Waste generation is the inevitable byproduct of every construction activity. The problem does not lie in the fact that waste is generated; the problem lies in the way it is managed. The trenchless industry has been significantly challenged when it comes to the disposal of waste generated during the drilling operation.
The use of drilling fluid is an essential part of trenchless technology as it provides the necessary lubrication to install the pipe and also acts as a carrier for removing the drill cuttings from the borehole. Drilling waste needs to be managed, and waste management methodology and guidelines vary with every project. Recycling the drilling mud has become popular because it saves costs associated with mud quantity and disposal.
Different technologies such as solids control, onsite disposal, incineration, bioremediation, waste recycling, and thermal desorption are now available for companies to implement and manage drilling waste.
What is Solids Control?
Waste management using solids control is the process by which solid particles generated from rock cuttings during the drilling process are separated from the drilling mud. The growing demand for minimizing the environmental impact associated with oil drilling has increased the importance of solids control.
The efficient removal of solids from the drilling mud significantly reduces the volume of waste generated by the drilling operation. A 10% improvement in drilled solids removal in deep well drilling can reduce the waste by 8000 to 10000 barrels. Removing solids from the drilling mud also makes it more suitable for recirculation into the system. Solids control equipment comprises of:
- Mud tanks for mixing and storage of drilling mud.
- Shale shaker to remove big sized solids.
- Hydroclones to speed up the settling process.
- Centrifugal pumps to provide feed pressure and volume required to operate the hydroclone.
- Desander to remove medium-sized solids.
- Desilter to remove small-sized solids.
- Mud cleaner.
- Vacuum degasser to remove air from the mud.
- Conveyance system to transport cuttings.
- Decanter centrifuge to recover barite and comply with environmental standards.
- Mud cooler for oil-based drilling muds and those with high-temperature applications.
- Cuttings dryer for removing moisture from cuttings below the 5% mark.
- Dewatering unit to remove fine solids (<5 microns).
- Containers for transporting cuttings.
- Cuttings collection and transportation system.
Understanding the Importance of Solids Control
A solids control system increases the usable life of the fluid twice or thrice as much, making it highly desirable to drilling companies. Mud dilution and mud dumping have become unacceptable due to environmental regulations requiring strict regulation of drilling waste disposal; it is also costly. Solids control is the best way to make a drilling operation and used mud disposal economical.
To maintain the mud quality and make it suitable for recirculation, the content of solids in the mud has to be kept below 5%. When the concentration of solids in the drilling mud is more, the quality of the filter cake reduces, increasing the thickness of the cake thereby affecting downhole filtration. Some problems associated with bad quality filter cake are increased torque and drag, sloughing, increased surge and swab pressures and stuck pipe.
Excess solids in the drilling mud also reduce the rate of penetration of drill bit, increases the risk of wellbore instability due to prolonged periods of open borehole and increases drilling costs.
Achieving Effective Solids Control
Solids can only be removed effectively if the system used to remove them is dependable and effective. Every step of the solids control system must be followed to ensure the solids of all sizes are properly removed. Since every project is different and sub-surface soil conditions vary, the solids control system should be adjusted to suit the site conditions best. This can be done by keeping a variety of meshes and screen sizes in hand including spare parts.
The solids control system should be hydraulically balanced, should be capable of making both scalp-cut and fine-cut, and should have sufficient capacity for mixing and recirculation of drilling fluid.
When the drilling fluid is first pumped out of the borehole, the first step of the solids control system is to remove as many of the large cuttings as possible without impacting the commercial drilling fluid solids. This can only be done if the solids control equipment is properly designed and installed, and sized to process 100 to 125% of the mud circulation rate.
The different equipment used in the solids control system has different removal capabilities. The following table gives a classification of the solids based on size:
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HDD solids classification
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API classification
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Micron range
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Inches
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Large drilled solids
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Intermediate and coarse
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>440
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>0.018”
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Sand
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Medium
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74 to 440
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0.0031” to 0.018”
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Silt
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Ultra-fine and fine
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2 to 74
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0.000083” to 0.0031”
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Clay
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Colloidal
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0.5 to 2
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0.000021” to 0.000083”
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Colloids
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Colloidal
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<0.5
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<0.000021”
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Treatment of Waste and Disposal
Once the solids control phase including solids collection is over, it is time to manage and dispose of the waste drilling fluid. While non-toxic waste does not require further treatment, oil-based and synthetic-based muds have toxic content and need to be treated before disposal.
The different options available to treat and dispose of the waste are cutting re-injection (CRI), thermal desorption, fixation/stabilization or encapsulation and dewatering. Effective solids control has a high impact on the cost-efficiency of a project. When the solids are removed from the mud, the volume reduces thereby reducing transportation costs.
It also reduces the need for the addition of mud products to maintain mud quality for the drilling operation by maintaining the rheology of the mud.