2. Synthetic Polymer type Fluid Loss Agents

Synthetic polymer type fluid loss agents mainly consist of vinyl polymers, which are typically formed from AM and various functional monomers through copolymerization reactions to form one or multiple polymers. The research on polymer based fluid loss agents has been relatively early abroad, especially with the successful development and promotion of commercial products like AMPS polymer series, such as the COP, Polydrill and other polymer based fluid loss agent series products from BASF in Germany, Pyro Trol and KemSeal polymer based fluid loss agent series products from Baker Hughes in the United States, and related products and technologies occupy an international leading position.

The research on polymer based fluid loss agents in China started relatively late, but with the increasing requirements for the performance of fluid loss agents under harsh conditions like high temperature, high pressure, and high salinity in the formation, synthetic polymer based fluid loss agents have gradually become a research hotspot in recent years due to their advantages such as good molecular chain thermal stability, strong molecular structure regulation, multiple types and sources of functional monomers, and relatively simple synthesis processes. Synthetic polymer type fluid loss agents mainly include linear polymer fluid loss agents, branched chain polymer fluid loss agents, and micro crosslinked polymer fluid loss agents based on the geometric configuration of molecular space.

2.1 Linear Polymer Fluid Loss Additives

Linear polymers are the most widely studied type of synthetic polymer based fluid loss agents. These polymers are often formed by polymerization of acrylamide monomers with strong hydration, adsorption, and rigidity monomers, and have characteristics such as high molecular weight, good water solubility, and easy swelling. The linear polymer molecular structure is flexible and easy to block the pores of the mud cake through deformation, with outstanding filtration reduction effect. The commonly used monomers for linear polymer synthesis in China include acrylamide monomers, anionic monomers, cationic monomers, zwitterionic monomers, cyclic monomers, organosilicon monomers, etc. The main types and functions of monomers are detailed in Table 2.

With the increasing number of harsh geological conditions such as high temperature and high salinity in deep and ultra deep drilling, the research and development of polymer based fluid loss additives is gradually moving towards resistance to ultra-high temperature and salt calcium. The type of polymer is gradually expanding from binary and ternary polymers to multivariate polymers.

Researchers have conducted extensive research on functional monomer screening, optimization of polymerization methods, evaluation of treatment agents, and exploration of action mechanisms. They have synthesized a series of polymer based fluid loss agents with good temperature and salt resistance by utilizing strong hydration monomers to enhance polymer hydration ability, cyclic monomers to enhance molecular rigidity, and strong adsorption groups to enhance clay stability. Significant progress has been made in related research, The product performance meets or exceeds that of similar foreign products.

Luo Mingwang et al. synthesized a quaternary polymer fluid loss additive (PANAD) using AMPS, DMAM, N-vinylpyrrolidone (NVP), and diallyl dimethyl ammonium chloride (DMDAAC) as raw materials through aqueous solution polymerization. This agent has a temperature resistance of 230℃, a NaCl resistance of 20%, and better salt resistance and filtration loss performance than the advanced foreign fluid loss agent product Driscal D. Yan Xing and others selected four monomers, namely DMAM, sodium styrene sulfonate (SSS), DMDAAC, and NVP, to prepare a high-temperature and saturated salt fluid loss additive FLA-1. After 16 hours of hot rolling at 220℃, the saturated saline slurry of this agent has a high temperature and high pressure filtration loss of 15.5 mL, showing good resistance to high temperature and saturated salt.

In recent years, researchers have explored methods such as introducing zwitterionic monomers to enhance salt resistance, and introducing organic silicon monomers to enhance the interaction with clay to enhance the temperature and salt resistance of polymers. There are two main ways to synthesize zwitterionic polymers: one is to introduce both anionic and cationic monomers into the molecular structure, and the other is to introduce monomers with both anions and cations in the molecular structure, such as betaine monomers; Organic silicon polymers are mainly modified by the use of silane coupling agents. Under high temperature conditions, polymer Si-H reacts with clay surface OH, forming strong chemical bonds to enhance the adsorption of clay. Outstanding progress has been made in related research results.

Zhang Meng et al. prepared zwitterionic copolymer fluid loss additive PADAN using AM, NVP, AMPS, and self-made betaine monomer (2-methylacrylamide propyl dimethylamino) propane sulfonate (DMAPMAS). This treatment agent has a temperature resistance of up to 200℃, resistance to saturated salts and 1% CaCl2, and has good inhibitory effects. Four monomers, vinyl trimethoxysilane (A-171), AM, N-vinylcaprolactam (NVCL), and AMPS, were selected by You Fuchang et al. to synthesize a new type of silicone fluid loss additive. This treatment agent has a temperature resistance of up to 200℃ and a CaCl2 resistance of up to 15%, exhibiting good high-temperature stability and salt resistance.

