Permanent Fiber-Optic System Monitors Oil-Rim Movement
Authors: Carpenter Chris 
Published: February 2019

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 188868, “Development and Field Application of a Permanent Fiber-Optic Wellbore-Fluid-Level Monitoring System,” by C. Staveley, SPE, C. Doyle, and C. Coetzee, Smart Fibres; A. Franzen, H. Den Boer, A. van Rooyen, and W. Birch, Shell; and A.B. Biderkab and E. Moes, Petroleum Development Oman, prepared for the 2017 Abu Dhabi International Petroleum Exhibition and Conference, Abu Dhabi, 13–16 November. The paper has not been peer reviewed.
In highly fractured carbonate reservoirs, the conventional method of monitoring oil-rim movement is running wireline gradiometric surveys periodically. However, some operators have found that this method is inconclusive and is unable to provide information in a manner timely enough to influence operations because the gradiometric surveys are only run a few times a year. In this paper, the authors describe a project to design, field trial, and qualify an alternative solution for real-time monitoring of the oil rim in carbonate reservoirs that overcomes these disadvantages.
Current Methodology
The methodology of performing gradiometric surveys can be applied in reservoirs successfully where the permeability of the formation is high and where the formation is fractured such that good communication exists between fluid within the formation and within the observation well (given that the well casing is highly perforated across the full length of the reservoir section). Under these conditions, the fluid levels measured by the gradiometric surveys give the operator enough information about the oil rim within the reservoir to adopt an active smart-field production method. The term “smart field” is used by the operator to describe oilfield operations where surface decisions are supported by measurements from within the reservoirs.
While the use of gradiometric surveys for production optimization is helpful, it does have disadvantages. Each survey requires a well intervention; as can be imagined, the process is relatively expensive and introduces health, safety, and environmental (HSE) risks while the well is open. Consequently, gradiometric surveys are generally commissioned at a low frequency, perhaps only one or two measurements per year within key observation wells.
New Methodology
In the early 2000s, a potential was recognized to provide an array of permanently deployed pressure gauges in the wellbore to measure the density of the media between adjacent gauges. From this measurement, the interfaces between the media of different densities could be derived accurately.
The idea of measuring pressure within a well is not new; pressure transducers based on numerous electronic measurement principles have been used by the industry for decades. However, these electronic gauges are not optimal for use in the described application because of their relatively short service life at the elevated temperatures found in wellbores and be-cause of the logistical difficulties and high cost associated with deploying a string of multiple electronic gauges in a single well. The operator’s in-well technology team examined the emerging use of optical-fiber Bragg gratings (FBGs) for pressure metrology applications in extreme environments. The gauges used in this technology required no downhole electronics and offered long service at elevated downhole temperatures. The technology offered the long-term stability required to detect the subtle pressure changes resulting from small fluid-level changes. Dozens of such gauges could be deployed in a series on one optical cable connected to a single surface-acquisition unit (interrogator).