Application of Solvent-Assisted Dual-Network Hydrogel in Water-Based Drilling Fluid for Lost Circulation Treatment in Fractured Formation.

During oil and gas well construction, lost circulation caused substantial nonoperation time and extra costs, and hydrogel, resilient and environmentally friendly, was one of the major types of material for lost circulation treatment. To migrate the weak bonding and hydrothermal degradation of conventional single network hydrogels, dual network (DN) hydrogel was prepared and immersed in solvents of polyethylene glycol (PEG), ethylene glycol, and glycerol. The swelling of DN gels at different temperatures was studied with water content and swelling rate tests, and the gel structural and morphology was characterized with attenuated total reflectance infrared spectroscopy (ATR-IR) and scanning electron microscopy test. Then, the compression test and fracture plugging performance test were conducted to study the strength of the gel. The results show that compared to those in ethylene glycol and glycerin, DN gel after immersion in PEG (DN-PEG) exhibits greater compression strength and better plugging performance even at high temperatures. The compression strength of DN-PEG was twice that of DN hydrogel before immersion, and its fracture plug breaking pressure can reach over 10.0 MPa. After undergoing hydrothermal treatment at 90 °C, the compression strength of the DN-PEG was nearly 20 times that of the DN hydrogel, and the fracture plug breaking pressure was still 2.81 MPa. According to ATR-IR spectroscopy, as the molecular weight of the solvent increases, more hydroxyl groups in the PEG have better ability to bind with hydrogen bonds, which greatly inhibits the swelling and polymer chain breakage, thereby reducing hydrothermal degradation in the strength of the dual-network hydrogel. Our work proposed an effective method to reduce the degradation of hydrogel in water at high temperature, and the prepared DN-PEG hydrogel was a promising material for lost circulation treatments in fractured formation.