Assessment of Initial Wettability Effects on Smart Water Injection Efficiency in Carbonate Reservoirs

Carbonate reservoirs, which hold a significant portion of the world’s oil reserves, are particularly challenging for enhanced oil recovery (EOR) due to their predominantly oil-wet nature and low permeability. Smart water injection (a low-cost, environmentally friendly EOR method) has demonstrated potential to enhance recovery by modifying rock wettability. While numerous studies have examined smart-water mechanisms, the specific role of initial wettability (including Swi and core preservation state) in controlling its efficiency remains insufficiently quantified. This study addresses this critical gap by systematically investigating how initial wettability affects oil recovery during smart water flooding in a Middle Eastern carbonate reservoir. Core flooding experiments were conducted using brines enriched with potential-determining ions (SO42−, Ca2+, Mg2+) under varying wettability conditions. These tests were performed under controlled initial wettability conditions (Swi and preservation state) to ensure consistent and representative comparison across brine types. Results reveal that initial rock wettability plays a pivotal role in dictating the extent of wettability alteration and oil displacement. In strong oil-wet samples, sulfate-enriched brines induced substantial wettability shifts, significantly enhancing recovery. Conversely, ion saturation effects were observed, limiting further improvement beyond a threshold. Quantitatively, spontaneous water-displacement tests on core 122 at ambient conditions yielded 8.1% of OOIP at Swi = 10%, approximately twice the recovery of the same core in a dry (Swi = 0%) condition. Under reservoir-temperature core-flooding, seawater increased oil recovery from 38.3 to 53.1% OOIP in sample 122 and from 42.2 to 54.1% OOIP in sample 188 relative to formation water, corresponding to incremental gains of about 10–15 percentage points. These findings highlight the critical role of initial wettability characterization in designing effective smart-water EOR strategies. Tailoring brine composition to reservoir-specific wettability conditions enabled recovery improvements of approximately 10–15 percentage points relative to formation water at reservoir temperature. The results provide clear mechanistic insight into ion-specific interactions and offer practical guidance for optimizing smart-water formulation and deployment in carbonate reservoirs.