# An Ultra-Stable Polysaccharide Gel Plugging Agent for Water Shutoff in Mature Oil Reservoirs
)1 State Key Laboratory of Shale Oil and Gas Enrichment Mechanisms and Effective Development, Beijing 100083, China; [EMAIL_ADDRESS] (P.L. ); [EMAIL_ADDRESS] (Y.W. )1 State Key Laboratory of Shale Oil and Gas Enrichment Mechanisms and Effective Development, Beijing 100083, China; [EMAIL_ADDRESS] (P.L. ); [EMAIL_ADDRESS] (Y.W. )1 State Key Laboratory of Shale Oil and Gas Enrichment Mechanisms and Effective Development, Beijing 100083, China; [EMAIL_ADDRESS] (P.L. ); [EMAIL_ADDRESS] (Y.W. )1 State Key Laboratory of Shale Oil and Gas Enrichment Mechanisms and Effective Development, Beijing 100083, China; [EMAIL_ADDRESS] (P.L. ); [EMAIL_ADDRESS] (Y.W. inorganic salts and double-layer compression effects contribute to insufficient thermal and salt resistance in polyacrylamide-based gel plugging agents [13]. Moreover, the flexible chain structure of polyacrylamide polymers makes them susceptible to molecular chain breakage under high-shear forces. Shearing through surface equipment, pipelines, perforations, and formations significantly reduces gel strength, indicating inadequate shear resistance. Currently, the functional modification of acrylamide-based polymers and multi-component compounding are the primary methods used to enhance the gel strength and stability of these systems. Introducing monomers with bulky steric hindrance groups effectively increases polymer chain rigidity, thereby significantly improving resistance to temperature, salinity, and shear forces [14; 15]. Additionally, incorporating materials such as clay minerals and inorganic nanoparticles with acrylamide-based polymers enhances gel strength and stability through physical filling and cross-linking [16; 17]. However, these technical measures do not fundamentally alter the flexible chain structure characteristic of acrylamide-based polymers, and their resistance to temperature, salinity, and shear forces still falls short of meeting the requirements of field operations. Biopolymers such as xanthan, guar, and cellulose are eco-friendly and non-toxic materials that have been used in oilfields for decades [18; 19; 20; 21]. However, existing biopolymer gel systems are mainly composed of biopolymers and inorganic cross-linkers [22; 23; 24], which makes the biopolymer gel system unsuitable for the use in high-temperature reservoirs. The absence of active groups (such as amide groups) in the macromolecular chains of commonly used biopolymers in oilfield operations is an important reason for the lack of biopolymer organically cross-linked gel systems. Chitosan (CTS), which is a biopolymer consisting of \(\beta\)-(1,4)-2-amino-2-deoxy-D-glucopyranoside and 2-acetamido-2-deoxy-D-glucopyranoside units, is obtained via the deacetylation of chitin--the second-most abundant biopolymer next to cellulose in nature; the molecular chains exhibit greater rigidity, resulting in enhanced resistance to temperature, salinity, and shear forces compared to acrylamide-based polymers [25; 26]. As a derivative of chitin, CTS is eco-friendly and has many excellent biological properties such as biocompatibility, biodegradability, and antibacterial characteristics [27; 28]. CTS shows favorable reaction activity due to the presence of lots of highly reactive amino groups on the macromolecular chains, which makes it possible to achieve organic cross-linking between CTS and organic cross-linkers. N,N\({}^{\prime}\)-methylene-bis-acrylamide (MBA) is a low-toxic organic cross-linker that has been used in many fields including water shutoff operations [29; 30; 31]. Gels used for water shutoff require exceptional adaptability for in situ crosslinking within underground reservoirs. Conventional polyacrylamide-based gels utilize inorganic or phenolic crosslinking agents to form network structures through ionic bonding or condensation reactions, which are challenging to control under complex environmental conditions.