Energy Harvesting of Cathodic Protection Currents in Subsea Wells

Abstract

Downhole wireless communication in oil and gas wells is a mature and proven technology but has yet to become ubiquitous in use. Operators prefer wireless communication over cabled solutions, where the advantages of wireless communication are access to locations where cables cannot be installed, lower installation cost, and reduced risk of leaks. Applications cover the well life cycle from drilling to abandonment, requiring both transmission of sensor data and control of equipment such as flow valves, for example. Power requirements for downhole wireless applications vary significantly, from low power, but frequent use for sensors, to infrequent relatively high instantaneous power for operation of valves. A key challenge is the lack of a reliable means to power equipment for the life of a typical well, which is on-the-order-of 25 years. At present, high temperature rated primary batteries are used, which limits operation to just a few years due to rapid self-discharge at the high downhole temperature. There is therefore a need to devise a suitable power source with the required longevity.

This thesis therefore introduces a new and generic method of energy harvesting that exploits ohmic (IR) voltage potential differences as current flows in a marine structure when a cathodic protection system is present. Modelling of this method is presented which verifies that it is viable to deliver of a constant long-term low power source suitable for most sensor applications. Validation is achieved by comparison of the predicted potential differences by a model with actual data obtained during deployment and operation of quasi-static wireless communication method systems. Data from four different wells, each with a significantly different geology, was compared with a bespoke model for each. Close correlation between predicted and actual potential differences was observed in all four wells.

Robust verification and validation of the concept to harvest energy from cathodic protection system currents provided sufficient confidence to proceed with design and development of an annulus pressure monitoring system for use in subsea wells. An overview of this design, including some elements of qualification, is provided.

In addition to verification and validation of a novel energy harvesting method, this work also introduces the concept of ‘impedance modulation’ to wirelessly communicate data. The principal here is to modulate the load presented by the energy harvester to encode data. This results in a detectable signal remote from the harvester location that can be demodulated to receive the encoded data.

An unexpected finding from this work is the observation of differences between conventional and the finite element analysis method for determining the resistance of long, slender electrodes in a conductive media. This could be significant as the conventional method has been used for many decades to specify the size of cathodic protection anodes on marine steel structures. Fortuitously, the conventional method over-specifies anode requirements and therefore adds to contingency.

To summarise, this thesis provides a new approach to harvest energy from stray cathodic protection currents in marine structures and thereby continuously power wireless sensors. A broad range of new applications is envisaged, including self-powered and lifetime monitoring of safety critical parameters on marine structures.

Date of Award	28 Jun 2023
Original language	English
Awarding Institution	University of Bath
Sponsors	Metrol Technology Limited
Supervisor	John Taylor (Supervisor) & Chris Bowen (Supervisor)