How to Evaluate Hydraulic Fracture Risk in HDD Design

If the drilling fluid pressure required to transport the cuttings along the drilled hole exceeds the strength of the soil, localized fracturing of the soil can occur. This is often referred to as hydraulic fracture or hydrofracture. If the drilling fluid from the drilled hole flows through these fractures to the ground surface, it is commonly called an inadvertent return (IR) or more accurately a drilling fluid surface release. It is also colloquially referred to as a frac-out.

There can be numerous causes of hydraulic fractures and drilling fluid surface releases including inadequate removal of cuttings and drilling through inherently weak soils. Thoughtful HDD design is critical to the reduction of the hydraulic fracture risk.

Origins of Hydraulic Fracture Evaluations

A method to evaluate the hydraulic fracture risk for HDD installations was first published by the United States Army Corps of Engineers (USACE) in 1998 as part of the Construction Productivity Advancement Research (CPAR) program (Bennett and Staheli, 1998). The evaluation method was based on cavity expansion research from the Dutch Delft University of Technology (Delft 1997, Luger and Hergarden 1988). Since publication, the Delft method outlined in the CPAR has become the standard in evaluating the risk of hydraulic fracture for HDD installations. Recent research, case studies, and further study of the cavity expansion theory have resulted in numerous publications seeking to better define limitations of the model, present appropriate model inputs, and posit potential alternate methods for evaluating the hydraulic fracture risk under specific subsurface conditions. These have contributed to better modeling by practitioners and provided additional tools to refine evaluations of hydraulic fracture risk. However, the Delft method remains the standard when evaluating hydraulic fracture risk.

The Delft model incorporates components for both, cohesive and cohesionless soils, but has some significant limitations that practitioners should consider. Because loading conditions for HDD operations induce rapid stresses on surrounding soils, undrained conditions should be assumed when selecting shear strength properties of soils. Also, care should be taken when selecting appropriate values for elastic soil modulus, shear modulus, Poisson’s ratio, and radius of the plastic zone. Inexperience in evaluating these properties can yield inaccurate or inappropriate results. Practitioners should also be reminded that the Delft model or any other model used to estimate the risk of hydraulic fracture are invalidated when full drilling fluid returns are not maintained. When drilling fluid returns cease or are diminished significantly, the drilling fluid pressures can increase rapidly resulting in hydraulic fracture even where the models estimate low risk.

Evaluating Hydraulic Fracture Risk

The procedure for estimating the risk of hydraulic fracture has been documented by numerous referenced authors. In general, the procedure includes the following three steps:

1.Estimate the total stress state and strength of a soil unit along the HDD profile (resisting force). This is often referred to as the formation limit pressure.

2.Estimate the drilling fluid static and dynamic pressure required to remove soil cuttings from the drilled hole (driving force) that is required to be applied in order to maintain a clean and properly conditioned hole.

3.Estimate the hydraulic fracture factor of safety by dividing the resisting force by the driving force.

The Delft model is most commonly used for estimating the formation limit pressure; however, practitioners should consider all models available and select the most appropriate model for the anticipated subsurface conditions. The pressure required to transport the cuttings-laden drilling fluid to the surface consists of two components. The first component is the hydrostatic drilling fluid pressure and is proportional to the depth or vertical distance measured from the top of the drilling fluid column to the drill bit. As the vertical depth of the drill bit increases, the drilling fluid pressure increases.

The second component is the hydrodynamic pressure associated with transporting the cuttings-laden drilling fluid through the annulus of the drilled hole. As the drilled length increases, the pressure required to remove the cuttings-laden drilling fluid increases with the increased frictional resistance to fluid movement. Estimating realistic drilling fluid pressure requires a thorough understanding of HDD construction practices, drilling fluid weight, plastic viscosity, drilling fluid yield point, and trenchless equipment tooling. Bentonite-based drilling fluid is a non-Newtonian fluid and requires specific rheological models to estimate drilling fluid pressures. The drilling fluid pressure required to circulate bentonite-based drilling fluid can be estimated by using either the Bingham Plastic or Power Law model (Bourgoyne 1991).

USACE Requirements For Work Under Levees

In recent years, the USACE has taken a keen interest in the evaluation of hydraulic fracture risk associated with HDD installations under or near regulated levees. Historically, the USACE has issued permits for HDD installations under levees where the Delft model was used with appropriate input parameters and adequate factors of safety applied. This approach changed in 2020 when the USACE abandoned the conventional Delft model and embraced a more conservative approach (USACE 2020). The USACE also released a 2023 engineering regulation seeking to clarify the 2020 guidelines and institute new regulations when evaluating the risk of hydraulic fracture associated with levees or other regulated structures (USACE 2023). Practitioners are in the process of evaluating the implications of these new regulations and how they will affect the permitting process for projects associated with USACE levees.

Targeted HDD Design to Reduce Frac-out Risk

During the design process, a trenchless engineer often has flexibility in geometric configuration to place the HDD profile at a targeted depth or location. When this flexibility allows the engineer to target a soil unit with a higher shear strength, the results include a higher resistance to hydraulic fracture. The higher factor of safety achieved through targeted HDD design can decrease the risk of hydraulic fracture and drilling fluid surface releases.

Conclusions

Hydraulic fracture and drilling fluid surface release resulting from an inaccurate evaluation of risk can result in significant unexpected costs to clean up, risk to the environment, danger to existing infrastructure, potential penalties and fines, productivity impact, and damage to the HDD industry’s reputation. The hydraulic fracture evaluation should be performed for each trenchless construction project by a qualified practitioner. The process to appropriately evaluate the hydraulic fracture risk continues to evolve as research and field measurements continue to increase our understanding and advance the field. Practitioners should continue to stay abreast of the continuing research by participating in the several professional organizations which continually publish new advances in evaluating the risk of hydraulic fracture.

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