Fibre-optic distributed sensing has the potential to revolutionize well and reservoir surveillance in the oil and gas industry. Benefits include the passive nature of optical fibre sensors, the potential for cost-effective installations, combined with the possibility of densely distributed measurements along the entire length of the fibre. Amongst a range of fibre-optic sensing technologies, Distributed Acoustic Sensing has the potential to provide a low cost alternative for conventional seismic technologies. To widen the geophysical application scope further, the fibre-optic sensing cable should be made more sensitive to incoming seismic waves that arrive at the cable perpendicular ("broadside") to its axial direction. We introduce the development of such cable concepts, and present results of a successful cable deployment in a surface seismic field trial. Efforts continue to realize cost-effective directionally-sensitive cables for geophysical use, for deployment down-hole and on surface. FIBRE-OPTIC SENSING IN THE OIL&GAS INDUSTRYThe passive nature and inherent long-term reliability of optical fibres, combined with the ability to string numerous individual sensing elements together in a single fibre, makes fibre-optic distributed sensing technology ideally suited for down-hole sensing along a full well path. A variety of fibre-optic sensors developed in the aerospace and defence industries are finding their way into the oil and gas industry, enabling measurement of quantities such as temperature, pressure, chemicals, strain and acoustics along the entire length of the fibre. As multiple fibres, each providing a specific distributed measurement, can be bundled in a single down-hole-deployable cable, the industry is encountering a unique opportunity for robust well and reservoir surveillance based on an abundance of measurements continuous in time and along the fibre's length 1 .In the oil and gas industry, seismic datasets provide a crucial source of information about the subsurface. The use of seismic surveys for finding new reservoirs with producible accumulations of hydrocarbons is widespread. More recently, seismic acquisition repeated during the productive lifetime of a field is becoming more common. This allows monitoring changes in the subsurface as a result of hydrocarbon production, and can help in optimising well integrity, subsurface models and production. Traditionally, electrical geophones have been used in onshore seismic surveys, either deployed on the surface or in a well-bore. Fibre-optic sensing technology provides an opportunity to replace these geophones by fibre-optic cables, offering the potential of reducing cost substantially while increasing the spatial sensor resolution dramatically.This paper focuses on the use of Distributed Acoustic Sensing for geophysical use. We will explain how the construction of fibre-optic cables confines the directional sensitivity, and we will present the development and testing of improved sensing cables.

Summary Fiber-optic (FO) -based sensing technologies such as distributed temperature sensing (DTS) or distributed acoustic sensing (DAS) for well surveillance are attractive because they offer a continuous collection of real-time downhole data without the need for well intervention, thus avoiding production deferment. An example is the application of DTS and DAS for gas lift performance monitoring in oil producers by measuring the thermal and acoustic effects from the flow of lift gas through the valves into the production tubing to determine the active, inactive, and possibly leaking valves, and, also, the unloading depth. An anomaly observed in DTS data of a deepwater Gulf of Mexico (GOM) gas lifted oil producer led to a significantly improved interpretation methodology that allows inferring both the lifting depths and the annular-fluid interface(s). These results were confirmed by DAS, by identifying gas flow through a valve in selected acoustic-frequency bands. The new insights have been applied to five wells in the GOM and Southeast Asia.

The objective of this paper is to demonstrate how both Distributed Acoustic Sensing (DAS) and Distributed Temperature Sensing (DTS) data, acquired using a fiber-optic cable installed and cemented behind a 7" production casing, could be used for single-phase production allocation in two conventional oil producers in the South of the Sultanate of Oman. DAS data can be processed, time-averaged, and filtered to specific frequency bands, to identify and monitor the acoustic frequencies that are excited by the flow through the perforation tunnels. It will be shown that under certain assumptions, the flow-induced acoustic amplitudes at the perforations can be calibrated and converted into actual flow rates, which allows for continuous production profiling across all intervals of interest. DTS data, acquired under transient conditions, can also be analyzed using a thermal simulation model, to allocate production to specific perforation intervals, provided an appropriate logging program is followed. DTS is not as good as DAS in capturing dynamic changes to the inflow profile, but does have a deeper depth of investigation and is less sensitive to the geometry of the perforation tunnels or possible flow obstructions in the wellbore. The two technologies are therefore complimentary and are best acquired simultaneously. This is the first case study in the Sultanate of Oman, where both DAS and DTS data sets were successfully acquired and interpreted for single-phase production profiling in a conventional oil producer with perforated casing. Moreover, it was also the first time in Oman that oriented perforation was achieved with full shot density, through a double perforation run with a slight offset in orientation angle between the two runs.

