Improvements in Perforating Performance in High Compressive Strength Rocks

Abstract

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 work 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. Introduction The basis for this project was to increase charge penetration depth to help optimize the completion efficiencies for the hard rock sandstone reservoirs in South America. Due to the unique properties of these quartz arenite sandstones, high compressive strengths up to 25,000 psi are common, and corresponding penetration depths are reduced. Additionally, these reservoirs have high permeability and modest porosities, resulting in large pore throats. Formation damage often occurs during the drilling of a well. Exposure to a drilling fluid generally results in the invasion of the rock matrix by mud filtrate and by mud solids. The extent of this invaded zone is dependent on several factors, such as the fluid loss characteristics of the mud system, the applied overbalance, the pore size distribution of the rock matrix and the time taken to drill the zone. The invaded zone may range from a few inches to a few feet around the well and usually results in a reduction of permeability. This permeability reduction, or damage, can have a dramatic impact on the potential productivity of the well. The most important consideration with respect to perforation length and well productivity is whether there is drilling damage and if the perforation length is sufficient to bypass such a damage zone. It is normally expected that effective perforating will bypass formation damage around a well if this damage is limited to a few inches. In hard rocks, the probability of bypassing the damaged zone is reduced due to the reduction in observed perforation length. For hard rocks with a significant depth of formation damage it is unlikely that the perforations will reach beyond the damaged zone. The effect of perforation length on well productivity has been reported by McDowell and Muskat, Harris and Klotz et al. These studies showed that the well productivity could only be maximized if the perforations penetrated beyond the damaged zone. Even when only a few perforations just pass the damaged zone the observed impact on productivity is significant. P. 33^

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