The Key Principles of Effective Solids Control
Knowing the right equipment and materials to use is key to effective solids control in trenchless construction projects.
Well-used drilling fluid will be full of unwanted solid content from the borehole after a drilling operation. The drilling fluid picks up all the soil and rock cuttings generated during the drilling process and effectively removes them using the slurry system.
The problem is, what should be done with the solids-ridden drilling fluid? As most industry professionals know, disposing of used drilling fluid is a major challenge for the trenchless industry.
However, there is a viable and effective solution to the problem – solids control. This technology removes solids from the drilling fluid and saves costs associated with mud quantity and disposal. This article will take an in-depth dive into solids control – what it is, the equipment used, the key steps involved, the benefits, and the role of drilling fluid.
What is Solids Control?
Solids control essentially means the management of cuttings in the drilling fluid. Why is this control necessary?
For one, it makes disposal of used drilling fluid much easier and manageable. The other reason is that, once the solids are removed from the used drilling fluid, the fluid can be recycled and reused by adding appropriate additives, substantially saving cost.
Efficient removal of solids significantly reduces the volume of waste generated. For example, a 10% removal of drilled solids can reduce waste by up to 1000 barrels. This is quite significant when it comes to reducing the carbon footprint of a project.
Solids control aims at removing as most of the large cuttings from the drilling mud when it is first pumped out of the borehole. However, this should not affect the good solids in the mud that are necessary for drilling. To accomplish this, the solids removal treating equipment should be properly designed and installed and sized to process at least 100 to 125% of the mud-circulation rate.
Equipment Used for Solids Control
The arrangement of the solids control equipment is done such that:
- The larger solids are removed first.
- Each piece of equipment discharges into the next compartment immediately downstream.
- Every compartment in the removal section (except the sand trap) features backflow from the downstream to the upstream compartment.
- Adequate agitation is provided to all tanks except the settling tank.
- All suction pits are agitated.
- No solids-removal equipment of the degasser has a settling pit for a suction pit.
Here are the key steps in a solids control equipment setup:
- The Shale Shaker - The shale shaker is the first piece of equipment in the arrangement and mainly removes large drilled solids from the drilling fluid. Its performance will depend on the shape and size of the screen openings, the drilling fluid properties, and the amount and type of cuttings generated. Only American Petroleum Institute (API) 13C qualified screens should be utilized.
- The Sand Trap - The sand trap design is such that it overflows into the next compartment downstream allowing solids to settle. Sand traps are particularly effective for water-based drilling fluid.
- Degasser - Used drilling fluid from petroleum prospective areas will have gas trapped in it. To remove this, the fluid is passed through a degasser after it has been passed through the shale shaker and the sand trap. Either vacuum or atmospheric degassers are installed on most rigs.
- The Hydrocyclones - Drilling mud is injected tangentially into the hydrocyclone, creating centrifugal forces that drive the solids to the walls of the cones. The solid particles containing small amounts of fluid are discharged from the bottom of the hydrocyclone and the processed fluid flows out from the top of the hydrocyclone to the next stage.
There are two types of hydrocyclones – desanders, and desilters. Desanders are of 6 inches diameter or larger and are used mostly with water-based muds. Desilters have a diameter of lesser than 6 inches and are designed to remove silt-sized particles from the drilling fluid.
- The Mud Cleaners - A mud cleaner is a combination of hydrocyclones and a fine shaker screen. The underflow received from the hydrocyclone contains a concentration of solids and drilling fluid. This is sieved through API 200 or 150 screens to remove drilled solids larger than barite. Barite, a weighting agent, is ground so that most of it is smaller than 74 microns (API 200), allowing most of it to pass through the screen. Mud cleaners are located on the surface tanks in the same position as desilters.
- The Centrifuges - Centrifuges remove ultra-fine or colloidal-sized solids that prove very problematic if allowed to stay in the mud system. The centrifuge is a high-speed rotating bowl and scroll assembly capable of separating down to 5 to 7 microns without using chemicals. The slurry is fed into the bowl where the centrifugal force propels the ultra-fine solids against the inner wall. A rotating conveyor drives these solids towards the discharge port where they are removed into a chute. The liquid retained is discharged through the liquid discharge ports at the other end of the bowl.
Benefits of a Good Solids Control System
- Reduces the need for replacement of fluid end parts.
- Reduces wear on the drill string.
- Less key-seating that can lead to stuck pipe.
- Easy casing installation.
- Extended bit life due to less abrasion.
- Increased rate of penetration.
- Dilutions to maintain low mud weights is reduced resulting in reduced cost, and reduced drilling fluid waste production.
- The addition of weighting agents is made easier.
- Setting and release of downhole tools becomes easier with less interference from drilled cuttings.
Role of Drilling Fluid
A good mix of drilling fluid helps in proper solids control and keeps the solids control equipment working efficiently. In addition, it:
- Cleans the borehole by removing cuttings.
- Keeps the cuttings in suspension when the fluid is static
- Decreases wear on the bit by displacing the drill cuttings from the bore face.
- Cools and lubricates tooling and the drill string.
- Increases the rate of penetration.
- Correct viscosity and gel strength prevent settling, inadvertent returns, and stuck pipe.
- Stabilizes the borehole.
- Keeps fluid loss at a minimum which is beneficial as it builds a thin, slippery wall cake that protects the formation, supports the borehole, and lowers the pullback pressure.
It is recommended to keep mud recipes simple to prevent complications. This also helps the mud hand to understand the mixing formula better and understand what roles the components perform. A simple mud recipe will generally have water as the base, bentonite clay, and polyanionic cellulose (PAC) polymers such as FlowMaster or UniDrill.
Tru-Bore, a highly concentrated bentonite-based drilling fluid is a non-toxic, environmentally safe, high-performance viscosifier that maintains borehole integrity during pullback. Soda Ash is often added to soften and condition the makeup water and raise pH to help maximize the performance of bentonite or polymer.