In the oil and gas industry, Enhanced Oil Recovery (EOR) plays a major role to meet the global requirement for energy. Many types of EOR are being applied depending on the formations, fluid types, and the condition of the field. One of the latest and promising EOR techniques is application of ion-engineered water, also known as low salinity or smart water flooding. This EOR technique has been studied by researchers for different types of rocks. The mechanisms behind ion-engineered water flooding have not been confirmed yet, but there are many proposed mechanisms. Most of the authors believe that the main mechanism behind smart water flooding is the wettability alteration. However, other proposed mechanisms are interfacial tension (IFT) reduction between oil and injected brine, rock dissolution, and electrical double layer expansion. Theoretically, all the mechanisms have an effect on the oil recovery. There are some evidences of success of smart water injection on the field scale. Chemical reactions that happen with injection of smart water are different in sandstone and carbonate reservoirs. It is important to understand how these mechanisms work. In this review paper, the possible mechanisms behind smart water injection into the carbonate reservoir with brief history are discussed.

Effective removal of drilling-mud filter cake during well completion is essential to reduce the formation damage caused by drilling activities in many production and injection wells. This task is very difficult to achieve, especially in horizontal/multilateral wells. Harsh chemical treatments (acids, oxidizers, and chelating agents) have been used extensively to conduct water-based mudcake cleanup treatments. However, these approaches have been limited due to the associated high corrosion rates and un-even mudcake removal. With their controlled reaction with the mudcake, mild chemical nature, better health, safety and environmental (HSE) profile, enzymes provide an excellent alternative to harsh chemical treatments in high temperature formations. However, their use has been limited to relatively low temperature applications due to their instability at elevated temperature values.In this work, two enzymatic systems were evaluated: old α-amylase system and new structurally reinforced α-Helix system. The old enzyme was found to form a potentially damaging precipitate at reservoir temperature above 100 o C. The degree of this damage was assessed using size-matching technique and core-flood experiments. This potential of secondary formation damage was drastically reduced in the new improved enzyme system. Enzyme denaturing was minimized by protecting the catalytic center using preferential hydration of proteins with a Polyol.The effectiveness of the new system was proven in the lab through comparative tests. Bioassay by reducing sugar estimation showed better biopolymer hydrolyzing capability of the new system at higher temperatures. In contrast to old enzyme system, core-flood experiments, conducted at high temperatures, using new enzyme system, showed the enzyme denaturing did not occur and the core oil permeability increased at stabilized pressure. In addition, this paper will also highlight the advantages and disadvantages of each enzyme system in terms of stability, compatibility, and mudcake damage reversal.

Carbonated water has proven advantages over conventional CO2 injection in terms of arresting free CO2 mobility, low-pressure injection, lower volume requirement, and higher efficiency. The term “engineered water” is designated to selective ion-spiked injection water with the advantage of the ion-exchange reactions with the rock minerals and releasing trapped oil. This article investigated the synergic effect of dissolved CO2 and engineered water for oil recovery and understanding inner mechanisms. Recovery efficiencies were evaluated through coreflood studies, which revealed that the hybrid water could recover 6–10% more oil than engineered water and about 3% more than carbonated water. HP-HT pendant-drop studies show the insignificance of IFT reduction. Wettability change from oil wet to near-water wet is attributed as a significant factor. The dissolution of Ca2+ and Mg2+ and deposition of SO42− observed in coreflooding may have a significant contribution to oil recovery. Pore enlargement evidenced in NMR-PSD and NMR-ICP results support this claim. The study confirmed that the EWI-CWI hybrid technique could be a promising EOR method, eliminating the requirement for high-pressure injection, the problems of gravity segregation, and the early breakthrough of CO2. It can also be an effective EOR solution, providing a significant cost advantage and higher oil recovery in addition to the environmental benefits of CO2 sequestration.

Hydraulic fracturing is a widely used technology to enhance the productivity of low-permeability reservoirs. Fracturing fluids using guar as the rheology builder leaves aside residual polymer layers over the fractured surface, resulting in a restricted matrix to fracture flow, causing reduced well productivity and injectivity. This research developed a specialized enzyme breaker and evaluated its efficiency in breaking linear and cross-linked guar-polymer gel as a function of time, temperature, and breaker concentration targeting a high-temperature carbonate reservoir. The study began with developing a high-temperature stable galacto-mannanase enzyme using the "protein-engineering" approach, followed by the optimization of fracturing fluids and breaker concentrations measuring their rheological properties. The thermal stability of the enzyme breaker vis-a-vis viscosity reduction and the degradation pattern of the linear and cross-linked gel observed from the break tests showed that the enzyme is stable and active up to 120 °C and can reduce viscosity by more than 99%. Further studies conducted using a high-temperature high-pressure HT-HP filter press for the visual inspection of polymer cake quality, filtration loss rates, and cake dissolution efficiency showed that a 6 h enzyme treatment degrades the filter cake by 94−98% compared to 60−70% degradation in 72 h of the natural degradation process. Coreflooding studies, under simulated reservoir conditions, showed the severity of postfracture damage (up to 99%), which could be restored up to 95% on enzyme treatment depending on the treatment protocol and the type of fracturing gel used.

Significant formation damage can occur during drilling operations because of the invasion of drilling fluid fines and filtrates that lead to pore blocking and saturation alteration mechanisms. This study demonstrates the ways to minimize drilling fluid-related damage and the removal of the deposited filter cake in the carbonate reservoir through judicious selection of bridging particles using “ideal packing theory” and formulation of an enzyme-based clean-up fluid with an acid precursor. The polymer-based drill-in-fluid with a mixed grade of CaCO 3 bridging particles resulted in a compact filter cake with reduced filtration loss preventing internal pore damage significantly. Several ester hydrolysis reaction kinetics were studied, and finally, one combination was chosen as the suitable acid precursor because of its ability to generate a required concentration of acid within the downhole condition. The return permeability of mud-damaged carbonate core plugs was higher than 95% after exposure to the clean-up solution. The corrosion rates were found to be significantly below the industry limits, and the use of acid corrosion inhibitors is eliminated.