Fluid loss reducer

In the drilling process, due to the role of pressure difference, the water in the drilling fluid is inevitably filtered through the well wall to the formation, resulting in the loss of drilling fluid water. As water enters the formation, clay particles in the drilling fluid adhere to the well wall to form a “filter cake”, forming a filter cake wall. Because the cake well wall is much denser than the original well wall, it prevents further water loss of drilling fluid on the one hand and protects the well wall on the other hand. However, during the formation of filter cake wall, too much water lost, too thick filter cake, and fine clay particles entering the formation with water will affect the normal drilling and cause damage to the formation.

Function of fluid loss reducer:

The filtration loss of drilling fluid and the formation of filter cake is inevitable in the drilling process, and the filter cake can be formed to protect the well wall through the filtration loss. However, excessive fluid loss can easily lead to shale expansion and collapse, resulting in wellbore instability. In addition, with the increase of filtration loss, the filter cake thickens, reducing the diameter of the well, causing greater torque to the rotating drill tool, causing swabbing and pressure fluctuation during tripping, which is easy to cause differential pressure sticking. Therefore, proper control of fluid loss is one of the important properties of drilling fluid. Obviously, the fluid loss is closely related to formation permeability. However, the formation of filter cake occurs at the same time of drilling fluid filtration loss, and when the drilling fluid filtration loss occurs again, it must pass through the already formed filter cake. Therefore, the main factor determining the size of filtration loss is the permeability of filter cake. How to form high quality filter cake with low permeability and prevent further loss of drilling fluid is one of the main problems to be considered in drilling fluid preparation.

Fluid loss can be divided into instantaneous, static and dynamic filtration. Instantaneous filtration accounted for a small proportion in the whole filtration process. Dynamic filtration is more consistent with downhole conditions, but it is difficult to measure both indoors and on site. Although there is a certain gap between static filtration and the actual situation, the evaluation method is simple and can reflect the filtration performance of drilling fluid well.

Therefore, in order to form a filter cake with low permeability and small filtration loss, two conditions must be met when preparing drilling fluid:

(1) Reasonable multi-level dispersed particle distribution. That is, there must be large, medium and small particles in the drilling fluid, and there must be a reasonable distribution. Practice has shown that the drilling fluid must contain bridging particles smaller than the maximum pore size of the formation to be drilled up to one-third of the maximum pore size of the formation. This is conducive to bridge 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 range of particles ranging in size from large to small to the equivalent of sol particles. In this way, in the process of drilling, the large to small particles successively bridge the pores from large to small, so gradually, the more the pores are blocked, the smaller the filter cake becomes denser and its permeability becomes smaller and smaller. The better the distribution of solid particle size in drilling fluid, the shorter the formation time and the lower the permeability of filter cake.

(2) 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 colloid particles. If the colloidal particles are flat and have good hydration, they are easy to deform under pressure, and the permeability of the filter cake formed is naturally low. In addition to the above conditions, the fluid loss reducer should be added to the drilling fluid to control the amount of fluid loss.