Frac-out is defined as an unintentional or inadvertent loss of drilling fluids during a drilling operation from the borehole to the ground surface from points other than its entry and exit points. Frac-out during a trenchless operation can happen due to various reasons.
Commonly, frac-out happens when downhole mud pressure exceeds the overburden pressure, or the drilling fluid meets a fault line, fracture or loose material. Frac-outs happen quite commonly during a horizontal directional drilling (HDD) project and are usually minor and close to the entry and exit points of the borehole.
Is Frac-out Serious?
HDD has become a choice method for contractors to carry out pipe installation because of its minimal surface disruption. The carbon footprint left by a trenchless installation method is much lower than the open cut method, provided serious frac-outs do not occur.
Minimal frac-outs do occur frequently near borehole entry and exit points, within the construction right of way (CROW), and usually, are not a cause for concern. However; a frac-out becomes a serious issue if it happens in the borehole path that passes through an environmentally sensitive location, such as wetlands, estuaries and freshwater systems.
The purpose of HDD is to avoid these areas, and a frac-out in these areas misses the point altogether.
Sometimes frac-outs can happen near infrastructures, such as highways and embankments. This can lead to damage to the structure by heaving or settlement.
With the number of regulations and acts surrounding these environmentally sensitive locations, it is necessary for contractors and owners to have contingency plans in place in case of frac-outs in these locations. The amount of time and cost associated with cleaning up a frac-out can take the profit out of the project. The best way to prevent a frac-out is to have a thorough knowledge of the geological stratification of sub-surface layers through which the borehole will pass.
This can be obtained through geotechnical investigation using methods such as ground penetrating radar (GPR), reconnaissance, boreholes, trial pits, laboratory testing, and also by obtaining core samples from directional core drilling. (Read Understanding Key Laboratory Testing Methods for Site Investigation.)
This knowledge is necessary while designing the right drilling fluid mix that will maintain sufficient downhole pressure to keep the borehole sealed. (Read Mixing the Mud: The Science of Drilling Fluid in HDD.)
How to Detect a Frac-out?
Frac-out can be easily detected by visual means if it happens near the vicinity or sight range of the drill rig. But what if the frac-out occurs out of sight? How do you know?
- The most obvious sign of a frac-out is loss of circulation pressure of the drilling fluid. Since the function of the drilling fluid is to maintain the downhole pressure and keep the borehole sealed, a loss of pressure is indicative of loss of drilling fluid circulation. Some loss of drilling fluid is normal during HDD because small fissures, fractures and loose sand encountered during the drilling process will be filled up by the drilling fluid.
- Another significant sign of frac-out is the drastic reduction in the quantity of drilling fluid returning to the site. This means that a good quantity of drilling fluid has escaped through some path in the geology and reached the surface.
- Yet another sign of a potential frac-out is when there is a loss of drilling fluid and the return drill cuttings do not carry sufficient gravel, indicating loss of containment pressure in the borehole.
How Should I Respond to a Frac-out?
When a frac-out is detected, the field team should be ready to respond immediately as per laid down state or federal regulations. Following necessary emergency actions that will not cause additional harm to the sensitive resources should be taken:
- Stop all directional boring activities such as drilling and drilling fluid pumps.
- Pullback the drill string and relieve pressure at the frac-out portion.
- Notify management and safety department and take necessary action.
- If frac-out is minor, not harming sensitive resources and easily contained, a leak stopping compound can be used to stem the frac-out.
- If the frac-out has reached the surface, the contaminated portion shall be excavated by hand up to a depth of two feet and disposed according to regulation.
- If the frac-out becomes widespread, a vacuum truck can be used to suck up the contaminated mud.
- The recovered drilling fluid can be recycled or disposed of at approved disposal facility.
7 Quick Tips to Prevent a Frac-out
Frac-outs are preventable if proper design parameters are adhered to, and proper drilling practices are followed. Here are a few ways frac-outs can be prevented:
- Conduct appropriate geotechnical investigation using recommended methods as per American Society for Testing and Materials (ASTM) to obtain detailed information on soil.
- Design correct drilling fluid mix with appropriate gel strength, plastic viscosity and yield point.
- Using parameters from the geotechnical investigation report, assess frac-out risk by using software such as DGeo Pipeline by Deltares or pressure calculations which compare maximum allowable fluid pressure against expected drilling fluid pressure.
- If reduced cover and bearing pressure exists at entry point, casing can be installed or relief wells can be drilled.
- Use correct drill bit size as per drill pipe dimension and use appropriate bottom hole assembly (BHA) for the formation.
- Continuously monitor drilling fluid parameters such as circulation pressure, gel strength, mud weight, and viscosity.
- Conduct regular inspection along the drill path during pilot hole drilling.
What We've Learned
Frac-outs can be easily prevented if contractors and owners do not downplay the importance of carrying out a thorough geotechnical investigation of the project site. Money saved by skirting this can prove to be a costly affair in case a serious frac-out occurs.
Since sub-surface conditions are not entirely predictable, it is also necessary that contractors have a contingency plan in place to deal with it in case frac-out does occur.