Smart Water Flooding for Enhanced Oil Recovery in Heterogeneous Carbonate Reservoirs: Experimental Evaluation of Engineered Brine Performance Under HPHT Conditions

Technical Summary: Smart Water Flooding for Enhanced Oil Recovery in Heterogeneous Carbonate Reservoirs

Problem Statement
Carbonate reservoirs, which constitute a significant portion of global oil reserves, present unique challenges for Enhanced Oil Recovery (EOR) due to their complex pore structures, high heterogeneity, natural fractures, and mixed-to-oil-wet surface conditions. Traditional waterflooding operations in these formations often suffer from low displacement efficiency, resulting in substantial volumes of trapped residual oil. While various EOR methods exist, they frequently face limitations regarding high chemical costs, operational constraints, formation damage risks, and unpredictability at the field scale. Consequently, there is a need for cost-effective, less damaging, and environmentally safer alternatives that can effectively alter wettability and improve oil recovery in these specific geological settings.

Methodology
This study presents an experimental evaluation of "smart water flooding" (engineered brine injection) using core samples from the WQ1-57 oilfield complex, a representative heterogeneous carbonate reservoir. The research was conducted under High-Pressure and High-Temperature (HPHT) conditions to simulate in-situ reservoir environments.

The experimental workflow involved:

1. Sample Preparation: Carbonate core samples were retrieved from four horizontal wells (5H, 43H, 44H, and 76H) within the WQ1-57 complex. These samples were cleaned, dried, and characterized for porosity (26–30%) and permeability (17–1020 mD).
2. Fluid Design: "Smart brines" were engineered by manipulating the concentration of potential determining ions, specifically Calcium (Ca²⁺), Magnesium (Mg²⁺), and Sulfate (SO₄²⁻), to facilitate wettability alteration.
3. Core Flooding Experiments: A series of displacement tests were performed using a specialized HPHT core-flooding apparatus. The process followed a sequential injection protocol:
   • Phase 1: Injection of formation water to establish baseline recovery and initial water saturation ($S_{wi}$).
   • Phase 2: Injection of treated (low-salinity/deionized) water as a control.
   • Phase 3: Injection of the optimized smart brine to induce wettability alteration and maximize recovery.
4. Data Acquisition: Throughout the experiments, differential pressure ($\Delta P$), effluent volumes, water cut, and fractional flows were monitored. Effluent samples were analyzed for ion exchange (Ca²⁺, Mg²⁺, SO₄²⁻) to confirm fluid-rock interactions. Relative permeability curves and residual saturations were derived from steady-state displacement tests.

Key Results
The experimental data demonstrated that smart water flooding significantly outperformed conventional waterflooding across the tested heterogeneous cores:

• Oil Recovery Enhancement: Cumulative oil recovery increased from a baseline of 31.7–50.4% of Original Oil in Place (OOIP) under formation water flooding to 47.2–65.0% OOIP under smart brine injection. This resulted in an incremental recovery of approximately 20–30% OOIP. The highest final recovery (65.0% OOIP) was observed in core 44H, attributed to its favorable permeability and heterogeneous pore structure facilitating ion penetration.
• Wettability Alteration: Analysis of relative permeability curves indicated a shift toward more water-wet conditions. The crossover point ($K_{rw} = K_{ro}$) moved to lower water saturation values, and residual oil saturation ($S_{or}$) decreased to the range of 0.20–0.25.
• Pressure Behavior: The pressure response during smart brine injection exhibited an initial increase followed by stabilization and a subsequent decrease. This behavior was interpreted as characteristic of gradual wettability changes and improved flow dynamics.
• Ion Exchange Mechanism: Effluent analysis confirmed the exchange of Ca²⁺, Mg²⁺, and SO₄²⁻ ions between the injected brine and the carbonate rock. This exchange facilitated the expansion of the electrical double layer and reduced the adhesion forces between oil and rock surfaces.
• Heterogeneity Impact: The study highlighted that reservoir heterogeneity profoundly influences recovery performance. Cores with intermediate permeability (e.g., 44H, 5H) showed the most significant incremental gains (23–29%), whereas high-permeability cores experienced early breakthrough, and low-permeability cores faced limited ion access.

Significance and Claims
The paper establishes that engineered brine design is a trustworthy and cost-effective EOR strategy for heterogeneous carbonate reservoirs. The authors claim that the primary mechanisms driving the observed improvements are wettability alteration (from oil-wet to water-wet), a reduction in interfacial forces, and enhanced microscopic displacement efficiency driven by specific ion interactions (synergy between sulfate and divalent cations).

The study concludes that smart water flooding offers a viable alternative to traditional chemical flooding methods, particularly for carbonate systems where conventional waterflooding is inefficient. By optimizing ion ratios (specifically Mg²⁺/SO₄²⁻ and Ca²⁺/SO₄²⁻), operators can potentially maximize recovery factors without the high costs or formation damage risks associated with other EOR techniques. The findings suggest that this approach has a high probability of successful application at the field level in analogous carbonate reservoir systems globally.