Mitigating Inadvertent Returns in Horizontal Directional Drilling

Abstract

Inadvertent returns during horizontal directional drilling (HDD) may occur when the downhole pressure is too high and can become a concern when the fluids reach the ground level or a enter a body of water. Bore planning is the first step in helping to avoid inadvertent returns. A well-crafted bore plan needs to account for soil conditions, product type, fluid calculations, estimated pressure and equipment tooling. There are best practices in HDD construction that can be used in conjunction with planning. This paper outlines best practices in HDD construction and considerations involved in planning and designing a bore to minimize situations that could lead to inadvertent returns. 1. What are Inadvertent Returns? This paper aims to serve as a resource for individuals in the utility installation industry as a reference for methods used in the reduction of inadvertent returns during the horizontal directional drilling (HDD) process. Inadvertent returns (often referred to as “frac-outs”) are defined as drilling fluids that take an unintended path during the HDD process. These fluids may return along the surface of the HDD area, return into an unintended waterway or become unaccounted-for fluid loss (e.g., discharged into an aquifer). 2. Potential Causes of Inadvertent Returns Inadvertent returns have various potential causes. Most inadvertent returns are caused by higher pressures than the geological formation can withstand, and there are several practices that can lead to increased downhole pressure. This paper discusses the following: • Fluid spoil carrying capacity • Improper rate of penetration (ROP) • Improper drilling fluid mixture • Poor fluid quality • Challenging geological formations • Annular space obstruction Page 1 of 9 No-Dig Down Under 2019 Mitigating Inadvertent Returns in Horizontal Directional Drilling Cory Mass Tod Michael a. Fluid spoil carrying capacity refers to the capability of the drilling fluid to suspend cuttings in the drilling fluid to remove from the bore path. Improper fluid spoil carrying capacity can lead to inadvertent returns during the HDD process. The first step is to determine the volume of fluid needed to remove cuttings from the borehole. Once the volume is calculated, a material factor can be applied based on soil types. This will help identify the amount of fluid needed to properly carry the cuttings. Material factors can be seen in Figure 1. Figure 1. Drilling fluid material factor for specific soils b. Improper rate of penetration (ROP) is something that is often misunderstood in HDD. Rate of Penetration (ROP) is defined as the speed at which the cutting bit enters the ground. In equation 1 we will refer to ROP as “Drilling rate”. ROP being too high can create a situation where the fluid flow from the drill cannot remove all cuttings from the borehole. If more cuttings are being created than the fluid flow can carry, the cuttings will collect near the drill rod and cutting tools. This can lead to an annular space obstruction and ultimately increased fluid pressure. This increased fluid pressure may result in an inadvertent return. Once the soil volume, material factor have been identified, the proper drilling rate can be calculated using the drilling fluid pump output capabilities of the drilling rig. Drilling rate can be calculated using equation 1. Utilizing the calculated drilling rate will help guide the driller in determining the drilling rate that will allow for proper spoils removal. Equation 1. Drilling rate c. Improper drilling fluid mixture can lead to inadvertent returns in cases of both high and low viscosity. High viscosity fluids require additional pressure to displace the viscous fluid to the expected return Page 2 of 9 No-Dig Down Under 2019 Mitigating Inadvertent Returns in Horizontal Directional Drilling Cory Mass Tod Michael

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