In the past decade, Fiber-Optic (FO) based sensing has opened up opportunities for in-well reservoir surveillance in the oil and gas industry. Distributed Temperature Sensing (DTS) has been used in applications such as steam front monitoring in thermal EOR and injection conformance monitoring in waterflood projects using (improved) warmback analysis and FO based pressure gauges are deployed commonly. In recent years 1 significant progress has also been made to mature other, new FO based surveillance methods such as the application of Distributed Strain Sensing (DSS) for monitoring reservoir compaction and well deformation, multidrop Distributed Pressure Sensing (DPS) for fluid level determination, and Distributed Acoustic Sensing (DAS) for geophysical and production/injection profiling. For the latter application, numerous field surveys were conducted to develop the evaluation algorithms or workflows which convert the DAS noise recordings into flow rates from individual zones. The applicability of a new graphical user-interface has been expanded to include smart producers and injectors that allows the user to visualize (in real time), QC and evaluate the DAS data. Also, the evaluation methods for the use of DTS for warmback analysis have been significantly improved.There are still improvements to be made in enabling Distributed Sensing infrastructure, such as handling and evaluation of very large data volumes, seamless FO data transfer, the robustness & cost of the FO system installation in subsea installations, and the overall integration of FO surveillance into traditional workflows. It will take some time before all these issues are addressed but we believe that FO based applications will play a key role in future well and reservoir surveillance.In this paper we present a recent example of single-phase flow profiling using DAS. The example is from a long horizontal, smart polymer injector operated by Petroleum Development Oman (PDO).

Petroleum Development Oman (PDO) has been utilizing several Enhanced Oil Recovery (EOR) methods in the Sultanate of Oman to accelerate (polymer/steam floods) and increase (Alkaline Surfactant Polymer, or ASP, floods) recovery from fields with challenging rock and fluid properties. Polymer-flood is a mature EOR technique, and has been trialled and implemented successfully on full-field scale in a sandstone reservoir of excellent quality in the South of Oman. As part of this continuing effort, a currently ongoing polymer pilot focuses on testing the ability to sustain polymer injectivity and to prove the polymer efficiency in a significantly lower permeability sandstone reservoir. This lower permeability reservoir introduces injectivity and conformance concerns. Continuous surveillance to monitor and assess changes in areal sweep, vertical conformance and fracture initiation play therefore a pivotal role in delivering on the success criteria of the pilot. The pilot was designed with this surveillance scope in mind. Producers are equipped with either a dual well-head or a permanent down-hole gauge, and well-testing is conducted with a dedicated [mobile] test-unit. In the injector wells verifying vertical conformance and matrix injection is critical but conventional surveillance (Production Logging) is unreliable as the spinner is affected by the non-Newtonian behaviour of the polymer solution. The two pilot injectors were therefore equipped with a fibre optic (FO) cable, clamped and cemented on the out-side of the 7″ casing, enabling continuous monitoring of the injection profile and thus injection conformance through temperature (Distributed Temperature Sensing, DTS) and acoustic (Distributed Acoustic Sensing, DAS) measurements. The focus of this paper is to explain the decisions taken based on the information obtained from the FO data during the pilot period. The data was used for several key decisions; to select which pilot injector to use for polymer injection and which to keep as a back-up by assessing Out-of-Zone Injection, matrix versus fracture injection, and to prove conformance and injectivity improvement after acid stimulation jobs. The most valuable information inferred from the FO data allowed confirmation of the improvement in injection conformance by the polymer-flood over the water-flood. Fibre optic surveillance thereby has truly proven its surveillance value, for this polymer pilot, and any potential up-scaling of the project.