The work addresses technical issues of waterflooding operations in offshore environments from the standpoint of fluid-fluid and rock-fluid interactions. Seawater is the preferred source of injection fluid for waterflooding in offshore reservoirs. Given the elevated CAPital EXpenditure (CAPEX), seawater treatment is generally driven by the need to mitigate injectivity loss or water compatibility issues such as the formation of hard scales. The emergence of engineered water as an enhanced-oil recovery (EOR) process has prompted the need to adjust seawater chemistry. Water chemistry modification, however, is a reservoir specific process in our view. Here, we analyze fluid-fluid as well as rock-fluid interactions for two heavy-oil carbonate reservoirs in the Gulf of Mexico to show that geochemical interactions upon adjustment of brine chemistry can lead to undesirable results depending upon lithological characteristics.
Crude oils from two offshore carbonate reservoirs in the Gulf of Mexico were used in experiments. Synthetic reservoir brine was prepared based on produced water analysis. Shear and dilatational rheology were employed to estimate interfacial moduli of crude oil-brine systems, for several water chemistries. These dynamic interface properties have been found to contribute to EOR mechanisms in low-salinity waterflooding. Three different carbonate outcrop samples were used to characterize single-phase rock-fluid interactions in coreflooding experiments. Effluent samples were collected to determine cations and anions concentrations and thereby calibrate geochemical models. We contrasted the needs of seawater modification with availability of viable technologies for offshore operations, such as reverse osmosis, to envision feasible optimization of brine chemistry.
Results show that geochemical interactions upon modification of equilibrium conditions in reservoirs resulting from injection of adjusted brine chemistry can lead to in situ alteration of water chemistry in a way that it impairs EOR benefits of adjusted brine chemistry injection. This is particularly severe when low-ionic strength is selected for injection in anhydrite-containing carbonate reservoirs. On the other hand, current sulfate-reducing strategies to avoid scaling in sandy reservoirs run contrary to proposed sulfate-enrichment in several oil-producing provinces. Current technologies appear to enable a broad spectrum of water chemistry modification, but implementation in offshore environments might be prohibitive for some designs.
The combination of experimentally calibrated geochemical modeling with interface rheological characterization, in light of current seawater treatment technologies, offers a novel strategy to guide optimum water design for adjusted brine chemistry waterflooding operations.