Inhibitory mud, such as lime mud, gypsum mud, calcium chloride mud and potassium-based mud, was widely used in mud shale formation to prevent collapse and stabilize borehole lining in the 1960s, which played a good role in treating unstable rock formations in water. Since the 1970s, the anti-collapse mud composed of polymer and potassium salt has been recognized as an ideal system, and nowadays potassium formate has been widely used.

1. Common methods of maintaining hole wall stability

(1) Cement slurry maintains hole wall stability

As a common consolidation material, cement has the advantages of wide supply, easy access, low cost, non-toxic, non-environmental pollution, easy use and easy to pour into the hole, compared with other anti-collapse slurry liquids. At present, cementitious materials, which are still widely used, are called "conventional weapons" to maintain hole wall stability.

Practice shows that cement slurry must be used in the case of serious collapse and leakage. As far as its characteristics are concerned, cement has certain permeability, good consolidation with rock, stable durability, high strength of hardened cement paste, and its fluidity and setting time can be adjusted artificially to meet the requirements of pouring technology under different drilling conditions. The lower maintenance hole wall stability has better adaptability. Therefore, the technology and method of using cement to maintain the hole wall stability has been widely valued and applied.

(2) Chemical slurry to maintain hole wall stability

At present, chemical slurry can be divided into inorganic and organic macromolecule according to its composition, and its performance can be divided into solidified and non-solidified. The commonly used water glass is the representative of inorganic slurry; the commonly used organic slurry is the representative of urea-formaldehyde resin and acrylamide; the commonly used cement is the solidified granular slurry, while clay and leachate are the non-solidified granular slurry and chemical slurry. In recent years, from the perspective of the development trend of chemical slurry, inorganic and polymer compound compound has developed into a variety of widely used materials for borehole wall protection and leak stoppage.

(3) Other methods of maintaining wellbore stability

Urea-formaldehyde resin cement balls, dry plugging materials - Drilling plugging sheets, asphalt materials have their own advantages and disadvantages. Generally speaking, the slurry is required to have low water loss, high salinity, high filtrate viscosity, appropriate specific gravity and proper viscosity. Low loss of water and high salinity mainly lie in reducing the hydration expansion of Unstable Strata in case of water; however, only controlling loss of water can not fundamentally prevent water from migrating into pore-wall strata, and the salt content of mud must be increased (i.e., salinity), so as to control surface hydration and permeability hydration. Direction makes water move from borehole to stratum instead of stratum to borehole.

Formate solid-free drilling and completion fluid system is a non-toxic water-based drilling fluid, which not only meets the requirements of environmental protection, but also has good temperature resistance, hydraulic characteristics, good inhibition and pollution resistance, especially good compatibility with reservoirs, and good reservoir protection effect. Potassium formate also has strong anti-collapse and inhibition ability, excellent rheological properties and water loss reduction ability.