and the flow rate was measured by an orifice meter. The gas was injected in the annulus between the tubing outside diameter and casing inside diameter. Waterflooding was also an integral component of the secondary recovery operations in Ras Budran.
At the request of EnSys Yocum, a wireline crew conducted a spinner survey; a weighted instrument was lowered down the annulus to determine which valves the injection gas was entering and the corresponding flow rate. High velocity gas in-flow was measured at the first gas lift valve and, to a lesser extent, at the second gas lift valve. As the spinner survey continued down the well shaft, results indicated the 5 lower gas lift valves were entirely closed to gas injection flow and therefore substantially limiting the oil ‘lifting.’ In addition, the quantity of gas entering the uppermost 2 valves significantly exceeded the injection gas design capacity, indicating that these two valves may have been damaged.
EnSys Yocum applied its WELLSIM model to simulate well performance and determined that the superficial velocity of the flowing gas-liquid mixture had to be significantly higher in order to increase oil production. This corresponded to decreasing the flowing fluid mixture density in order to decrease the well elevation head. There is an inherent tradeoff between elevation and friction pressure drop which occurs in the reservoir and the well (fluid mixture density decreases with increasing gas injection rate). WELLSIM was applied to calculate the well elevation and friction pressure drops in order to locate the optimum balance and to determine the optimum annular gas injection rate corresponding to maximum oil production. The below graph computed by EnSys Yocum WELLSIM for an Asian annular gas injection well is shown for illustration purposes.
EYI Well Simulation Results Applied to Improve Well Performance
The gas injection well in Ras Budran was reworked per the results from the spinner survey and WELLSIM. This involved a column of water being injected to temporarily seal off the well and then the tubing being lifted to the surface (30 foot sections stacked on the work over rig). The tubing sections containing the gas lift valves were retrieved and placed in a sulfuric acid bath. After cleaning, the valves were examined on the test rack. The 2 upper valves were, in fact, damaged. The gas injection was essentially flowing into the tubing with no control from the bellows. The bellows pressures were subsequently reset using high pressure nitrogen. These pressures were set based on the hydraulic calculation of the temperature at each valve location based on a maximum producing rate estimated at four times the current producing rate.
It was also decided to install a one inch tubing orifice ten feet above the reservoir formation. The tubing was reinstalled in the well. The two upper gas lift valves were replaced. The valve controlling the gas injection flow rate and pressure was opened to 70 percent lift. The gas lift valves now operated sequentially and the water cut column was subsequently lifted out of the tubing and the reservoir flowing fluid started to flow up the well once again.
As the pressure in the tubing reached the conditions required to close the bellow at each gas injection point, the gas lift valve was closed. Thus, gas injection was finally flowing through the tubing orifice at the bottom of the well, requiring a day to equilibrate. Setting the gas injection rate at the optimum indicated by the WELLLSIM simulation, the gas injection well now flowed at 9,600 barrels per day—approximately four times the original oil production rate of 2,200 barrels per day.
EnSys Yocum has performed analyses similar to that described above for a number of oil fields, using WELLSIM to optimize the gas injection rate. This has resulted in significant producing company profits reflecting the following:
- Increased oil production
- Less ‘wasted’ gas and reduced energy consumption where gas was over-injected