Although the use of drilling fluids within the horizontal directional drilling (HDD)/trenchless industry is still relatively new, recent trends suggest that this standard is shifting as solids control is one of the most important phases in the drilling process. This shift is due to a growing awareness that drilling fluid maintenance and disposal can become exponential problems as they are both harmful to the environment and extremely costly.
With the correct solids control system, fluid life can be doubled, if not tripled. Since whole mud dumping and dilution are unacceptable from both a cost and environmental standpoint, effective solids control equipment has become an increasingly important aspect in any drilling process.
One of the most important objectives in solids control is to remove as many of the large drilled solid particles (i.e. contaminants), as is practical, the first time that these solids are pumped out of the borehole. This must be done without significantly impacting the commercial drilling fluid solids. This requires properly designed and installed solids removal treating equipment, sized to process a minimum of 100 up to 125 percent of the mud circulation rate. Solids that are not removed during the first circulation through the surface equipment are subjected to mechanical degradation by the drill bit, reamers and mud pumps during each circulation cycle until they are too fine for removal by traditional mechanical (i.e. Primary cleaning systems utilizing shaker and hydrocyclone technology) means.
In order to evaluate the removal capabilities of the various pieces of mechanical treating equipment, it is necessary to consider the source of the solids and classify them according to the following sizes. Table 1 provides a cross reference to commonly used terminology used within the HDD industry:
Often good solids control practices are ignored simply because they are not understood. Typically, an HDD solids control system will employ a combination of shaker, hydrocyclone, mixing, pump, and compartmentalized tank technologies.
The shale shaker is vital to the solids control system and is considered the “first line of defense”. Shakers act by removing 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 elected) will simply be overloaded beyond their design capacity.
Shale Shaker Screens
Basically, a screen acts as a guard of whether a particle is small enough to pass through a screen or not. Screening surfaces used in solids control equipment are generally made of multilayered woven wire screen cloth. The screening surfaces are considered the center of the shaker; therefore, the quality of the shaker is thus defined by the quality of screens it utilizes. 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 RP designation as defined by the American Petroleum Institute.
Hydrocyclones are simple mechanical devices that do not contain moving parts and are 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. Drilling mud enters the cyclone under pressure from a centrifugal pump. The velocity of the mud then 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 underflow. Hydrocyclones are important in a solids control system because of their ability to efficiently remove particles smaller than the finest mesh screens.
Another important aspect of a well-engineered solids control system is the use of centrifugal pumps. Centrifugal pump not only mix the mud to be pumped down the bore hole, but also provide the feed pressure and volume required to operate the hydrocyclones. Maintenance of these pumps is essential to the operation of your solids control equipment.
Centrifuges are another essential part of any effective solids control system. They have a great advantage of achieving extremely fine cut points. The rotating speed is transformed into centrifugal force inside the rotating assembly to accelerate particle settling in accordance with Stoke’s law. Essentially, a centrifuge acts as a small settling pit which alloys rapid settling of solids under controlled conditions.
In order to achieve effective solids control there are several steps that should be taken. The first and most critical step is to obtain an effective, dependable and durable solids control system. Special attention should be given to ensure that the system is hydraulically balanced, capable of making both a scalp cut and a fine cut, and that the system has sufficient drilling fluid mixing and re-circulating capacity. Shale shakers and other solids control equipment should never be bypassed while drilling. Various mesh sizes should be used on the shaker as conditions change from formation to formation requiring operators to maintain a variety of screens to be kept on hand. Furthermore, an adequate inventory of recommended spare parts and screens should always be kept enhancing operations.
To achieve effective solids control, the following should be deployed:
1. Obtain an effective, dependable, and durable solids control system.
2. Ensure that the solids control system acquired is:
a. Hydraulically balanced
b. Is capable of making both a scalp cut and a fine cut.
c. Has sufficient drilling fluid mixing and re-circulating capacity.
3. Do not by pass the shale shaker or other solid control equipment while drilling.
4. 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.
5. Maintain an adequate inventory of recommended spare parts and screens.
6. Certify and assign rig personnel to be responsible for equipment operation & maintenance.
a. Request equipment commissioning and training for your OEM.
b. Discuss possible written training programs that may be available from your OEM.
By employing each of these steps, operators can achieve enhanced rig performance, improved rates of penetration, reduced waste disposal costs, and reduced mud costs.