Abstract:

Tannin extract can be used to address the problem of gas channeling during steam huff-and-puff process. However, it faces difficulties in gel formation at low temperatures and poor stability at high temperatures. Through a systematic study on the high-temperature gel-forming properties of valonea tannin extract, myrica tannin extract, and acacia tannin extract, and comprehensively considering factors such as stability, structural characteristics, and economic factors, myrica tannin extract was finally chosen as the key component of the gelling agent. Experiments were carried out in which the mass fraction of myrica tannin extract in the investigated system was in the range of 4%-8%. The results showed that the optimized formula of the gelling solution consisted of 6% myrica tannin extract, 4% water-soluble phenolic resin, 3% nano-SiO₂, and 0.1% catalyst. The gelling solution containing 2%-6% tannin extract had a long gelling time at low temperatures. After heat treatment at 250 ℃ for 3 days, the elastic modulus of the gel strength was in the range of 8-10 kPa, the degree of dehydration after 30 days was less than 15%, and the structure was stable after long-term heat treatment, which was suitable for the plugging of steam channeling in offshore oilfields. By observing the microstructure of the gel with Cryo-SEM, it was found that nano-SiO₂ could transform the gel from a loose structure into a dense three-dimensional network structure, which contributed to its enhanced thermal stability. By using thixotropic loop method, it was found that when the amount of nano-SiO₂ exceeded 3%, the gelling solution had thixotropy, which was beneficial for formation plugging. Elastic modulus, viscous modulus, and creep tests showed that adding nano-SiO₂ could increase the elastic modulus of the gel. When the amount of nano-SiO₂ was 3%, the peak value of elastic modulus of the gel after 30 days of aging could reach 0.256 34 MPa, which could significantly delay the strain overshoot. When the amount of nano-SiO₂ was 5%, the yield stress of the gel could reach a maximum of 1 500 Pa. This research could provide technical support for solving the problem of steam channeling during steam huff-and-puff process in oilfields.

Key words: steam channeling, gel of tannin extract, nano-SiO₂, gelling regularity, thermal stability