Rock nailing of boreholes is considered a means to reinforce the rock and increase its borehole failure strength. Nail reinforcement improves stability by increasing the normal force and hence the rock shear resistance along potential slip surfaces and localization zones. In a continuum sense, nail reinforcement is modeled as a body force. A numerical finite element formulation is developed that couples the rock and the nail behavior and solves the equations simultaneously. The numerical results are verified against analytical solutions for reinforced elastic boreholes and hollow cylinder geometries. Comparison of the reinforcement effect in boreholes and hollow cylinders shows that the hollow cylinder test has to be specially designed to model realistically the field conditions.

1. Introduction

Rock bolts, soil nails, geotextiles and fibers are used as reinforcement elements of slopes and underground excavations. Here rock reinforcement is used on a different scale, namely for supporting hydrocarbon producing boreholes in shale and weak rocks. Nailing of a borehole (Figure 1) could provide the necessary confinement and strengthening of the weak rock and provide inside the softening zone arching and frictional constraint of the grains to reduce wellbore instability problems and sand production. The reinforcement is passive and develops its reinforcing action through nail-formation interactions as the formation deforms during and following drilling and/or production. Nails work predominantly in tension but may also work in bending/shear in certain circumstances. The mechanism of reinforcement of the nails is similar to the action of frictional rock bolts like the Swellex and split set bolts.

Nail reinforcement of a free surface or a borehole has been analyzed in the past [2, 3]. Elastic solutions with infinitely stiff nails were presented where the action of nails was modeled in a continuum sense through a distributed body force. For elastoplastic rock, a finite element formulation is developed and the results are compared with the analytical solution for elastic rock. The stability of a borehole in an elastoplastic rock can then be analyzed. The Finite Element Method (FEM) formulation is presented and it is followed with verification results for an elastic borehole with rigid nails where analytical solutions exist. Using the finite element model, the reinforcing effect in a hollow cylinder in the laboratory or a borehole in the field are analyzed. The idea is to use the hollow cylinder test as a means to design and calibrate experimentally nail reinforcement applications in boreholes.

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