Solid-free drilling fluids are composed of inorganic salts and different kinds of polymers, which have constant rheological properties and filtration characteristics. Potassium formate drilling fluid has the characteristics of high solubility, high density, high PH value and low crystallization point. Appropriate water loss is conducive to improving drilling rate and is a new type of drilling fluid with film-forming wall protection effect.

Potassium formate has a density of 1.91g/cm³ and is easily soluble in water, non-toxic and non-corrosive. The water solution is colourless and transparent, and the saturated solution has a specific gravity of 1.58g/cm³. The basic components and functions of potassium formate drilling fluid are as follows:

1. Potassium formate drilling fluid is moderately crosslinked with organic polymers to increase the viscosity (mainly structural viscosity) of the solution and reduce the water loss of the solution.

2. Potassium formate drilling fluid regulates the salinity of the solution to balance the chemical activity of the formation and inhibit the formation expansion, dispersion, fragmentation and collapse.

3. Potassium formate drilling fluid can also adjust the PH value of the solution.

Potassium formate drilling fluid system has good reservoir protection, anti-collapse and inhibition, and its mechanism is mainly embodied in three aspects:

Firstly, potassium formate drilling fluid is a solid-free system, which has no solid damage to oil and gas reservoirs.

Secondly, the E potential on the surface of the cuttings soaked in potassium formate drilling fluid is very low and the hydration film is very thin, so the cuttings are not easy to hydrate and disperse.

Finally, potassium formate drilling fluid system is conducive to the improvement of mudstone membrane efficiency, thus promoting wellbore stability.