Identifying Reservoir Pathways in a Fractured Dolomitic Heavy Oil Reservoir

Abstract

AbstractThe Issaran field, situated in the Egyptian Eastern Desert, is one of the few heavy oil fractured carbonate reservoirs in the world (Refs.1 to 3). Oil production and recovery from such fractured heavy oil carbonate reservoirs is generally lower than conventional reservoirs, with primary recovery being as low as 1% in some cases, with ultimate recovery generally being lower than 30%, as per studies.Issaran is estimated to contain 700 MMBBLS of 10–12 degree API heavy and highly viscous crude in 3 main reservoirs. These are the Upper and Lower Dolomite reservoirs and the deeper Nukhul (fractured Dolomite) reservoir. The field produced 5000 STBOPD in 2008 and is forecasted to produce 8500 STBOPD in 2009 after further development. A Cyclic Steam Stimulation (CSS) program was started in the field in 2006.Scimitar planned to develop the Nukhul formation with cold production in vertical open-hole well completions, as the initial wells gave high liquid rates, possibly due to the high fracture permeability and gravity assisted drainage. However, these super-K layers also had the unwanted effect of channeling water very rapidly to the wellbores, resulting in fast increasing water cuts in the field. Also, if steam stimulation was attempted in these wells, then the steam rapidly channeled through the fractures and the heavy oil could not be heated effectively in order to reduce its viscosity. Viscosity reduction is a key factor in increasing recovery efficiency from such reservoirs.The first step in remediating the situation was to clearly diagnose the super-K layers in a dynamic flowing condition using production logs. Since the wells were producing under ESP’s, Scimitar designed a special Y-tool that would enable production logging (PL) to be carried out. Normal PL density sensors will be unable to differentiate effectively between oil and water in this environment, as the heavy oil density is very close to the water density. Hence a special Production Services Platform (PSP*) tool containing electrical micro-probes was used to differentiate between oil and water and get an accurate estimate of the water holdup in the well.Production logging was successfully carried out in 3 wells of the field using the above toolstring. Surveys were carried out at different flowing rates in order to identify the high permeability layers dominating the production and calculate their pressures/productivity indices. The layerwise contribution of oil/water was calculated and layers producing high water rates were identified. Shutin surveys were successful in detecting crossflowing layers. The results matched well with surface measured production rates.The results calculated from production logging had a significant effect upon the Issaran field development plan. It was found that water was being produced from layers having high resistivities up to 30 ome. This led to an increase in the resistivity cutoff for hydrocarbon production and a reduction in watercut. Hydrocarbon producing zones were better identified and Scimitar decided to abandon open hole completions in future wells and go in for cased hole completions to control the water production.