INVESTIGATION OF MAXIMUM ALLOWABLE MUD PRESSURE
WITHIN SAND AND CLAY DURING HORIZONTAL
DIRECTIONAL DRILLING
by:
HONGWEI XIA
A thesis submitted to the Department of Civil Engineering
In conformity with the requirements for
the degree of Ph.D.
Queen’s University
Kingston, Ontario, Canada
(October, 2008)
Abstract
Horizontal Directional Drilling (HDD) has become commonplace for the trenchless installation of utility conduits and other buried pipe infrastructure while minimizing social, economic, and environmental impact on the surrounding community relative to the conventional “cut-and-cover” installation method. Pressurized drilling fluid is used during drilling process to maintain borehole stability and transport the soil cuttings return to the ground surface. However, mud loss problem can occur through tensile failure (hydrofracture or frac-out) and/or shear failure (unconfined plastic flow or blowout) because of excessive mud pressure in the borehole. A clear understanding of the mud loss problem is necessary to advance the application of HDD to install underground infrastructure.
The objectives of this research have been to study the mud loss problem through finite element analysis (both three-dimensional and two-dimensional) and scaled laboratory experiments (both small scale and large scale laboratory experiments), to quantify the maximum allowable mud pressure within sand and clay, and to evaluate the performance of the “Delft solution” which was adopted as a design solution by the US Army Corps Engineers.
The recent laboratory experiments studies (both small scale and large scale tests) on mud loss within sand performed at Queen’s are reviewed. Finite element analyses are carried out to interpret these scaled laboratory experiments, and to facilitate evaluation of the Delft solution. Two additional large scale laboratory experiments are performed to understand further the moss loss phenomenon within a poorly graded sand. Comparisons are made between the maximum allowable mud pressures obtained from the experimental measurements, theoretical studies using plane strain analyses like the Delft solution and the finite element analyses. Comparisons indicate that the finite element method provides an effective estimation of maximum mud pressure, and the Delft solution overestimates the maximum mud pressure by more than 100%. The surface displacements (ground heave) for the large scale laboratory experiments are calculated using Particle Image Velocimetry (Geo-PIV) program. The interpreted surface displacements exhibit a “bell” shape with the maximum surface displacement located around the center of the borehole, and tapering down almost to zero with the distance from the centerline.
A parametric study is carried out using plane strain finite element analysis to investigate the effect of various parameters (Young’s modulus E, unit weight , friction and dilation angles , the depth of burial H, the borehole diameter D and the coefficient of lateral earth pressure at rest K0) on the maximum allowable mud pressure within sand. An approximate equation (a function of H/D and K0 ) is then developed to facilitate design estimates of the maximum allowable mud pressure within sand.
A new approach is introduced to consider the effects of coefficient of lateral earth pressure at rest K0 on the blowout solution for purely cohesive soil (clay) through approximating the plastic zone as circular. Plane strain finite element analyses are carried out to evaluate the effectiveness of the new approach. The evaluations indicate that the new approach provides more reliable maximum allowable mud pressure within purely cohesive soil (clay) when initial ground stress conditions are anisotropic (K0 ≠1). Suggestions for future investigation on mud loss within clay through hydrofracture/blowout experiment are made.