2.2 Branched chain polymer fluid loss agent

Branched chain polymers introduce long side chain groups onto linear polymers to enhance the rigidity of their molecular structures. Under high temperature and high salt conditions, polymers which have long side chains, high steric hindrance,are difficult to curl molecular chains. They have a large hydrodynamic radius, thus can maintain good viscosity and reducing filtration efficiency. Branched chain polymer fluid loss agents mainly include comb-type polymers and hydrophobic association polymers.

Domestic researchers mainly synthesize comb shaped polymers by introducing monomers such as allyl polyoxyethylene ether (APEG) into the molecule. Hydrophobically associating polymers are a type of branched polymer prepared by introducing long chain alkyl groups onto the polymer chain. After reaching the critical association concentration in aqueous solution, such polymers are prone to form intermolecular physical cross-linking network structures, which is beneficial for improving the temperature and salt resistance of the polymer. The main hydrophobic monomers include long-chain alkyl cations, styrene, etc. The research on branched polymers provides new ideas for the development of temperature and salt resistant polymer fluid loss agents, and has broad application prospects. Xu Yunbo et al. aimed at the problem of insufficient resistance of linear polymers to high salt calcium, synthesized a comb type polymer fluid loss additive (DMP-1) using APEG, AM, and AMPS. The viscosity reduction rate of this additive after 180℃ hot rolling was less than 42%, and the amount of DMP-1 added to the base slurry was 0.5%. The fluid loss reduction effect was basically stable at 200℃, and it could resist saturated NaCl and 3% CaCl2 at 180℃. Quanhongping et al. synthesized a five component comb polymer fluid loss additive using APEG, AM, AMPS, NVP, and ethylene glycol ethylene ether as raw materials through aqueous solution polymerization, which the medium pressure filtration loss is 25 mL after hot rolling at 240℃, and has excellent resistance to saturated salts, high temperature thermal stability, and filter loss reduction performance under 150℃ conditions. Li Dong et al. synthesized a new type of hydrophobic association polymer using monomers such as AM, AMPS, and octadecyldimethylallylammonium chloride (DMAAC-18). As the salt concentration increases (NaCl<20%), the polymer's filtration reduction effect first decreases and then increases. This treatment agent has a temperature resistance of 180℃ and a resistance to NaCl of 35%.

2.3 Micro crosslinked polymer fluid loss agent

Micro crosslinked polymers form moderately crosslinked polymers by adding crosslinking agents in the polymerization reaction. The main research and development strategy for this type of polymer is to increase its molecular weight, enhance the rigidity of the polymer molecular chain, and form a moderately cross-linked three-dimensional network structure while ensuring water solubility, thereby improving the temperature and salt resistance of the polymer.

Rong Kesheng et al. used AM, AMPS, NVP, and crosslinking agent TDED as reaction monomers to prepare a micro crosslinked fluid loss agent PTAPN. After aging at 240℃, it had good filtration and rheological properties, good compatibility, and a high-temperature and high-pressure filtration loss of 12.6 mL after high-temperature aging. The filtration loss reduction effect was significantly better than linear polymers. However, this type of polymer has shortcomings such as high selection requirements for crosslinking agents and poor reaction controllability, so there are few related research reports.

Synthetic polymer based fluid loss agents have achieved significant results in product development and promotion, occupying a certain market share in China. However, there is still a gap in temperature and salt resistance compared to advanced foreign technologies. In terms of theoretical research, there is a lack of systematic research on the mechanisms of polymer failure between polymer molecular chains, between polymers and clay under high-temperature and high salt conditions. In terms of product performance, there are mainly problems such as insufficient product quality control, relatively lagging industrialization of high-quality functional monomers, and relatively single product structure and preparation process.

In the future, it is necessary to strengthen the research on relevant basic theories, in order to guide the molecular design and structural optimization of polymer fluid loss additives, and thereby reduce low-level repetitive research work. In addition, domestic environmental protection requirements are becoming increasingly strict, and research on the environmental non-toxic properties of drilling fluids is gradually increasing. Developing environmentally friendly synthetic polymer fluid loss additives and reducing waste drilling fluid pollution is expected to become one of the research directions for the development of water-based drilling fluid water loss additives in the future.