Understanding the Corrosivity of the Flowback Fluid Following an Acid Job

Acid stimulation is a widely established technique to prolong the life of oilfield wells and generate increased levels of hydrocarbon production. High strength acids are injected into formations to facilitate the dissolution of fines in the vicinity of the wellbore and/or create channels in the reservoir itself to improve permeability. The majority of previous acidizing research has focused on the ability of chemical inhibitors to protect the downhole assets during this injection process. Very little research has studied the corrosivity of the fluid which flows back once production restarts following an acid job. After the acid has been shut in the wellbore, production restarts and there exists a period over which unreacted acid is transported through the production pipeline along with the process fluid. The concentration of acid (and any remaining inhibitor) then gradually declines over time until normal production resumes. Such a process is termed ‘acid flow-back’ and can be particularly detrimental to the integrity of production pipelines.

This thesis addresses the corrosion process associated with the flowback fluid following an acid stimulation procedure. By testing a range of solutions with much lower acid concentrations than the injected acid it was hoped that the corrosivity of the flowback fluid could be better characterised. However, established mass loss and short term electrochemical tests were found to be inadequate for characterising the flowback process. This research offers a new perspective on how to quantify the corrosivity of the flow-back fluid using a continuous flow cell integrated with in-situ electrochemistry and capable of operating at temperatures of up to 80˚C. It was observed that if the acid and inhibitor concentration of the solution is diluted to between 100-1,000 times less than the injected acid/inhibitor concentration then the corrosivity of the solution is significantly higher than that of the injected acid. Two clear peaks in the corrosion rate of the steel were observed during the flowback process. The first peak was found to be controlled by a critical inhibitor concentration and the second peak was controlled by a critical solution H+ concentration.

Unlike previous methodologies, the new test methodology is able to profile the full flowback process in a single test. This has significant cost and time saving implications for testing specific flowback profiles encountered following different acid jobs.