Experimental investigation of nail reinforcement of a free surface

ABSTRACT: Maintaining wellbore stability is important in both drilling, completion, and production phases of petroleum wells. We have investigated an innovative way of stabilizing rock formations using steel nails akin to concrete or soil reinforcement. In this paper we focus on its applications to high-porosity chalk as an alternative to traditional completion methods. High depletion of mature chalk reservoirs creates difficulties in producing intervals (solids production and chalk influx) but can also present stability challenges in infill drilling. We have conducted an experimental campaign to investigate the stiffness, strength, and failure characteristics of chalk specimens with and without different types of nail reinforcement of a free surface. Reinforcement was done either by pre-drilling and inserting nails into tight holes or dynamically inserting (shooting) nails without pre-drilling. Prismatic specimens were loaded axially and confined on three lateral surfaces while the anterior - the free surface - remained unconstrained. A glass window on one lateral surface allowed direct observation of fracture development. Specimens reinforced with nails inserted into pre-drilled holes showed similar or slightly higher maximum axial stress than unreinforced specimens, while dynamic penetration seemed to reduce maximum axial stress. At post-peak stresses, all reinforced specimens showed higher and more sustained load-bearing capacity and reduced free surface deformation compared to unreinforced specimens. The post-yield reinforcement may have important operational implications such as limiting the risk of stuck pipe or chalk influx events.