In the process of drilling, due to the effect of pressure difference, the water in the drilling fluid will inevitably leak into the formation through the wellbore, resulting in the loss of water in the drilling fluid. With the water entering the formation, clay particles in the drilling fluid will adhere to the well wall to form a "filter cake", forming a filter cake well wall. Because the well wall of filter cake is much denser than the original one, it prevents further water loss of drilling fluid on the one hand and protects the well wall on the other hand. However, in the process of the formation of the filter cake wall, too much water is lost, the filter cake is too thick, and the fine clay particles enter the formation with water, which will affect the normal drilling and cause damage to the formation.

（1） The filtration of drilling fluid and the formation of filter cake are inevitable in the process of drilling. However, if the filtration rate of drilling fluid is too large, it is easy to cause shale expansion and collapse, resulting in wellbore instability. In addition, with the increase of the filtration rate, the filter cake thickens, which makes the well diameter smaller, causing larger torque to the rotary drilling tool, causing swabbing and pressure fluctuation during tripping, which is easy to cause differential pressure sticking. Therefore, it is one of the important properties of drilling fluid to control the filtration properly. Obviously, the filtration rate of drilling fluid is closely related to formation permeability. However, filter cake is formed at the same time of drilling fluid filtration, and the filter cake must be passed through when the drilling fluid is filtrated again. Therefore, the main factor determining the filtration rate is the permeability of filter cake. How to form high quality filter cake with low permeability and prevent further filtration of drilling fluid is one of the main problems to be considered in drilling fluid preparation.

A. Static filtration equation the filtration of drilling fluid can be divided into instantaneous filtration, static filtration and dynamic filtration. The proportion of instantaneous filtration loss in the whole filtration process is small; the dynamic filtration loss is more in line with the downhole situation, but it is difficult to determine whether it is indoor or on-site; although there is a certain gap between the static filtration and the actual situation, the evaluation method is simple and can well reflect the filtration performance of drilling fluid.

B. In the process of filter cake formation, the static filtration rate is often measured by the filtration tester in the laboratory. In the experiment, the drilling fluid in the filter cup flows out through the filter paper. Fig. 1-4 schematic diagram of solid phase invasion of drilling fluid in formation 1-4 the outflow rate of filtrate is very fast before the formation of filter cake. After 1-2 s, the outflow rate of filtrate gradually slows down and tends to be uniform after a certain period of time. The filtration loss of filter paper before the formation of filter cake is called instantaneous filtration (also known as initial loss), and the amount of filtration during this period is called instantaneous filtration. This process also exists when drilling through permeable formations. In order to reduce the instantaneous filtration loss, the drilling fluid should contain appropriate large solid particles to bridge and block the rock pores. When these larger particles form a bridge plug at the throat of the rock, the smaller particles plug into the pores formed between the larger particles, and continue until the small sol particles block up the small pores of the filter cake. At this time, only water can penetrate into the formation. In this way, the particles in the drilling fluid form three kinds of distribution in the wellbore and formation (Fig. 1-4).

a. The outer filter cake on the well wall.

b.  Internal filter cake formed by particles entering the formation.

c. These particles will block the formation pores and damage the reservoir after migration.

Therefore, in order to form filter cake with low permeability and small filtration loss, two conditions must be met in the preparation of drilling fluid.

A.  Reasonable multi-stage dispersion of particles distribution. That is to say, there must be large, medium and small particles in the drilling fluid with reasonable distribution. Practice shows that the drilling fluid must contain bridging particles which are smaller than the maximum pore size of the formation and up to one third of the maximum pore size of the formation. This is beneficial to bridge and plug the large pores of the newly drilled formation as soon as possible and reduce the instantaneous filtration loss. In addition, the drilling fluid must contain a series of particles with the size from large to small, and even small to equivalent to sol particles. In this way, in the process of drilling, the particles from large to small successively bridge the pores from large to small. In this way, the pores become smaller and smaller, the filter cake becomes denser and denser, and its permeability becomes smaller and smaller. The more appropriate the size distribution of solid particles in drilling fluid, the shorter the time of forming dense filter cake and the lower the permeability of filter cake.

B.  Type of colloidal particles. The permeability of filter cake is not only related to the size distribution and quantity of colloid and fine particles contained in drilling fluid, but also closely related to the type of colloidal particles. If the colloidal particles are flat and have good hydration property, they are easy to deform under pressure, and the permeability of the filter cake formed is naturally low. In addition to the above conditions, fluid loss additive should be added to the drilling fluid to control the fluid loss.