Theory and Practice of a Flexible Fiber-Optic Cable in a Horizontal Well Used for Crosswell and Microseismic Hydraulic Fracture Monitoring

Abstract

Summary A flexible optical fiber cable, either as a wireline or a disposable fiber deployed using a pumped fiber payout shuttle, in a horizontal well, can be used to measure distributed near-static or dynamic strain. These measurements can be used to monitor the hydraulic fracturing treatment of nearby wells. We present a theoretical framework for the understanding of the cable behavior and to compare the theory with field measurements. The theory predicts the conditions under which slippage occurs between the optical fiber cable and the wellbore, if coupling is provided by Coulomb friction. For near-static strain, as used in crosswell strain monitoring, the theory explains the broadening of the strain zone detected with a wireline cable or disposable fiber when slippage occurs. However, for a grease-covered disposable fiber, the theory underestimates the coupling where the grease provides a level of adhesion between the fiber and the wellbore. Understanding the fundamental theory explains measured data and enables confident data interpretation regardless of sensing cable configuration. For the dynamic strain, the theory confirms the generally good response observed using the flexible cables for microseismic monitoring due to the low amplitude of the dynamic strains involved. The low amplitude of the strains means that the strain gradients and inertial forces are also small, such that Coulomb friction is sufficient to provide the needed coupling. An interesting result of the theory is the existence of a resonance condition allowing for large amplitudes to be detected faithfully even if only Coulomb friction is present. This resonance does not depend on signal frequency but on the match between the intrinsic travel speed of a disturbance on the cable and the apparent phase velocity of the seismic signal in the well direction. Most importantly, the theory enables (i) a comparison of different cable types for the near-static and dynamic strain applications, and (ii) better data interpretation and associated decisions. Field examples are provided to show both when the theory is applicable and where the obtained coupling exceeds what is predicted by the theory. The novel aspect of the paper is the first presentation of a theoretical background for the understanding of the performance of flexible cables inside horizontal wells used as static or dynamic strain sensors for the monitoring of hydraulic fracturing jobs.