Advanced distributed fiber optic sensors for monitoring real-time cementing operations and long term zonal isolation

Abstract

This paper presents an advanced fiber optic distributed temperature and strain sensing (DTSS) system designed for real-time in-situ monitoring of cement presence and integrity in the annuli of an oil and gas well. The DTSS system is used to monitor cement quality and zonal isolation by application of hybrid Brillouin and Rayleigh technology. This sophisticated technology allows for separate strain and temperature measurements using a single optical fiber.The DTSS system enables monitoring of the exothermic cement hydration reactions after a cement slurry is displaced into an annular space and allowed to set. In the laboratory, temperature and strain profiles were measured using the DTSS system at various external temperatures to quantify the effect of heat release during cement hydration. The temperature measurements showed excellent agreement with those obtained from thermocouples embedded in the same cement samples for comparison. Contamination of cement due to incomplete displacement of drilling fluid by cement slurry was also considered in the experiments. Contamination with synthetic-based drilling fluid was found to decrease the amount of heat generated during the cement hydration reactions. The degree of cement hydration and the presence of cement in the annular space - or lack thereof - can therefore be quantified. It is shown that the temperature profiles measured using the DTSS system can be used to detect both the top of cement (TOC) as well as annular sections that are either left uncemented or contaminated with drilling fluid.In addition to monitoring cement hydration, specially designed fiber optic sensing cables were developed to detect the presence and migration of hydrocarbons in cemented annuli, which may occur either because the cement is absent or compromised by cracking/fracturing. The fiber optic cables were modified with hydrocarbon-sensitive coatings generating a swelling effect when exposed to hydrocarbons. Such swelling effect could be measured and quantified using an induced strain on the fiber optic cable, which is monitored and recorded by the DTSS system. The modified cable was also evaluated for its response to various well construction fluids (such as synthetic based drilling fluid, spacers etc.).The ability to monitor cement hydration and hydrocarbon presence behind casing using cement-embedded fiber optic sensors that provide real-time, continuous responses without the need for wellbore re-entry presents a powerful new tool to assess and verify the quality of zonal isolation over the entire lifecycle of the well.