Abstract

High temperature and high salinity environments in deep well drilling can easily lead to the failure of drilling fluid performance, leading to accidents such as well collapse and leakage, and affecting drilling safety and efficiency. As one of the most important additives in drilling fluid systems, fluid loss agents reduce the filtration rate of drilling fluid through methods such as gel protection, viscosity increase, and plugging, achieving the effect of reducing formation damage and maintaining wellbore stability.With the continuous increase in the operation volume of deep and ultra deep wells in China, the number of drilling conditions such as high-temperature and high-pressure formations and salt gypsum layers is gradually increasing. The research and development of temperature and salt resistant fluid loss agents has become a key issue in tackling high-temperature drilling fluid technology in complex wells.

As one of the most widely researched and widely sold core materials in drilling fluid treatment agents, fluid loss agents form a stable spatial network structure through adsorption, hydration, and clay particle combination, forming a dense low permeability mud cake on the wellbore, significantly reducing the difficulty of circulating fluid pumping and the risk of wellbore instability caused by the invasion of drilling fluid filtrate into the formation and excessive filtration. The temperature and salt resistance of fluid loss additives is the key to ensuring the stable performance of water-based drilling fluids in salt bearing formations during high-temperature deep well drilling. In recent years, it has become a hot topic in the research of water-based drilling fluid technology both domestically and internationally, and has been highly valued by relevant domestic scientific researchers and manufacturers.

At present, there are many types of temperature and salt resistant fluid loss agents, but there is a lack of in-depth understanding of the relationship between the main components, molecular structure, and temperature and salt resistant performance of the products in the industry, which has affected the research on temperature and salt resistant fluid loss agents. Therefore, water-based drilling fluid loss agents widely used domestically and internationally have been classified into three categories based on their main components: natural polymer materials, synthetic polymers, and organic/inorganic composite materials.

This article provides an overview of three types of temperature and salt resistant water-based drilling fluid loss additives, mainly including natural polymer and modified material fluid loss additives, synthetic polymer type loss additives, and inorganic/organic composite material loss additives. This article summarizes the research results of temperature and salt resistant drilling fluid loss agents from the aspects of raw material composition, synthesis (modification) methods, and product performance. It explores the development direction of temperature and salt resistant drilling fluid loss agents in China, providing theoretical guidance for the research and application of temperature and salt resistant fluid loss agents.

1. Natural Polymers and Modified Materials Fluid Loss Additives

Natural polymer materials have advantages such as abundant sources, green environmental protection, and low prices, and are widely used in the petroleum industry. However, natural polymer materials have significant deficiencies in temperature and salt resistance, making it difficult to meet the actual needs on site. Through chemical modification of natural materials, the temperature and salt resistance of natural polymer materials can be effectively improved, and the filtration loss reduction performance can be improved. At present, natural polymer material fluid loss agents mainly include three types: starch, cellulose, and humic acid.

1.1 Starch based fluid loss agents

Starch is produced by α-A natural polysaccharide polymer formed by glucose polymerization, with C2, C3, and C6 hydroxyl groups on the glucose monomer serving as active sites for chemical modification. According to the modification method, starch based fluid loss agents can be divided into etherified starch, grafted copolymer starch, cross-linked starch, etc.

Etherified starch is a modified starch prepared by modifying and etherifying starch with epoxy propane, sodium chloroacetate, etc. Products with high commercialization mainly include carboxymethyl starch (CMS), hydroxypropyl starch (HPS), etc. CMS is an anionic etherified starch with a temperature resistance generally between 120~130℃ and a certain degree of salt resistance, but its resistance to high valence salts is weak. CMS products containing potassium salts also have the effect of stabilizing shale and reducing pore size expansion. HPS is prepared by reacting starch with substances such as epoxy propane, without ionic groups, and has better salt and calcium resistance than CMS. Although the etherification products of starch have good salt resistance, they lack temperature resistance, and the maximum usage temperature generally does not exceed 130℃.

By using chain transfer agents to generate free radicals in the main chain of starch, graft copolymerization of starch and vinyl monomers can be achieved, thereby obtaining graft copolymerization modified starch. Vinyl monomers carry functional groups, so grafting copolymerization can improve the temperature resistance of starch. Zhang Yaoyuan et al. synthesized a modified starch fluid loss agent St-AANDP. By introducing a benzene ring structure into the molecular chain, the temperature resistance of the starch material can be increased to 160℃ and the salt resistance can reach saturation.

