Recommended Practice for Overbalanced Perforating in Long Horizontal Wells
Authors: Bybee Karen 
Published: February 2015

This article, written by Assistant Technology Editor Karen Bybee, contains highlights of paper SPE 94596, “Recommended Practice for Overbalanced Perforating in Long Horizontal Wells,” by F.F. Chang, SPE, Schlumberger; A.M. Mathisen, SPE, Norsk Hydro A/S; N. Kågeson-Loe, SPE, BP plc; I.C. Walton, SPE, Schlumberger; G. Svane, M-I Norge; and R.R. Midtbø, I. Bakken, J. Rykkje, and O. Nedrebø, Norsk Hydro A/S, prepared for the 2005 SPE European Formation Damage Conference, Scheveningen, The Netherlands, 25–27 May.
A 2-year extensive research program was carried out to quantify formation damage caused by various perforating practices. The testing was conducted under downhole conditions in a large-scale laboratory setup that physically simulated the wellbore and reservoir, as well as the perforating and production processes. Tests were designed to study pressure dynamics during perforating to gain understanding of the effect of perforating pressure dynamics on fluid-loss control and formation damage, to select the most suitable kill fluid, and to provide overbalanced-perforating design guidelines for production optimization.
Introduction
One of the two major conventional techniques for completing long horizontal wells calls for an underbalanced operation, in which the upper completions are installed before running the guns to depth and perforating. The well is controlled on the surface after perforating and during gun retrieval. This is believed to create less formation damage because the entire completion process is conducted underbalanced. However, cost and time of the operations are of concern to many operators, and this completion procedure may not be feasible in some situations.
The second technique calls for overbalanced operations that require killing the well after perforating to maintain well control while retrieving the guns and installing the completion system. This perforating process often is referred to as “shoot-and-pull.” Some operators prefer to run shoot-and-pull perforating in a thoroughly cleaned wellbore environment filled with clear completion brine to minimize formation damage. Then, a specially formulated pill is spotted to kill the well. During the kill process, the completion brine is displaced into the formation, causing undesirable formation damage in a carefully perforated wellbore.
Experimental
Fluids used in the testing program included water-based kill pills, oil-based muds (OBMs), and kerosene to simulate reservoir oil. The laboratory test of the shoot-and-pull completion was conducted under downhole conditions in a large-scale core-flow setup that physically simulated the wellbore and the reservoir around a single perforation. Perforating, shut-in, and production flow tests were conducted by use of outcrop cores that matched the reservoir-rock properties. For Field 1, Castlegate sandstone with a 1,500-psi unconfined compressive stress (UCS) was used. The permeability of the cores ranged from 600 to 900 md. For Field 2, Berea sandstone with an 8,000-psi UCS was used. The permeability of the Berea cores ranged from 100 to 200 md. Each experiment included the following standard procedure.