The productivity of wells damaged during drilling is directly dependent on the depth of the damage and the performance of the perforating guns. If the perforations by-pass the damaged zone then the well will have a low mechanical skin. Conversely, if the depth of damage is greater than the perforation length, the skin factor will be much higher, especially when the drilling damage is severe. While we normally associate drilling damage with low strength rocks, there are reported hard-rock fields with extensive drilling damage.
The performance of shaped charges is significantly affected by the compressive strength of the rock to be perforated; consequently, the ability to bypass drilling damage in formations with high rock strength is reduced. Previously reported work1,2 has shown a 75% reduction in total target penetration, compared to API Section I, in rock with an unconfined compressive strength of approximately 25,000 psi.
This paper describes the development and field testing of alternative charge designs aimed at improving performance in high compressive strength formations. So that the adverse effects of drilling damage can be reduced, computer simulations and laboratory tests showing the improvements achieved are presented. Field testing of the new charges and results achieved are shown.