Crosslinked starch is a modified starch prepared by using crosslinking agents such as epichlorohydrin and sodium trimetaphosphate to chemically crosslink starch molecules. By forming a stable spatial network structure, chemical bond breakage can be reduced at high temperatures, significantly enhancing the temperature resistance of starch and having a certain degree of sealing ability. Li Jiaqi et al. synthesized a cross-linked starch drilling fluid loss agent. Compared with carboxymethyl starch fluid loss agent, this loss agent has significantly improved its high-temperature resistance, with an API filtration capacity of 11.0 mL at 160℃. Chen Siqi et al. synthesized a starch microsphere using N-hydroxysuccinimide (NHC) as a crosslinking agent. After aging at 150℃, the API filtration of 10% saline based slurry and 1% CaCl2 based slurry decreased by 55% and 60%, respectively, and microcracks were also sealed.

1.2 Cellulose based fluid loss agents

Cellulose and starch are both polysaccharide macromolecules, but the monomers of cellulose are β-Glucose which has a higher degree of polymerization and strong intramolecular and intermolecular hydrogen bonding, making cellulose materials superior to starch in temperature resistance. However, its water solubility is poor and chemical modification is necessary before it can be applied to water-based drilling fluids. Cellulose modified fluid loss agent products mainly include cellulose ethers and grafted copolymer modified cellulose.

Cellulose ether based fluid loss agents mainly include carboxymethyl cellulose (CMC), hydroxyethyl cellulose (HEC), and polyanionic cellulose (PAC). 

CMC fluid loss additive was first applied in drilling fluids, but its temperature resistance is weak, generally not exceeding 130℃. PAC and CMC have similar chemical structures and belong to the category of anionic cellulose ethers. However, PAC molecules have a high degree of substitution, a more uniform distribution of substituent groups, and better temperature and salt resistance. The temperature resistance of commercially available PAC products can reach 180℃. HEC belongs to non ionic cellulose ether, which has stronger salt resistance and certain calcium resistance compared to anionic cellulose ether. It is suitable for drilling fluids in high salt environments.

There is relatively little research on cellulose graft copolymerization modification of fluid loss agents, mainly based on the modification of CMC and PAC. Zhao Baoquan grafted acrylamide (AM), 2-acrylamido-2-methylpropane sulfonic acid (AMPS), and methacryloyloxyethyltrimethylammonium chloride (METAC) onto CMC to prepare a grafted copolymer modified cellulose fluid loss additive. The fluid loss additive has a temperature resistance of 150℃, resistance to 30% NaCl, resistance to 1.5% CaCl2, and good biodegradability, which can be applied in environmentally friendly drilling fluids.

1.3 Humic acid based fluid loss agents

Humic acid is a macromolecular polymer with aromatic rings as the main body and many phenolic, ketone, carboxyl and other functional groups connected. As a drilling fluid treatment agent, it not only has the function of reducing filtration loss, but also has anti collapse and viscosity reduction effects, and is widely used in the field of drilling fluid. Humic acid molecules have strong hydrophilicity, adsorption ability, and large specific surface area, and have good coordination, chelation, and chemical reaction abilities. The early application product of humic acid fluid loss agent is sulfonated lignite, which can withstand temperatures up to 200℃, but has poor salt resistance and fluid loss reduction effect. Currently, copolymerization with resins or olefins is commonly used to improve its performance.

Wei Yan introduced AM, N, N-dimethylacrylamide (DMAM), and sulfonation agent (JSJ) into the humic acid structure through graft copolymerization to prepare a modified humic acid fluid loss agent CPHA. The performance of this fluid loss agent remains stable after 72 hours of aging at 200℃, and is suitable for saline systems containing 15% salt. Wang Zhonghua synthesized a low molecular weight humic acid graft copolymer (AOBS-AM-AA) by grafting and copolymerizing humic acid with acryloyloxybutyl sulfonic acid (AOBS), AM, and acrylic acid (AA). This fluid loss agent can withstand temperatures up to 240℃ and can further improve the rheological properties of drilling fluid under high temperature conditions. It can still exhibit good fluid loss reduction effect in saturated saline slurry.

Table 1 summarizes the modification methods and characteristics of commonly used natural polymer materials in China. Although there are many modification methods for starch and cellulose materials, which improve temperature and salt resistance on the basis of raw materials, the performance improvement is limited, especially insufficient temperature resistance, which is generally below 180℃, making it difficult to meet the needs of deep well drilling.

From Table 1, it can be seen that the modification method of grafting copolymerization using natural polymer materials and olefinic monomers has significantly improved the temperature and salt resistance of the fluid loss agent. However, in some studies, the proportion of grafted polymer in natural polymer materials modified by grafting is relatively high, and some monomers are prone to self polymerization during the copolymerization process, leading to a weakening of the environmental protection characteristics of natural polymer materials. In the future research on natural polymer fluid loss agents, while pursuing higher temperature and salt resistance performance, the advantages of natural polymers as green, environmentally friendly, and economically effective should also be fully utilized. This requires exploring new modification methods for natural polymer materials.