Well Cementing in an Abnormal Pressure Zone of the Talara Basin:Overcoming Engineering Challenges

摘要

The Talara Basin in Perú is a mature oil field where production is extremely marginal,and well-constructiondesigns are tailored to this condition to optimize well costs.However,one well recently drilled in this areawas identified with the original reservoir pressure,and the well design was not based on this pressure.As aresult,drilling issues such as fluid loss and lost circulation were carefully monitored while trying to controlreservoir pressure in a long open-hole section.The challenges faced while cementing a high-pressure zonewith controlled fluid losses and achieving well objectives in terms of safety,reliability,and lifetime zonalisolation of the well are discussed.At the planning stage,a mud density of 13 lbm/gal was considered adequate to control downhole pressure(overbalanced); however,before reaching total depth,signs of high pressure were observed while drillingthrough a low-permeability formation.Mud density was significantly increased to control the well whilemonitoring the fluid return.When the mud weight reached 15.5 lbm/gal,losses were reported.Finally,the well was stabilized at 15.8 lbm/gal using different lost circulation pills.This event complicated thecementing operation because it substantially increased the necessary slurry density(16.1-lbm/gal lead and17-lbm/gal tail).Nevertheless,proper cementing design and effective slurry placement using conventionaltechniques resulted in successful cementing operation with full cement return to the surface.High-density cement slurries were successfully mixed and pumped regardless of their highly viscousproperty.The cementing unit and personnel were able to manage this vital operation without facing majorproblems.Before bumping the plug,circulating pressure was close to the theoretical value,confirming thatthe cement slurry column was as per the design without noticeable losses into the formation.Full cementreturns confirmed a successful cement placement.Final circulating pressure was 2,370 psi,and it increasedto 2,900 psi when the plug was bumped.The cement evaluation log obtained after 24 hours indicated goodbonding,effective mud displacement,and a successful cement operation.The well was then fractured andput on production.Neither corrective work nor a well-integrity remediation operation was necessary.Drilling the abnormally pressurized well was extremely challenging because of unexpected high pressureand associated problems,including working with high-density slurries and using various fluid additives toachieve adequate zonal isolation for future stimulation works.Techniques involved and experiences gainedwhile undertaking this considerably challenging project are discussed in this article.