The desilter, also known as a hydrocyclone desilter, drilling fluid desilter, or mud desilter, is a device that uses centrifugal pumps to provide pressure, allowing drilling fluid to pass through a set of small-diameter hydrocyclones combined with fine mesh screens. It is designed to separate solid particles ranging from 15 to 47 microns in size from the drilling fluid.

1. The Role of the Desilter in the Solids Control System

In a standard five-stage solids control system, the desilter functions as the fourth-stage purification unit, with a clearly defined operational role:

First-stage purification: Shale shaker – primarily removes large particles larger than 74 microns.

Second and third-stage purification: Degasser and desander – mainly handle harmful gases and fine sand particles ranging from 47 to 74 microns in the drilling fluid.

Fourth-stage purification: Desilter – the drilling fluid purified by the desander flows directly into the desilter compartment. From there, the fluid is pumped by the desilting pump into the desilter, where smaller solid particles (14–47 microns) are separated, completing the fourth-stage purification.

Fifth-stage purification: Centrifuge – used to separate finer solids (≥2 microns) and colloidal particles.

The desilter effectively bridges the gap between coarse and fine treatment stages, serving as the primary equipment for controlling the fine particle content in the drilling fluid.

2. Core Differences Between the Desilter and the Desander

Although both the desilter and the desander operate based on the hydrocyclone principle, they differ distinctly in processing targets, technical parameters, and system positioning. The two devices work in coordination, not as substitutes for one another.

 Difference in Particle Separation Size

Desander: Primarily separates larger particles ranging from 47 to 74 microns (approximately 200–325 mesh) — these are generally considered “sand-sized” particles.

Desilter: Primarily separates finer particles ranging from 14 to 47 microns (approximately 600–1000 mesh) — these are “silt- or mud-sized” particles.

 Difference in Equipment Structure and Size

Desander: Uses larger-diameter hydrocyclones, designed to handle higher flow rates and larger particles.

Desilter: Uses smaller-diameter hydrocyclones, where the smaller size helps generate stronger centrifugal forces, enabling the effective separation of finer solids.

Difference in Position Within the Solids Control Process

Desander: Installed after the shale shaker and degasser, but before the desilter, to perform initial fine particle removal.

Desilter: Installed after the desander and before the centrifuge, performing finer purification and preparing the drilling fluid for final-stage separation.

In summary, the desander and desilter form a continuous purification sequence, transitioning from “sand removal” to “silt removal.”

The desander performs coarse separation, while the desilter performs fine separation, working together to ensure that harmful solid phases in the drilling fluid are removed step by step, efficiently and thoroughly.

3. Core Value of the Desilter in Drilling Operations

Protect Hydrocarbon Reservoirs: Prevent fine particles from clogging reservoir pores, thereby preserving production capacity.

Increase Mechanical Drilling Rate: Low solid content significantly improves drilling efficiency and shortens the drilling cycle.

Maintain Downhole Safety: Reduces the risk of stuck pipe and other hazards caused by thick filter cake.

Control Overall Costs: Minimizes equipment wear and reduces the frequency of drilling fluid dilution and chemical treatments.

Conclusion

The drilling fluid desilter and desander are two complementary and functionally distinct key devices within the solids control system. Through the coordinated process of “sand removal” followed by “silt/mud removal,” the solid content of drilling fluid can be controlled progressively and precisely, providing a solid foundation for efficient, safe, and cost-effective drilling operations.