Perforation Damage Studies in Unconsolidated Sands: Changes in Formation Particle Sizes and the Distribution as a Function of Shaped Charge Design
Authors: Snider P.M. , Benzel W.M. , Barker J.M. , Leidel D.J. 
Published: December 2007

Studies were undertaken in which different shaped charge perforating systems were fired into simulated unconsolidated sand formations to examine the amount of material crushed by the explosive event The changes in sand grain sizes were determined using a laser particle size analyzer and sampling methods to "map" the damage created over the length of the perforation. This is believed to be one of the few studies to examine the crushed material created by perforators on an individual grain basis, and results are quite disturbing when related to the potential impact on well completion operations. For example, one conventional "big hole" perforation charge in a 4-% inch (117 mm.) perforating system was found to damage as much as 5 pounds of formation material and cement; generating as much as 30 % -40% fines. It becomes apparent that several thousand pounds of damaged formation material can be created in the near-wellbore area on a high shot density perforating job of significant length. The damaged material is significant in total volume, and the generated fines can create filter cakes which limit fluid injection during subsequent operations such as gravel packing and acid stimulation. This paper discusses the test results for typical charges utilized in the industry today, and the potential benefit of a revised strategy to develop charges designed to reduce the amount of damaged formation material to less than 20% of the amount observed with some conventional shaped charges. The revised strategy focused on developing charges to create very large holes in the casing and through the cement, yet minimize formation penetration. These revised charge designs have already been utilized on a few field applications with improved results.

Introduction

The industry has known for many years that the perforating charge created a "crushed zone" of formation material around the perforation tunnel. Completion procedures generally included some method to attempt to remove this damaged material. Underbalanced perforating strategies are common and the level of underbalance suggested by King and Tariq are general guidelines still utilized today. In unconsolidated formations, Crawford proposed a method to determine the maximum allowable underbalance that could be achieved, while preventing the perforating guns from becoming stuck. Still today, subsequent to perforating operations in unconsolidated sands, field data suggests some type of damage mechanism exists between the wellbore and formation that is counter productive to many completion operations. Data indicates techniques such as underbalance perforating, surging, perforation washing, and flowing the wells to remove the crushed zone result in higher fluid loss rates than perforating at balanced conditions. However, in many instances these rates are but a fraction (10% -30%) the calculated rate based upon completion fluid viscosity, formation permeability, reservoir pressure, perforated interval, and wellbore hydrostatic pressure. Likewise completion efficiency on these same high permeability gravel packed wells can be less than 50%.

Experimental work conducted on weak Castlegate sandstone rock samples (unconfined compressive strength = 1200 psi (8,270 kPa)) by Blok, Welling, Behrmann, and Venkitaraman cited perforating as having a major impact on gravel pack impairment. Fines generated by perforating not only limited the ability to inject gravel pack fluids into the formation, but also impacted the long term producibility of gravel packed wells. These fines were quite difficult to remove from the perforation tunnel utilizing a variety of techniques and one of the study's conclusions was that mechanical methods to form a hole through the casing, cement, and into the formation could be superior to the use of conventional perforating charges.

Review of field operations show fluid injection into the formation is a completion step following perforating in many instances. While some wells are produced as "natural completions". near wellbore stimulation is a common practice. It is desirable to inject fluids into the formation for gravel packing, acidizing, hydraulic sand fracturing, and water/gas injection wells. P. 677^