Advanced Distributed Fiber Optic Sensors to Monitor Cement Hydration and Detect Annular Hydrocarbon Migration for Enhanced Zonal Isolation

Abstract

Abstract This paper presents an advanced fiber optic distributed temperature and strain sensing (DTSS) system designed for real-time in-situ monitoring of cement annuli in an oil and gas well. The DTSS system used in this study of cement quality and zonal isolation uses both Brillouin and Rayleigh backscattering phenomena, allowing one to separate strain and temperature using a single optical fiber. The first part of the paper focuses on monitoring the exothermic cement hydration process after a cement slurry is displaced into an annular space. In the laboratory, temperature and strain profiles were measured independently using the DTSS system and thermocouples at various temperatures to quantify the effect of heat release during cement hydration. These independent temperature measurements showed excellent agreement. Contamination of cement due to incomplete displacement of drilling muds by cement slurry was also simulated in the experiments. Contamination was found to decrease the amount of heat generated during the cement hydration process. The degree of cement hydration and the presence of cement in the annular space – or lack thereof - could thereby be quantified. It is shown that the temperature profiles measured using the hybrid-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. This is highly relevant information, because contamination of cement is one of the main factors that negatively affects zonal isolation directly by creating paths for hydrocarbon migration or indirectly by weakening the cement, making it susceptible to damage from cyclical pressure/temperature loads. In addition to monitoring cement hydration, specially designed fiber optic sensing cables were used to detect the presence and migration of hydrocarbons in cemented annuli, either because the cement was absent or compromised by cracking/fracturing. The presence of hydrocarbons in cemented annuli could be detected even after the cables were exposed to well construction fluids such as synthetic-based drilling mud and spacer fluid. Moreover, the materials used were sensitive to the type of hydrocarbon, allowing indirect hydrocarbon fingerprinting and determination of the origin of the hydrocarbons behind pipe. This presents a powerful new tool to assess the quality of zonal isolation behind casing, with the ability to monitor in real-time over the entire lifecycle of the well. In this capacity, it may provide guidance on such important decisions as the need for well intervention and remedial cementing.