Reliability analysis of permanent downhole monitoring systems
Authors: van Gisbergen S.J.C.H.M. , Vandeweijer A.A.H. 
Published: October 2008

Permanent downhole gauge (PDG) systems have been installed in many oil and gas wells to date. These systems enable improved reservoir management and production optimisation. Currently, more complex downhole monitoring and control systems are being installed, in what is often called 'Intelligent Wells'. For all these systems reliability is a critical aspect.

We have evaluated the performance of 952 pressure and temperature monitoring systems installed since 1987. It was found that the 5-year survival probability of these systems improved from 40% for systems installed in the period 87/88 to 75% for the period 91/92. As from 1992 no further improvements have been observed resulting in a 5-year survival probability of 69% for the period 1993–1998.

Failure modes have been evaluated and could mostly be attributed to the downhole gauge and to the cable. Based upon these findings, recommendations have been made to achieve reliability improvements.

Introduction

Permanent downhole gauge (PDG) systems have been installed in several hundreds of oil and gas wells so far. These systems form an alternative to wireline-conveyed downhole surveys. In comparison, PDG systems avoid hazardous operations and offer continuous measurements, which enable better reservoir management and production optimisation. An increasing number are being installed, mainly in wells where wireline surveys are expensive (subsea, offshore, remote) or where they result in high deferred production (e.g. horizontal wells). Electric pressure and temperature (p,T) monitoring systems are most common. Other systems that have been installed are:Permanent downhole flowmeters for liquid-only mixtures,PDG systems in ESP-lifted wells that use the ESP power cable for data transmission,The Fibre Optic Well Monitoring (FOWM) system for measuring pressure and temperature.The Tucatran system for cableless communication.The SCRAMS system.

The installation of electric gaslift valves, all electric inflow control devices and reservoir monitoring systems are planned in the near future. Once all these systems are available, the socalled 'Intelligent Well' concept, which can be defined as a modular combination of downhole monitoring and control systems will become reality.

For all these systems reliability is key: Shell's targets are a 90% probability to survive 5 years for monitoring systems and a 90% probability to survive 10 years for actuators. In this study, we have performed an evaluation of the reliability of installed systems, where we have focused on electric pressure and temperature measurement systems, since sufficient have been installed to allow a statistical evaluation.

The paper contains a description of a PDG system and its components, the method of obtaining field data, the statistical analysis and the final results. Conclusions are then drawn from the results.

System description

A schematic of an electrical PDG system for pressure and temperature measurement is depicted in Fig. 1.

The sensing element is an electronic gauge that measures the pressure and temperature in the tubing. The gauge is mounted in a mandrel, which forms part of the tubing string.

The topside of the gauge is formed by the cablehead. Here, the gauge is connected to the electrical metal-sheathed cable that runs along the tubing to the hanger.

The cable is coaxial and consists of an insulated monoconductor that is encapsulated in a metal sheath. This metal sheath forms the return wire and protects the cable from the medium in the annulus. The sheath is in most cases encapsulated in a thermoplastic to prevent