Extreme-Overbalance Perforating Extends Perforations without Vertical Growth
Authors: Guoynes J.C. , Palmer W.M. , Grantham J. , Peters J.B. , Friend B.L. 
Published: December 2007

Abstract
A new completion method known as extreme-overbalance perforating, was introduced by Handren, Jupp, and Dees in 1993 at the 68th SPE Annual Technical Conference and Exhibition. This new method improved initial completion efficiency, demonstrating negative skin effects in 14 of the 16 wells tested. Extreme-overbalance perforating (EOP) involves perforating or surging existing perforations at pressures above the fracture gradient of the formation. This practice uses compressed gas at high pressures and a small volume of liquid in the wellbore to extend the perforation tunnels past damage.
EOP was used to develop a new completion procedure on the Glenwood formation in Clare County, Michigan. The EOP completion design extended beyond wellbore damage and allowed observers to determine if the vertical growth associated with previous hydraulic fracturing techniques would occur. In the case history presented, a cost-effective initial completion was achieved with negative skin effect, and the stimulation did not experience vertical growth.

Introduction
A major operator investigated the performance of EOP and compared their technique with earlier industry-accepted perforating methods. Before 1993, the industry widely accepted underbalanced perforating as the best process for reducing near-wellbore skin effect during completions. Underbalanced perforating has been especially popular since Roy Vann's introduction of tubing-conveyed perforating (TCP) in the 1980s. Benefits of the new EOP process include higher initial production, lower treating pressures associated with subsequent hydraulic fracturing, quick on-line production, and rapid evaluation of a well's producing capability. By consolidating EOP with other completion services, operators may be able to reduce total well completion costs and increase net present value.
Since EOP is a relatively new process, treatment processes have not been well documented in certain formations. The Ordovician age Glenwood formation in the Michigan Basin consists of interbedded sandstones, shales, and carbonates. Core analysis from this well indicates that porosities within the sandstone, which is the productive facies in the Glenwood, range from 5.0 to 9.2%, while permeabilities range from 0.15 to 1.32 md. The sandstone in this formation is primarily composed of fine- to medium-grained quartz sand with a significant amount of potassium feldspar and authigenic chlorite and illite clays. These clays contribute to significant formation damage in the Glenwood and Prairie du Chien formations.
Virtually all of Michigan's Glenwood and Prairie du Chien wells have had some form of positive skin damage. The Glenwood formation has historically been known for its near-wellbore damage, which could only be relieved by fracturing. Further treatment of the zone after the initial completion has induced additional damage, especially when water-based treating fluids were used. Completion methods in the Glenwood and Prarie du Chien have included underbalanced perforating with and without tubing-conveyed guns, ballout acid treatments, solvent/alcohol washes, and fracturing treatments. All of these methods have had marginal to no impact in removing skin damage. Until the introduction of EOP, hydraulic fracturing treatments with condensate were the only processes that allowed the wellbore to connect with the undamaged reservoir. Vertical growth, however, is relatively common with condensate fracturing, which can pose the risk of additional reservoir damage if water-bearing zones are near the productive interval.
The objective of the new Glenwood completion method was to treat the formation without water-based fluids, extend perforations past wellbore damage, and limit the vertical growth associated with previous hydraulic fracturing treatments without killing the well. This case documents results of the EOP method and examines the vertical growth and skin damage after the initial completion. The case results were evaluated with logging and well-test analysis techniques.
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