As world energy demand continues to increase, the oil and gas industry is moving to develop the less conventional resources. Technology is opening new exploration options, including heavy oil, which makes up 15% of the world’s oil resources.

Oil in the desert

Operators in Egypt’s Eastern Desert are facing the challenge of recovering the region’s large heavy oil resources, which are contained in fractured carbonate reservoirs. Such reservoirs offer pessimistic probabilities, with primary recovery rates as low as 1% and ultimate recovery under 30%. In the case of the Issaran field, however, steam EOR has been used to change the production outlook.

The work undertaken in the Issaran field has demonstrated the importance of sophisticated logging and production technology that can withstand the extreme operational conditions of unconventional oil production. In the Issaran field, the technology improved reservoir characterization and extended production in heavy oil reservoirs that might otherwise not be viable.

To exploit the production from oil zones in conditions of high fracture intensity and water cuts, it was essential to precisely identify high permeability layers and calculate their corresponding reservoir pressure. In this case, unique production logs were combined with high-capacity electric submersible pumps (ESPs). The conundrum was to find an ESP that could provide enough drawdown while retaining the ability to perform accurate production logging in high-rate flowing conditions. Operator Scimitar partnered with Zenith Oilfield Technology to develop a new Y-tool that can operate at four times the rate limit of competing products in the market.

The Y-tool customized for Scimitar’s requirements is part of Zenith’s complete ESP bypass system. Using patented Saddle technology revolutionizes bypass system installation, halving the actual installation time and significantly reducing rig costs and time-to-production delays.

Setting the stage

The Issaran field, discovered in 1981, is operated by General Petroleum Co. and Scimitar Production Egypt. The field, which holds an estimated 1.6 MMbbl of 10- 12°API crude, covers 20,000 acres 290 km (180 miles) southeast of Cairo and 3 km (1.9 miles) inland from the western shore of the Gulf of Suez.

Initially the wells yielded an average of 30 b/d. Cyclical stream stimulation initiated in 2006 boosted production to a peak of 8,000 b/d in 2007, but decreased to 5,000 b/d in 2010 due to water breakthrough.

Part of the difficulty in producing the field was the dissimilar reservoir formations, each of which required different management. Formations included Miocene dolomites and limestones (upper dolomite, lower dolomite, Gharandal, and Nukhul limestones) and sandstones (Zeit).

The Upper Dolomite formation had the unfavorable oil recovery factor of oil-wet reservoir rock. Similar factors plagued the Lower Dolomite sections, and because of their shallow depth, cold production was not an option for these formations.

Three limestone bodies comprise the Gharandal formation, all with permeability of less than 20 md. Due to its highly fractured nature, the deepest Gharandal formation, called the Nukhul zone, has been the largest producer of cold oil in the field. Unfortunately, high fracture permeability caused water flow into the wellbore in some areas of the reservoir, resulting in high water cuts. Steam channeled through these fractures resulted in heat depletion within the reservoir, hampering heavy oil viscosity reduction.

Breaking barriers for a solution

High water production was the result of two main factors – water and steam channeling through fractures and difficulty in determining the correct pay resistivity cut off. It was crucial to distinguish between water and oil zones to shut off the breakthrough and maximize production.

An ESP was deployed to maximize production, but it presented a new set of problems. The Y-tools available on the market at the time reached their limits at 1,000 b/d of fluid, but a capacity four times that amount was required to meet Scimitar’s objectives. Zenith developed a Y-tool to allow production logging with the higher rate and optimize production.

The Y-tool was built for 7-in. casing, and the tubing size was designed to optimize production. Given the diameter parameters, concerns arose during risk assessment about fishing operations and also the risk of plugging with the heavy oil. The wireline production logging toolstring was run through the completion using the Y-tool to ensure it would go through. This allowed a trial run offline to ensure potential problems could be addressed prior to the tool-string going into action.

Logging the problems

Identifying these problem high-permeability layers was central to halting water and steam breakthrough. Wells were experiencing 98% to 100% water cuts, so flowing surveys and shut-ins were conducted to determine the optimum conditions in the varying composite areas of the reservoirs.

The best possible results for the well were attained using the Y-tool for the first time, achieving the designed well production rates with the ESP. It also allowed water production to be isolated so oil zones could be targeted within the reservoir.

Production log results had momentous impact on the field development plan. Identifying water-bearing layers by detecting different layer pressures and identifying water production zones led to the increase in the resistivity cut off for hydrocarbon production. Recompletion of these wells was successful by closing off the lower resistivity zones, which saw water cut drop from above 90% to less than 50%.

The upper and lower dolomite wells have been completed as cased/openhole steam injection wells, producing by cyclical steam stimulation. After production logging, new cased-hole wells were trialed for these formations, confirming increased oil production. Steam injector wells were used in the existing openhole wells in these formations, avoiding highly fractured zones, which resulted in incremental oil production of around 40 b/d per well.

The Nukhul reservoir housed some problematic open-hole wells, one of which was experiencing 98% water cut and another 100%. Four reservoir zones could be identified in production logging, and a selective inflow profile identified individual layer pressures so oil contributing intervals could be targeted to increase oil rate to 70 b/d. Another well that was contributing to water cut in surrounding wells due to downward cross flow was plugged back and subsequently recompleted to produce from the Gharandal formation.

Field development success

Using the Y-tool in conjunction with the wireline production logging toolstring enabled Scimitar to identify and shut off the high permeability layers and deter early steam and water breakthrough in production. Processing a higher rate of fluid using the ESP was only possible using the monitoring Y-tool, which had the capacity to process this volume of activity. Having accurate information in hand allowed the operator to conduct tests and gather results to make informed decisions about operational changes.

The results from the Nujhul well were pivotal to the success of the entire field. Increasing the drawdown in this formation increased oil production. And using the ESP with the Y-tool increased production to 20,000 b/d while decreasing water cut in surrounding wells.

The ability to monitor even when using high-rate ESP equipment proved pivotal for the Issaran field. It enabled the operator to improve results in a highly complex reservoir scenario where various factors came into play to test the technology and processes used to extract the heavy oil.