Principles of Solids Control

Relative to the use of drilling fluids within the oil and gas industry, the use of drilling fluids within the horizontal directional drilling (HDD) industry is new. Even today, the amount of research and development aimed at HDD drilling fluids is considered a rounding error in the oligopoly of drilling fluid manufacturers. However, the trends strongly suggest that the future of HDD drilling will follow a similar evolution as experienced by the traditional oil and gas industry.

Couple the evolution of drilling fluids with the escalating cost of waste management and disposal, the economic practicality of utilizing drilling fluids, is fully dependent on the ability to recover and recycle. Though the cost of barite and bentonite continue to be commoditized, additives continue to become more and more expensive. Add the escalating cost of qualified personnel, HDD rig operators are left with a critical challenge: making profits, when the cost of business is increasing.

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In addressing the economics of drilling fluids, HDD rig operators must clearly understand the basic operating principals behind the available recycling processes. The following provides for a practical understanding of the key principles behind HDD solids control and drilling fluid management. With that said, the following discussion should not be considered exhaustive. Ultimately, the manufacturer of the drilling fluid and the solid control and waste management equipment should be consulted.

Solids Control System
When deploying a “solids control system,” there are several key hydraulic design factors that must be considered. These factors simply provide a “hydraulically balanced” solids control system, however, buyers of solids control systems should not overlook the durability, dependability and return on investment that the features and benefits bring.

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A typical HDD solids control system will employ a combination of shaker, hydrocyclone, mixing, pump and compartmentalized tank technologies. The shaker is vital to the solids control system and is considered the first line of defense.  Located directly downstream from the shaker will likely be a hydrocyclone assembly that will employ desilters, desanders or both. Though still considered rare, centrifuge-enable solids control systems are becoming more popular.  By adding a centrifuge, the solids control system will achieve the highest level of cut available within the market today.
Hydraulic Design Criteria
The first line of defense for a properly designed solids control system is the shaker.  Shakers remove solids from drilling fluid as the mud passes over the surface of a vibrating screen. Particles smaller than the openings in the screen pass through the holes of the screen along with the liquid phase of the mud. Particles too large to pass through the screen are thereby separated from the mud for disposal.  Without proper screening of the drilling fluid during this initial removal step, downstream equipment will experience reduced efficiency and effectiveness. The downstream hydrocyclones and/or centrifuges (if employed) will simply be overloaded beyond their design capacity.

There are a number of commercial claims that indicate that shakers can achieve solid cuts lower than 74 microns, however, when it comes to the practical installation and use of solids control systems (i.e. relative to flow rates, drilling fluid viscosity, screen condition, volume of solids being managed, etc.) operators should conservatively assume 100 microns as the performance limit. Shaker Screens and Hydrocyclones

Shaker Screens

Basically, a screen acts as a “go/no-go” gauge: Either a particle is small enough to pass through the screen or it is not. Screening surfaces used in solids control equipment are generally made of multi-layered woven wire screen cloth and are the “heart and soul” of the shaker.  Fundamentally, the quality of a shaker is defined by the quality of screens it utilizes. When selecting a solids control system it is important that the quality of the screen manufacturing and level of experience by the in manufacturing screens is considered.

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If the system is appropriately set up with a scalping system and a fine screen system, the scalping screens must be sized just coarse enough to ensure that drilling fluid does not sheet off the shaker (i.e. whole mud losses). This happens when the scalping screen is too fine to allow the drilling fluid to pass into the solids control system’s dirty tank. Typical scalping screen configurations for HDD applications range from 50 mesh (API 50) to 120 mesh (API 100).  Typical fine screen configurations range from 160 (API 120) to 200 mesh (API 170). The API 13C classification is gaining traction in the HDD industry as more contractors focus on the micron cut point they are trying to achieve in their fluid.

Relative to the use of shaker screens, it is the size of the screen openings, not the mesh count that determines the size of the particles separated by the screen. It is because of this fact that HDD system users should compare and specify screens based on their API 13C designation.

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Hydrocyclones are simple mechanical devices, without moving parts, designed to speed up the settling process. Feed pressure is transformed into centrifugal force inside the cone to accelerate particle settling in accordance with Stoke’s Law. In essence, a cyclone is a miniature settling pit that alloys rapid settling of solids under controlled conditions. Hydrocyclones have become important in solid control systems because of their ability to efficiently remove particles smaller than the finest mesh screens. There are a number of industrial claims that indicate that hydrocyclones can achieve solid cuts lower than 25 microns, however when it comes to the practical installation and use of solids control systems (i.e. relative to pump impeller wear, drilling fluid viscosity, internal hydrocyclone wear, etc.) operators should conservatively assume 30 microns as the performance limit.

Drilling mud enters the cyclone under pressure from a centrifugal feed pump. The velocity of the mud causes the particles to rotate rapidly within the main chamber of the cone (i.e. like a tornado in a bottle). Light, fine solids and the liquid phase of the mud spiral inward and upward for discharge through the liquid outlet. Heavy, coarse solids tend to spiral outward and downward for discharge through the solids outlet or under flow.

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Like hydrocyclones, centrifuges are designed to speed up the settling process. The rotating speed is transformed into centrifugal force inside the rotating assembly to accelerate particle settling in accordance with Stoke’s Law. In essence, a centrifuge is a miniature settling pit that alloys very rapid settling of solids under controlled conditions. Though much more complicated than a hydrocyclone, centrifuges have the advantage of achieving extremely fine cut points.

Centrifuges can improve the cut point by a factor of 6X when compared to a prototypical shaker/hydrocyclone system and can dramatically increase the drilling fluid life. KEMTRON has experienced applications in which drilling fluid life was increased by a factor of 10 when utilizing centrifuges in cooperation with the standard solids control equipment (Total improvement in drilling fluid life is dependent on the particle size distribution of the solids generated and the degradation life cycle of the solids within the drilling fluid).

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Summary of Effective Solids Control System
To achieve effective solids control, there are several steps that should be taken. To start, obtain an effective, dependable, and durable solids control system. Ensure that the solids control system acquired is; hydraulically balanced; is capable of making both a scalp cut and a fine cut; and has sufficient drilling fluid mixing and re-circulating capacity. Do not by-pass the shale shaker or other solid control equipment while drilling. Use the smallest mesh screen possible on the shale shaker; this will change from formation to formation and will require operators to maintain a variety of screens on hand. Always maintain an adequate inventory of recommended spare parts and screens. Last and not least, a good solids control system is only effective when you certify and assign rig personnel to be responsible for equipment operation and maintenance. By employing each of these steps, operators can be achieve improved rig performance, improved rates of penetration, reduce waste disposal costs, and reduce mud costs.

Michael Rai Anderson, P.E., is president and CEO of KEMTRON Technologies Inc.

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