Improving Volumetric Sweep Efficiency in a Heavy Oil Tight Matrix Fractured Carbonate with Continuous Steam Injection

Steam injection whether cyclic or continuous, combined with selective well-stimulation and / or other well intervention techniques, are essential for developing heavy oil, tight carbonate reservoirs. The success of steam injection processes relies on achieving optimal areal and vertical sweep efficiencies, as well as optimal displacement efficiency. The Issaran field on the Egyptian Gulf of Suez, is producing heavy oil, majorly, from naturally fractured dolomite reservoir. Continuous steam injection is the one of the key long term thermal development strategies the company adopts to achieve sustainable oil production rates and achieve a best-in-class ultimate recovery factor for the specific complexities of Issaran. The presence of a highly conductive fracture system in the reservoir with a major NW-SE strike direction is evident and based on 52 image logs, acquired in nearly every well drilled over the past 6 years. This fracture system is combined with a relatively tight matrix system. This combination of tight matrix and a significant fracture system greatly affects the volumetric sweep efficiency. This is because wells located in the preferential flow direction have premature steam breakthrough and significant hot water production, resulting in energy loss. This means a live steam chamber is not built and subsequently no efficient heating of the matrix blocks occurs impeding the drainage of oil from the matrix system. The study area of this work is the Issaran Upper Dolomite reservoir at a depth of around 1000’ss in the south of block G. This sector of the reservoir is characterized by a relatively low fracture density. However, also in this sector, the dual permeability system is one of the major challenges for the development of the Issaran reservoirs as most of the oil is found in the matrix (estimated to be more than ~98-99% of the STOIIP). Based on field development studies, including the construction of a dynamic reservoir sector model, it was decided that continuous steam injection (CSI) would be implemented in the form of an inverted 7-spot pattern. Preferential flow was observed in the NW-SE fracture direction within one month of commencement of the CSI in the central well. To enhance areal sweep efficiency, several optimization actions have been performed in the production wells in the CSI pattern, all aimed at distributing the steam in different directions. To improve volumetric sweep, selective well intervention for the non-affected zones in the producers surrounding the injector was carried out. This was done to mitigate the low sweep efficiency along the main fractures’ direction. Additionally, carefully designed steam cycles at well-defined time periods acted as conditioning activation slugs resulting in a decrease in the mobility contrast between oil and water. This approach helped to alter the preferential flow path in the CSI pattern, thus improving areal sweep efficiency by contacting and heating up previously uncontacted matrix blocks. This approach has been systematically applied and resulted in impressive results in not only arresting the oil decline but also tripling the oil production rates. This resulted in a decrease in the steam-to-oil ratio, a decrease in reservoir pressure depletion rate, as well as tripling the Estimated Ultimate Recovery (EUR) for the pattern. This achieved not only a significant increase in the longevity of this development project but also confirmed the validity of the company’s propriety workflow across Block G and wider. The findings of this case-study highlight the effectiveness of proactive reservoir management, the production/injection rates in a continuous steam injection pilot as well as the effectiveness of customized stimulation of the surrounding producers to enhance volumetric sweep efficiency and optimize steam injection in heavy oil, tight carbonate reservoirs. These insights can contribute to the development of improved strategies for maximizing oil recovery and injection efficiency in such complex reservoirs.