Among the various enhanced oil recovery (EOR) processes, CO₂ injection has been widely utilized for oil displacement in EOR. Unfortunately, gas injection suffers from gravity override and high mobility, which reduces the sweep efficiency and oil recovery. Foams can counter these problems by reducing gas mobility, which significantly increases the macroscopic sweep efficiency and results in higher recovery. Nevertheless, CO₂ is unable to generate foam or strong foam above its supercritical conditions (for CO₂, 1100 psi at 31.1 °C), and most of the reservoirs exist at higher temperatures and pressure than CO₂ supercritical conditions. The formation of strong CO₂ foam becomes more difficult with an increase in pressure and temperature above its supercritical conditions and exacerbated CO₂-foam properties. These difficulties can be overcome by replacing a portion of CO₂ with N₂ because a mixture of N₂ and CO₂ gases can generate foam or strong foam above CO₂ supercritical conditions. Although many researchers have investigated EOR by using CO₂ or N₂ foam separately, the performance of mixed CO₂/N₂ foam on EOR has not been investigated. This study provides a solution to generate CO₂ foam above its supercritical conditions by replacing part of CO₂ with N₂ (mixed CO₂/N₂ foam). The mixed foam not only generates strong foam above CO₂ supercritical conditions but also remarkably increases the oil recovery. This solution overcomes the difficulties associated with the formation of CO₂ foam at HPHT conditions enabling the use of the CO₂-foam system for effective EOR and other applications of CO₂ foam such as conformance control.

Organoclays (OCs), formed by surface modification of clay minerals using organic compounds, are typical additives for providing rheology for oil-based drilling fluids (OBDFs). There are different studies on the effect of OCs on the rheological properties of oil-based systems under high-pressure, high-temperature (HPHT) conditions, but finding new OCs as rheology control agents is attractive for drilling fluid engineers. This work reviews different OCs used in OBDFs, namely, organo-montmorillonite (OMMT), organo-sepiolite (OSEP), and organo-palygorskite (OPAL). Furthermore, the structure of OCs in OBDFs, their rheological properties, and the thermal stability of OCs were investigated. Besides, the role of fibrous and layered OCs in enhancing the rheological properties of OBDFs is illustrated. Finally, the synergistic use of different OCs to enhance the thermal stability and rheological properties of the OBDFs is presented. The study highlights research gaps and recommendations for research approaches and potential areas that need further investigation. The application of OCs in OBDFs is a wide field and has huge potential to be developed. The use of OCs in OBDFs will promote development in the oil and gas industry.

High-temperature conditions drastically compromise the physical properties of cement, especially, its strengths. In this work, the influence of adding nanoclay (NC) particles to Saudi class G oil well cement (OWC) strength retrogression resistance under high-temperature condition (300 °C) is evaluated. Six cement slurries with different concentrations of silica flour (SF) and NC were prepared and tested under conditions of 38 °C and 300 °C for different time periods (7 and 28 days) of curing. The changes in the cement matrix compressive and tensile strengths, permeability, loss in the absorbed water, and the cement slurry rheology were evaluated as a function of NC content and temperature, the changes in the structure of the cement surfaces were investigated through the optical microscope. The results revealed that the use of NC (up to 3% by weight of cement (BWOC)) can prevent the OWC deterioration under extremely high-temperature conditions. Incorporating more than 3% of NC severely damaged the cement matrix microstructure due to the agglomeration of the nanoparticles. Incorporation of NC particles increased all the cement slurry rheological properties.

Durability and long-term integrity of oil well cement are the most important parameters to be considered while designing the cement slurry, especially in the high-pressure and high-temperature (HPHT) environments. In this study, the effect of adding the polypropylene fiber (PPF) to Saudi Class G cement is evaluated under HPHT conditions. The effect of the PPF on the cement compressive and tensile strength, thickening time, density, free water, porosity, and permeability was studied. The effect of the PPF particles on the cement sheath microstructure was studied through powder X-ray diffraction (XRD) and scanning electron microscope. The results obtained showed that PPF did not affect the cement rheology, density, and free water. The addition of PPF considerably decreased the thickening time and improved the tensile and compressive strength of the cement. 0.75% by weight of cement (BWOC) of PPF reduced the thickening time by 75%, from 317 to 78 min. The compressive strength of the cement increased by 17.8% after adding 0.5% BWOC of PPF, while the tensile strength increased by 18% when 0.75% of PPF is used which is attributed to the formation of stable forms of calcium silicate hydrates because of the ability of PPF to accelerate cement hydration process as indicated by the XRD results. The ability of the PPF to decrease the cement thickening time along with its ability to improve the cement strength suggests the use of PPF as an alternative for silica floor in shallow wells where a reduction in thickening time will decrease the wait on cement time. Porosity and permeability of the base cement were also decreased by incorporating PPF because of the pores filling effect of PPF particles as indicated by the microstructure analysis.