Underbalance or Extreme Overbalance

Abstract

Abstract Application of an extreme overbalance, sufficient to fracture the reservoir formation, applied either during or after perforating was first presented as an alternate completion technique during the 1993 Fall SPE Conference. During the ensuing two years, a number of jobs have been run by different operators in a variety of formations with varying applications. The primary question asked by an operator relates to the benefit of Extreme Overbalance (EOB) versus current perforation techniques of underbalance (UB) or modest overbalance for well control. Of course this question spawns a whole set of new questions about when and how to use and the benefits and risks of the EOB. EOB, as used in this paper, includes Extreme Overbalance Perforting (EOP) and Rapid Overpressured Perforation Extension (ROPE). This paper compares and contrasts UB and EOB strengths and weaknesses and provides recommendations of when and why to use the different techniques. Field examples are provided of successful and unsuccessful application of EOB completions. Introduction Perforating, fracturing and acidizing are simply a means of providing and enhancing flow communication between the reservoir and the wellbore. The objective of these different completion operations is to cost effectively maximize the producibility of the well. Perforating is used to provide the basic fluid communication between the reservoir and the wellbore. Underbalance perforating and acidizing are typically used to bypass or eliminate near wellbore damage. Fracturing is generally used to expose a larger reservoir flow area to the wellbore. Use of propellant tools and now EOB produce short fractures and thus complements underbalance perforating and acidizing to bypass wellbore damage. However, the process of perforating also creates a permeability damaged region around the perforation commonly called the "crushed zone". Underbalance perforating has been used to remove the perforating damage zone in reservoirs with sufficient pressure and permeability. Since perforating is the primary means to provide the initial fluid communication with the reservoir and where applicable is performed underbalance, the relevant issue is when-where-how to use BOB as an addition to perforating. We will first review where UB perforating is applicable, then discuss the interaction between the perforation and the fractures from an EOB process and the implications/questions with regard to the fracture flow path. Several field examples will then be discussed. Finally, based on the physics of EOB, recommendations of applicability will be made. One word of caution, the following discussions do not apply to unconsolidated or very weak rock. P. 117