Water Injection Conformance Control using Nitrogen Foam – A Successful Field Trial in a Tight Carbonate Reservoir in Northern Oman

Abstract Natih reservoir of the D oilfield of block 5 in northern Oman is a tight carbonate reservoir characterized by low permeability of (1-5) md, with a formation temperature of 70°C and high formation water salinity of around 180,000 mg/L. After 15 years of water-flood with horizontal well patterns, field performance review shows typical challenges such as high water-cut and ineffective water injection caused by the water circulation. Due to low permeability and strong reservoir heterogeneity, conventional water conformance methods such as polymer are not applicable. As an alternative, nitrogen foam flooding – a technique using nitrogen and surfactant to create foam to improve water injection conformance – offers a promising solution to control water channeling, reduce water cut and enhance oil recovery. This technology has two main features: First, nitrogen and surfactant generate a large volume of foam within the reservoir, which temporarily blocks and diverts water flow paths, thereby expanding the sweep efficiency. At the same time, the surfactant reduces the interfacial tension between crude oil and formation brine, enhancing the mobility of crude oil, ultimately improving oil recovery. Second, nitrogen not only replenishes reservoir energy by providing drive, but also tends to accumulate at the top of the reservoir to displace residual oil in the upper sections, hence creating a secondary profile modification. The formula of foam surfactant was optimized in the lab based on the field crude oil and brine sample. The selected surfactant offers excellent tolerance to high temperatures and high salinity, enabling its application in reservoirs with temperatures up to 100°C and formation water salinity reaching 200,000 mg/L. In September 2024, a total of 355,000 Nm3 of nitrogen and 3,240 m3 of surfactant solution were injected into a horizontal well pattern with one injector and two producers, which resulted in an approximate 10% reduction in the water cut. A comprehensive monitoring program – including well production testing, injection wellhead pressure monitoring and time-lapse production logging – was executed to systematically evaluate well performance. Following foam injection, a clear and sustained increase in injection well pressure was observed, lasting for approximately three months. Concurrently, memory production logging (MPLT) results indicated an improved and more uniform water injection profile as a result of foam conformance control. By June 2025, the pattern had achieved a cumulative oil gain of 6,630 barrels, meeting the expected incremental oil and UTC targets. This field trial has demonstrated the feasibility and potential of nitrogen foam technology for water injection profile control, providing valuable field data and a solid foundation for its future expansion in such tight carbonate reservoirs.