Impact of Casing Pressure Test After Waiting on Cement on Cement BondQuality

摘要

A high percentage of continuous cement bond with casing is an indicator of high-quality cementing anda good approach to cost efficient construction of oil and gas wells.However,the pressure created insidethe casing after waiting on cement(WOC)leads to expansion and contraction of the pipe which has aconsiderable negative effect on the cement bond quality.This article describes the effect of a casing pressuretest on the result of cement bonding,integrity of annular spaces and isolation of formations.Ensuring the cementing quality prior to the cement job requires an engineering approach to the designof the entire job,namely:duration of circulation after casing running,volume of spacer fluids,designand weight of cement slurry,type and quantity of casing equipment,casing reciprocation or rotation,displacement fluid pumping rate,etc.However,even with all these conditions optimized,the blowoutpreventer(BOP)/casing pressure test after WOC will apply destructive casing tensile forces on hardenedcement because of excessive internal pressure resulting in the loss of a significant percentage of good cementbond.A study was performed and special attention was drawn to the fact that the casing test after WOCresults in instances of micro-annuli which are recognized by the cement bond log(CBL)tool as no bondor partial bond,however these micro-annuli may be non-permeable.The micro-annuli occurrence processis confirmed by hydraulic calculations.As a result,a joint decision was made to carry out experimental work with casing pressure tests at thebump pressure point and further BOP tests against a wellhead packer,to eliminate casing pressurization.According to the Oil and Gas Industry Safety Regulations this would require the cement slurry to bedisplaced with water instead of the drilling mud usually pumped for displacement.Immediately afterbumping the plug the casing shall be tested for integrity by pressuring up to the required pressure andholding it for 30 minutes.Then,after BOP nippling up,a special wellhead packer is run and the BOPs arepressure tested without any impact on the casing.CBL results for the operations performed showed a significant increase in good cement bond with casing(from 31% to 56%)as well as a reduction of absent bonding(from 10% to 0.5%)and microannuli.This topic is important,as in most cases BOPs are pressure tested together with the casing.This results in lowerquality of the well cementing.In this paper,details of the increase in the bond quality through adjustmentsto the well construction process are shown to improve well profitability by preventing micro annuli andincreasing the productive life of the well.