Evaluating a Concept of Using Distributed Optical Fiber Temperature and Strain Sensor for Continuous Monitoring of Casing and Completion Mechanical Deformation in Intelligent Wells
Authors: Li X. , Parker Tom , Farhadiroushan Mahmoud , Blacklaw David 
Published: October 2008

This paper presents the findings of a project sponsored by the BP Technology Innovation Board with the objective of developing a technology for continuously monitoring casing and completion mechanical deformation in intelligent wells. The innovation idea: " a permanent strain-sensing system in intelligent wells to continuously monitor the axial, lateral and cross-sectional deformation of casing and completion, with the desire to have the combined features of gyroscope, multifinger caliper, radioactive logs, and conventional mechanical deformation monitoring systems" was proposed to and funded for research by the BP Technology Innovation Board in 2000. The preliminary development evaluation was completed in late 2002.

Production from soft reservoirs can suffer significant compaction resulting in casing and completion damage. This often leads to well failures with the consequent production loss and costs of replacement or repair.

Current reservoir compaction and wellbore deformation monitoring technologies suffer from several drawbacks. A new technology is needed that combines characteristics of long distance profile sensing, 2 to 3 feet spatial resolution, the ability to measure large deformation (at least 2% strain), and multiple deformation modes.

Based on a review of available sensing technologies, Sensornet's DTSS (simultaneous Distributed Temperature and Strain Sensing) technology18,19,20 was selected for the evaluation as it appeared to meet some special sensing requirements of this application. A large scale mechanical test (15 feet long, 7 inch OD, P110 casing deployed with both fiber optic sensors and conventional strain gauge rosettes) was designed and executed with the aim of evaluating:the capability and technical limit of the DTSS technology for large and sophisticated strain profile sensing;the effectiveness of load transfer; andthe reliability of the designed downhole sensing system.

The experiments produced some very interesting results:It was for the first time, to our knowledge, that pure mechanical compressional strain was directly measured (without the need for pre-tensioning the fiber) by a fiber sensor using Brillouin scattering;For the first time temperature and sophisticated deformation modes were simultaneously and independently measured in a single fiber;The DTSS system measured strain up to the breaking point of the casing testing system (2.83% stretch), which is the largest strain ever published in the literature measured by fiber sensor.

The experiments also helped to outline further work needed for adopting this technology for downhole applications.

Introduction

Production from soft reservoirs, especially those that are over pressured and thick, can cause significant amounts of reservoir compaction, leading to surface subsidence, overburden deformation, casing damage (offset, bending, crushing/ovalization, and restricted entry, see Figure 1), and well failures (lost access to the well due to severe casing damage such as severing of casing in the overburden, casing and completion buckling/collapse in the producing intervals resulting in completion failure such as loss of sand control integrity, and intolerable annular pressure build up due to loss of seal integrity, see Figure 2)1–6.