Foamed fluids have been used for decades to diminish formation damage in nearly all kinds of unconventional reservoirs with a wide range of pressures. Although water-based fluids are widely used in the oil industry as one of the most economic hydraulic fracturing methods, foams are another substitute to fracture water-sensitive reservoirs at which damage to pore throats is caused by swelling clays or fines migration. The mixture of CO2 and surfactant as a CO2-foam not only reduces formation damage by minimizing the quantity of aqueous fluid which enters the formation, but significantly improves sweep efficiency. Even though it is common to utilize surfactants in order to generate and stabilize foams, surfactants tend to degrade at high temperatures and in high salinity environments. Adding nanoparticles can solve the aforesaid problems and can increase foam stability.
The choice of surfactant concentration is a critical step in preparing more stable foams. In the present work, using CO2/alpha olefin sulfonate (AOS) solution as a new foaming solution is introduced for optimizing surfactant concentration in order to generate a stable CO2-foam in unconventional reservoirs. Several experimental studies were conducted to obtain the optimal surfactant concentration using a pendant drop method for CO2/solution and CO2/nano solution. Moreover, the effects of temperature, pressure, salinity, and surfactant concentration on surface tension and the critical micelle concentration (CMC) value were studied at high pressure and high temperature (HP/HT). In these experiments the temperature ranged from ambient conditions to 302°F, while the pressure increased from atmospheric up to 435 psi. AOS solutions were prepared using different brine concentrations ranging from 1 to 10 wt% of NaCl and different surfactant concentrations from 0 to 1 wt%.
Experimental results indicated that the CMC value increases as temperature increased. It also decreased while salt concentration increased. Furthermore, for a given temperature and salinity, the results did not exhibit changes in the CMC value when the pressure increased. The addition of nanoparticles decreases the CMC value.
A number of research studies have been conducted to investigate the CMC value and surface tension for AOS at ambient conditions using N2. However, minimal work has been performed in order to determine such characteristics at reservoir conditions. The present work will provide a new foaming solution in order to evaluate and optimize surfactant concentrations. The present work will also investigate the effect of mixtures of surfactant and nanoparticles on the formation of stable CO2-foam in unconventional reservoirs.