Drilling fluid (drilling mud) is an essential element of many trenchless construction projects. Over the years, the science of drilling fluid has developed extensively, allowing contractors to realize substantial benefits in the execution of a project.
Choosing the right fluid for the ground conditions as well as having effective fluid management and recycling systems are crucial elements of a successful project. The project will benefit from maximizing the uptime of the drilling rig, therefore reducing schedule and costs. At the same time, there will be minimal drilling fluid losses and thus, lower operating costs.
Some estimates show that drilling fluid costs could reach as high as 50% of a contractor’s expenses, which highlights the importance of good fluid management and prevention of losses.
The Purpose of Drilling Fluid
Drilling fluid performs three main functions in a trenchless construction project:
- It creates borehole stability by lining the internal wall and resisting the collapse of soil into the borehole.
- It suspends cuttings, meaning that the fresh cuttings from the drilling process are suspended in the drilling fluid solution and do not settle on the bottom of the borehole.
- It removes cuttings from the hole. The entire drilling fluid system is a recycling loop where cuttings are first removed from the borehole. Then the drilling fluid is cleaned before being used in the borehole again.
Types of Drilling Fluid
Over the years, the science of drilling fluid has improved significantly. Each type of soil can be matched to the most appropriate drilling fluid for that project. This fact raises the importance of performing geotechnical surveys before a project begins. Starting with a clear picture of the types of soil along the proposed route allows engineers to specify the best drilling fluid for the job.
While drilling fluid consists mostly of water, some additives adjust its properties according to the project needs. The two main additives are bentonite and polymers or a combination of both. Loose soils like sand respond well to bentonite. It helps form a filter cake which stabilizes the borehole. More compact soils like clay or shale have the risk of swelling, but polymer additives help to prevent this.
Fluid Management and Recycling Systems
A typical drilling fluid management system consists of a mixing tank, a pump, a shaker and a mud cleaner.
The mixing tank is where the clean recycled drilling fluid returns. It can also be used to make up fresh drilling fluid or top up the volume in circulation. Regular tests of drilling fluid composition in the tank enable contractors to dose additives to achieve the desired ratios. The pump circulates the drilling fluid through the borehole and back to the cleaning system.
Drilling fluid cleaning systems usually work in two stages. The first is a simple shaker to remove large cuttings. From there a desanding process removes finer particles from the fluid. Drilling fluid management systems aim to maximize the recycled fluid and maintain a consistency of composition.
Maximizing Work and Minimizing Cost
An efficient drilling fluid management and recycling system reduces operating costs in several ways:
- It reduces the need for fresh water to make up the drilling fluid volume. Water is expensive to buy and may need to be transported to the site, which adds to the costs.
- It reduces waste disposal costs. Drilling fluid must be handled following local authority regulations, and this includes its disposal. Too much drilling fluid loss in the cuttings removal and filtration will result in higher volumes of waste and higher costs.
But it is not only the management of drilling fluid that influences the cost of a trenchless construction project. The performance of the drilling fluid also plays a significant role.
If the composition of the drilling fluid does not match the soil conditions, the fluid may not hold cuttings in suspension. Over time, they will build up in the borehole, causing a loss of circulation in the fluid system. The borehole will also narrow making pullback of the product pipe difficult.
High pressures in the borehole can also cause inadvertent returns (frac-outs). Drilling fluid pushes its way out of the borehole and into the surrounding ground or to the surface. An inadvertent return event shuts down the project for repairs and costs the contractor in terms of drilling fluid losses.
Slow penetration rates and drilling equipment wear are other consequences of poor drilling fluid management. Instead of improving the execution of the project, the wrong drilling fluid mix can actually slow it down.
Conclusion
Drilling fluid is a critical element of many trenchless construction projects. The benefits of using the right drilling fluid composition are significant in terms of project efficiency and reducing costs. However, there are also risks associated with drilling fluid systems. The wrong composition can slow a project down.
Doing a detailed geotechnical survey helps engineers to specify the right drilling fluid upfront. Having an efficient management and recycling system helps minimize waste and keeps the drilling fluid performing as expected.