# Synthesis of Oligomeric Silicone Surfactant and its Interfacial Properties
## 1 Introduction  
Many low permeability gas-condensate reservoirs possess small reservoir pores and throats wetted by water, which can generate high capillary pressure. The mud filtrate, acidizing liquid, and fracturing fluid invasion during the drilling process cannot entirely flow back, so the reservoir water saturation increases, as the gas relative permeability significantly decreases near the wellbore. The reduction of surface tension of invaded fluids is the most effective way to decrease the damage caused by water blocking. As the temperature and pressure drop below the dew point near the wellbore during drilling, a special gas accumulates and condensates into liquid. Since gas condensate cannot spread on the pore surface wetted by water, it results in the throat being plugged leading to a sharp decrease in gas relative permeability. The rate of such permeability damage to the reservoir can reach 70-90%, with the reduction of the gas well production of less than one-third [1; 2; 3]. Numerous methodologies, including solvent injection, microwave heating, and hydraulic fracturing, have beenused to reduce condensed liquid damage [4; 5; 6]. Furthermore, wettability alteration from water-wet to preferential gas-wet is also employed to boost gas condensate flow [7]. Capillary pressure is the pressure between two immiscible fluids flowing through a narrow space, resulting in the interaction of forces between the fluids and the solid walls of the tube. Moreover, a decrease in capillary pressure can be achieved by using an adsorbing surfactant at the gas-liquid interface [8; 9; 10]. Gas reservoir wettability is the ability of the mud filtrate, formation water, and gas condensate to stick to and spread onto the pores, which influence various fluid flow and irreducible water saturation by decreasing capillary pressure and surface energy. Wettability alteration can be achieved by using an adsorbing surfactant at the solid-liquid interface [11; 12; 13]. Since the solid-liquid and gas-liquid interfaces appear simultaneously in the flow back of invaded mud filtrate, capillary pressure reduction and preferential gas-wetness alteration can be achieved with the addition of a surfactant to the drilling fluid. Surfactants are organic compounds composed of hydrophilic polar and hydrophobic non-polar groups, with only a small amount necessary to significantly change the interfacial properties. Currently, hydrocarbon and fluorocarbon surfactants are the most commonly utilized surfactants in oil and gas development. Hydrocarbon surfactants, such as Emulsifier OP-10, Sodium dodecyl benzene sulfonate (or ABS in short), and Lauryl benzene sulfonate diethanolamine salt (or ABSN in short), are inexpensive and environmentally friendly, they can reduce the surface tension between gas and liquid to 30-35 mN/m, and they are widely used in emulsions, foaming agents, and capillary pressure reduction. However, they are unable to alter the wettability of reservoirs from water-wet to preferential gas-wet by adsorption. On the other hand, fluorocarbon surfactants are expensive and hazardous to the environment but possess good surfactivity, which can decrease the surface tension between gas and liquid below 20 mN/m, promoting defoaming and wettability alteration. Silicone surfactants have excellent thermal stability, high surface activity, and outstanding ecological safety and biodegradability, which are mainly attributed to the feature of the siloxane chain [14; 15; 16], and they have a very high molecular weight [17]. Reports have shown that sulfonate silicon surfactants can be synthetized via free radical copolymerization of polysiloxanes and sulfonic acid monomers. The hydrophobicity and oleophobicity of the surfactant increase with the polysiloxane chain length. The hydrophobic groups are cross-linked with relatively low silicon content. Due to siloxane hydrolysis, the interfacial tension of the sulfonate trisiloxane surfactant, prepared by hydrosiylation reaction, increases after a prolonged period of storage at normal temperature [18].