The treatment agent in drilling fluid includes inorganic treatment agent and organic treatment agent. The effect of high temperature on the inorganic treatment agent is mainly to intensify the thermal movement of inorganic ions, so as to enhance its penetration ability. This paper focuses on the effect of high temperature on organic treatment agents.

High temperature degradation

The phenomenon of molecular chain breakage of organic polymer compounds due to high temperature is called high temperature degradation. For drilling fluid treatment agent, high temperature degradation includes two aspects: the fracture of polymer main chain and the fracture of hydrophilic group and main chain connecting chain. The former reduces the molecular weight of the treatment agent and loses some or all of its high molecular properties, resulting in most or all of its failure. The latter reduces the hydrophilicity or adsorption capacity of the treatment agent, so as to reduce the salt and calcium resistance and efficiency of the treatment agent, and even lose its function.

Any polymer compound will undergo high-temperature degradation, but the obvious degradation temperature is different with its structure and environmental conditions. Therefore, high temperature degradation is another important problem that must be considered for high temperature resistant drilling fluid. Due to the great relationship between high-temperature degradation and medium, this paper only discusses its degradation in aqueous solution. Among them, the main factor affecting high-temperature degradation is the molecular structure of the treatment agent, which is determined by the high-temperature thermal stability of various bonds of the treatment agent molecules in aqueous solution. For example, ether bonds are easily oxidized in aqueous solution, and high temperature and pH value will promote this effect. Therefore, all polymer compounds connected by ether bonds are unstable and easy to degrade at high temperature, and this degradation is mostly related to oxidation, so it is called thermal oxygen degradation. Obviously, if we can try to stop or weaken this effect (such as adding antioxidants), the trend of high-temperature degradation can be reduced. In this medium, the hydrolysis of ester becomes more serious at high temperature. The second is the level of temperature and the length of action time. The temperature at which various polymers degrade obviously under different conditions is different from each other. The temperature at which the treatment agent degrades obviously in its solution is commonly used to express the temperature resistance of the treatment agent. The pH value and mineralization conditions in the solution also affect the degradation. Generally speaking, high pH value promotes the occurrence of degradation. Degradation is a gradual process, so it has a great relationship with the action time of high temperature, which must be seriously considered. Degradation is also related to other factors, such as bacteria, oxygen content, stirring shear and so on.

Since the thermal stability of the treatment agent is related to its molecular structure, the main chain, hydrophilic group, adsorption group and main chain connection bond of the high temperature resistant treatment agent molecule should try to use “C-C”, “C-N”, “C-S” and other bonds instead of “- O -” bonds.

Practice has proved that high-temperature degradation may also be reduced. Now the effective way is to use antioxidants. For example, phenol and its derivatives, aniline and its derivatives, sulfite and sulfide can improve the temperature resistance of cellulose treatment agents from 120 ℃ – 140 ℃ to 180 ℃ – 200 ℃. On the other hand, high temperature degradation can also be skillfully applied to better adjust and maintain the performance of drilling fluid, which has successful experience at home and abroad.

High temperature crosslinking

There are various unsaturated or active groups in the treatment agent molecules. Under the action of high temperature, it can promote various reactions and mutual connection between molecules, so as to increase the molecular weight. This effect is called high temperature crosslinking. Obviously, it can be regarded as the opposite effect of high-temperature degradation of treatment agent. General organic polymer treatment agents (especially natural polymers) can have high-temperature crosslinking, and high-temperature crosslinking may produce two results:

1. If the polymer crosslinks excessively and forms a three-dimensional spatial network structure, which is called bulk polymer, the treatment agent loses its water solubility. The whole system is called gel, and the treatment agent is completely ineffective.

2. The crosslinking of the treatment agent is appropriate, the molecular weight is increased, and the destructive effect of degradation is offset, so as to maintain or even increase the efficiency of the treatment agent. On the other hand, the proper crosslinking of the two treatment agents can make their hydrophilic ability and adsorption ability complement each other, and the result is equivalent to the further modification and efficiency increase of the treatment agents.

The influence of high-temperature crosslinking on the performance of drilling fluid has two aspects:

1. If the crosslinking is excessive, the treatment agent will be completely ineffective, the drilling fluid will be completely destroyed, the filtration loss will increase sharply, and the cementing of the drilling fluid (low amount of soil is inevitable). The water-insoluble polymer can be clearly seen in the drilling fluid;

2. If the crosslinking is appropriate, it is greatly conducive to the performance of drilling fluid, and the performance of drilling fluid is getting better and better under the action of high temperature. The result must be that the field application effect is better than the indoor experiment. In a certain range, the deeper the well, the higher the temperature, and the better the effect. Since high-temperature crosslinking can actually offset the high-temperature degradation, organic crosslinking agents can be added to effectively prevent the high-temperature degradation of treatment agents. However, due to the lack of research on high-temperature crosslinking and its influencing factors, there is no mature view and method on how to control it properly. Then, the understanding of high-temperature crosslinking and the establishment of relevant concepts at least give researchers the possibility of high-temperature crosslinking reaction to improve the deep well drilling fluid system, so as to transform the damage of the performance of high-temperature reactor deep well drilling fluid into the improvement of drilling fluid system at high temperature, This opens up a new way for deep well drilling fluid work.