Optimizing Underbalanced Perforation Performance, a New Model to Precise Adjustments of Downhole Condition
Authors: Mohammadsalehi Mehdi , Saghafi Hamidreza , Teymaarishamasbi Mehdi 
Published: March 2015

Abstract
Perforating creates a low-permeability crushed zone in the formation around the perforating tunnels that also constitutes a flow restriction, which can impair the well's productivity or injectivity. A precisely designed underbalanced perforation job could effectively minimize or remove perforation damages and thus optimizes the production rate.
Previous works on calculation of required underbalanced pressure were presented by J.G. Flores and H.R. Crawford. These models did not consider all of factors affecting the selection of final perforation performance, thus there will be some associated errors and uncertainties in outcomes. In this paper, studied reveal that, in addition to formation permeability and perforation gun parameters, amount of formation porosity and wellbore fluid rheological properties have great impacts on final perforation tunnel clean up, thus on production rate enhancement.
This paper represents a new formulated methodology of optimum selection of underbalanced pressure values and wellbore fluid properties prior to gun petonation. To achieve the most precise results, data from various locations and laboratory works have been collected, analyzed and modeled. Based on theoretical background, the most suitable correlations were derived and the model has been corrected and calibrated to compensate affects of the new mentioned factors. As a Result, better perforation damage removal achieved. This methodology is valuable both for more exact prediction of underbalanced perforation parameters and for providing guidance in further evaluations of perforation job efficiency.

Introduction
The use perforation as a completion method is not new in the petroleum industry. The concept of underbalanced perforation has been applied around the globe and shows promise of solving many of the inherent perforating problems for which solutions have been needed. This method is a key to successful exploration, economic oil and gas production, long-term well productivity and efficient hydrocarbon recovery. It has proven that it can offer a cost-effective, safe method as this technique also acts as a near wellbore damage clean up tool leads to significant improve in well productivity. Additionally, its usage can eliminate other service needs that often occur subsequent to conventional perforating jobs.
Today, underbalance is more common to minimize or remove perforation damage. The potential of underbalance perforating was recognized in the 1960s. Wells perforated with underbalance tended to show production increases. In the 1970s and early 1980s, researchers recognized that the flow efficiency of perforated completions increased when higher underbalance pressures were used. However there were some cases that underbalanced perforation results did not effectively increased well productivity.

Crushed Zone
The extent of perforation damage is a function of lithology, rock strength, porosity, pore fluid compressibility, clay content, formation grain size and shaped-charge designs. The crushed zone can limit both productivity and injectivity. Fines and debris restrict injectivity and increase pump pressure, which decreases injection volumes and impairs placement or distribution of gravel. Erosion of the crushed zone as well as removal of debris from perforations by surge flow is essential to mitigate perforating damage and ensure well success.