Thermal EOR by Cyclic Sequential Injection in Carbonate Reservoirs
Authors: Al Lawati J. M. 
Published: November 2024

Although thermal recovery has been proven to be an effective enhanced oil recovery (EOR) method for heavy oil, challenges are always greater when managing carbonate reservoirs. Carbonate reservoirs are heterogeneous in nature and tend to have features such as fractures and vugs that increase the complexity when operating such fields. Additionally, fluid properties add to the challenges and must be considered in determining the most effective EOR method.

In the case of heavy oil, steam injection is an option used to alter fluid properties and increase recovery. The industry offers different methods of steam injection, including steamflood, cyclic steam stimulation (CSS), steam-assisted gravity drainage (SAGD), and sequential steam injection (SSI).

The objective of this trial is to determine the optimum thermal recovery method in the Kahmah and Khuff fractured carbonate reservoirs in the Mukhaizna field in the Sultanate of Oman. Both reservoirs are 15-20 ft thick with oil viscosity ranging between 4,500-8,000 cp at downhole conditions. Because communication between wells has been observed due to existing fractures, the wells are grouped (and treated as groups instead of as individual wells), therefore avoiding steam breakthrough. CSS, steamflood, and SSI were all trialed in different groups historically; however, group cyclic steam stimulation (GCSS) was the main strategy employed for production from the Kahmah and Khuff. Even though GCSS was an effective method of operation, too many wells were put on a steam cycle at the same time, causing down oil (amount of oil lost due to wells being down). Therefore, in this trial, both CSS and SSI techniques are merged in a process called steam cyclic sequential injection (CSI). The process involves dedicating one injector with a specific injection rate in a group for a specified time while monitoring the offset wells' performance. After the specified time, the selected injector is returned to production while nominating another injector in the group, which will then inject steam at the specific rate and for the specified time. This results in continuous injection using different wells sequentially, leaving the previous injector with CSS impact (i.e., stimulated with steam), thus achieving CSI.

The trial resulted in a 10-20% increase in oil production, improved water cut, and increased active well count. Furthermore, pump performance was enhanced, heat was well maintained for stable pump operation, and the pump failure rate was lowered. This has also optimized steam injection rates and requirements, as a lower steam rate is required in a group to achieve these results. Moreover, the steam-oil ratio (SOR) trend is closely monitored as the main key performance indicator (KPI) for the overall economics. As a result, steam CSI has become the main recovery technique in the Kahmah and Khuff.

This paper describes the challenges associated with heavy-oil recovery in carbonate reservoirs and a new process to add to existing thermal recovery methods and to help manage the reservoirs effectively.