Formation reinforcement for wellbore stability and sand control

The major activities of the project have been both experimental and theoretical/numerical. The experiments have concentrated in optimizing the placement of nails in specimens of the two main rock types, i.e. chalk and sandstone. The chalk was an outcrop from Mons, Belgium, which is an analogue of high porosity North Sea reservoir chalks. The sandstones were the relatively weak Saltwash South and the more competent Saltwash North. Initial testing showed that Saltwash South was too weak for placing nails without fracturing the specimen. It was thus decided to concentrate on Saltwash North. The placement of nails has proven a challenging task and it was decided to follow two different approaches. In the first, nails were shot into the specimen with a Hilti gun while in the second the nails were pushed into pre-drilled holes in the rock using a load frame. A special cell was constructed where confining stress could be applied to minimize fracturing of the specimen during nail penetration. In addition, the cell is used to perform pullout tests to optimize the nail selection for each rock and calibrate the rock-nail contact model. The first set of pullout experiments have been performed and the results are currently analyzed with analytical and numerical models before the second set of pullout tests to obtain the optimum nail parameters. Experiments were performed on Saltwash North sandstone and Mons chalk in the Surface Instability Apparatus at the University of Minnesota under the supervision of Prof. Joe Labuz. The tests were nalyzed and gave indications about the optimum nail density and length. On the theoretical level, analyses have been performed with a finite element method where the process of nail pullout has been modeled. The interaction between nail and rock was simulated using a contact formulation with a friction law, while the rock material has been assumed to obey an elastoplastic constitutive law. Numerical analyses were compared to analytical results and key parameters were varied in order to assess their effect on pull-out resistance. Cox's shear lag model provided accurate results for interface stiffness when the appropriate boundary conditions were used, but for conditions resembling those used in many experimental pull-out test s, stiffness is highly overestimated. Nail contact length, interfacial friction, and initial pre-drilled hole diameter were found to have the largest effect on maximum pull-out force.

Soil nailing and rock bolting is a widely used technique in geotechnical and rock engineering, mining and tunneling, for the reinforcement and strengthening of the ground by installing closely spaced steel bars (nails or rock bolts) into a slope or excava tion. Here it is proposed to use nailing on a smaller scale to support hydrocarbon producing wellbores in shale and reservoir sandstones and chalks. Due to the inevitable scale effect, the intense fluid flow conditions and the high in situ stresses, no di rect transfer of the existing technology is possible and new developments are necessary. In formation nailing, nails are placed into the rock to create a reinforced section that is itself stable and able to retain the material behind it without reducing its permeability. The reinforcement is passive and develops its reinforcing action through nail-formation interactions as the formation deforms during and following drilling and/or production. The activities to establish the technique are based on experi ments on three relevant formation rocks, viz. shale, sandstone and chalk. Theoretically, a constitutive model is developed for the nail-rock interaction to be used for analytical and numerical finite element analyses of the nail reinforcement problem unde r various conditions. A user-friendly Nail Predictor Software Tool is developed for field applications.