Electric Submersible Pumps in the Zeit Bay Offshore Oil Field (Gulf of Suez)
The Suez Oil Company sought to develop the offshore oil field Zeit Bay (Gulf of Suez) and, in the process, encountered significant technical hurdles. EnSys Yocum was contracted to assist with the design of the field facilities and evaluate the results of various installation options.
Field Background: The crude was 31˚ API with a gas-oil ratio of approximately 565 scf/bbl but the field possessed very low productivity index (largely attributed to the low permeability and porosity of the reservoir. The sensible option was to install electric submersible pumps (ESPs) to handle the very low productivity index and, over a three year period, 208 ESP’s were installed for field production of approximately 75,000 bpd.
While ESP’s have production difficulty and declining pump efficiency with increased gas content, most ESP’s can comfortably handle 10 percent gas by volume. Since the bubble point pressure was roughly 2,100 psi, only a moderate amount of the gas was evolving. Throughout the several year period of field development, EnSys Yocum invoked WELLSIM to model and optimize the performance of the ESP wells.
EnSys Yocum WELLSIM maintains superior accuracy for computing 3 phase flows across ESP wells (and other well configurations) operating below the bubble point for several reasons:
- It uses an advanced approach to modeling reservoir fluid properties which results in higher accuracies than achievable with generalized correlations. This includes computation of the gas volume fraction. Critically, the pressure/volume/temperature (PVT) correlations are based on reservoir fluid PVT assay characteristics from the field/well allied to 30 unique equations that compute six key physical properties.
- WELLSIM is thus an accurate predictor of when free gas is present and quantifies the volumes. Any free gas flowing from the reservoir that is non-associated with the crude oil is also accounted for in WELLSIM and added to the associated gas evolved from the crude oil.
- The exacting physical properties representation combines with rigorous iterative 3 phase flow calculations across each well increment, breaking a flow system down into a large number of increments (typically 50), and delivering an exhaustive overall pressure flow balance to establish high levels of accuracy compared to other simulators.
- The EYI WELLSIM analysis on the Egyptian ESP wells and, more recently, Kuwaiti ESP wells, highlight the importance of capturing this below-the bubble-point issue. WELLSIM output indicates whether the well is operating below the bubble point when entering the ESP and computes multiphase flow rates within typically 10% of well test data (despite the presence of free gas).
- As gas volume fraction at an ESP inlet rises, WELLSIM accuracy may decline but we would not expect to see an ESP well operating at high gas volume fraction as this would be highly inefficient due to the drop in the ESP efficiency.
Twelve Zeit Bay wells were allotted to each small offshore platform. The wells were located in 100-240 foot depth water and were drilled to approximately 6,500 ft. depth. The production traveled up to the platform via riser where a high pressure gas-oil separator removed a significant amount of light ends to enable the pressure crude to travel approx. 20 miles to shore. The separator operated at approximately 250 psi so as to feed the gas compression turbines operating at this pressure. Effectively, the gas was utilized offshore and only the pressure crude traveled to shore albeit with a significant amount of gas-carry-under. A pumping station was located onshore which then powered the way to enable the crude to travel onshore to a refinery approx. 30 miles away. At the refinery, low pressure separation stages removed the rest of the light ends and used the gas and crude to turn into a wide variety of refined products.