Recommended Practice for Overbalanced Perforating in Long Horizontal Wells

Abstract

Completing offshore horizontal wells often requires killing the wellsimmediately after perforating before pulling the gun out of the hole andinstalling the rest of the completion hardware.Perforating practicevaries from operator to operator. Some operators prefer to perforate in a clearfluid and spot a kill fluid for well control afterwards.While othersperforate directly in a solid containing kill fluid.Both methods are donein overbalance. A two-year extensive research program was carried out toquantify the formation damage caused by these perforating practices. The testing was conducted under downhole conditions in a large-scalelaboratory setup, which physically simulates the wellbore and the reservoir, aswell as the perforating and production processes. Tests were designed to studythe pressure dynamics during perforating; to gain understanding of the effectof the perforating pressure dynamics on fluid loss control and formationdamage; to select the most suitable kill fluid; and finally to provideoverbalanced perforating design guidelines for production optimization. The core test results showed that perforating design not only determines theskin damage caused by debris and grain crushing, but also plays a key role incontrolling fluid loss, which strongly influences the permeability andproductivity of the reservoir. Therefore, proper perforating design not onlyinvolves selecting the right shaped charges and gun hardware, but also it needsto take into account the perforating pressure dynamics, fluid formulation, andcompletion sequence. In addition to the core flow testing, petrographic studies were performed onthe perforated core samples to allow visual observation of the rock alterationscaused by perforating and kill fluids. This paper presents the findings in formation damage mechanisms duringoverbalanced perforating and kill processes and provides recommended practicesin both job design and fluid selection for this type of operation in longhorizontal wells. Introduction Conventionally there are two major techniques in completing long horizontalwells.One calls for underbalanced operation, in which the uppercompletions are already installed before running gun to depth andperforate.The well is controlled on surface after perforating and duringthe gun retrieval.This is believed to create less formation damagebecause the entire completion process is conducted underbalanced.However, cost and time of the operations are of concerns to manyoperators.Besides, this completion procedure may not be feasible at allin some situations due to the capacity of the required hardware.The othertechnique calls for overbalanced operations which requires killing the wellsafter perforating to maintain well control while retrieving the gun andinstalling the completion system. This perforating process is often referred asshoot-and-pull.Some operators prefer to run shoot-and-pull perforating ina thoroughly cleaned wellbore environment filled with clear completion brine tominimize formation damage.A specially formulated pill is then spotted tokill the well afterwards.During the kill process, the completion brine isdisplaced into the formation, causing undesirable formation damage in acarefully perforated wellbore[1].Hence the concern for this overbalancedoperation is its productivity impairment potential.

Overbalanced Perforating Yields Negative Skins in Layered Reservoir

“…To find a better solution, a two-year extensive research program was carried out to quantify the formation damage caused by these perforating practices. 12,13 From the above references, the key to obtaining low skin values from an overbalanced operation is to create a dynamic underbalance while shooting through a fluid system that can rapidly seal the rockface to control fluid losses and, therefore, dramatically reduce or eliminate formation invasion around the perforation tunnels. The seal that is formed to protect the formation must then be easily removed with minimum drawdown after the well is placed on production.…”