WITSML Distributed Temperature Survey 

Usage Document

This document contains the following sections:

• Preface
• Introduction
• E&P Objects and Concepts: Distributed Temperature Surveying
• Requirements
• Use Cases
• DTS Schema Concepts
• DTS Schemas, XMLdocuments, and Stylesheets
• Reused Schemas, XMLdocuments, and Stylesheets

Preface

This document contains descriptions of data types produced by optic fiber based Distributed Temperature Survey data from oil wells.

Introduction

 This document describes a proposed transfer standard for Distributed Temperature Survey (DTS) data profiles measured using optic fiber techniques along wellbores drilled for the production of oil and gas.  The DTS data are specified re-using many relevant WITSML definitions.  

POSC gratefully acknowledges the contributions of intellectual property and expertise made by members and member companies of the DTS workgroup, especially BP, Halliburton, Sensornet and Shell.

Intended Audiences

It is anticipated that three kinds of audience will read this document.

• First, petroleum geoscientists and engineers will seek understanding of the temperature data profiles which they use to monitor conditions in the reservoir and wellbore.  In addition to the actual temperature data, the standard enables description of the equipment and software that has been used to record and process them, along with contextual data about the wellbore in which the measurements were made and when.  

• Second, software engineers who develop the data acquisition systems, the data transfer systems and the persistent data stores and application systems will also need to understand the content and format of the DTS data.

• Third, data managers will need to understand the content and format of the data so that they can store it in persistent data stores and make it available to the applications used by the geoscientists and engineers. 

E&P Objects & Concepts: Distributed Temperature Surveying

For a fuller description of the technology of Fiber Optic thermometry, and its application to oil and gas production, please refer to the Shell docment "A DTS Primer for Oil & Gas Production".  POSC gratefully acknowledges the availability and use granted by Shell International E&P of this DTS Primer as part of the basis of the proposed standard.

The application of optic fiber thermometry in oil and gas is a young and rapidly evolving technology.  For this reason, any standard may anticipate several iterations and evolutions in its early years, and should be designed with a degree of flexibility.  The challenges of installing and operating a delicate sensor - the optic fiber - in a remote and hostile environment - the oil well - are still being mastered.  The survey process still has a significant experimental aspect to it, in addition to the natural variability of the processes the technique is being used to monitor.

Well and Wellbore

The WITSML definition of wellbore as "a unique, oriented path from the bottom of a drilled borehole to the surface of the Earth" is particularly suited to DTS in that at least one end of the sensing fiber in a wellbore will always extend to the surface to connect with the Instrument Box.  A fiber will not be physically joined or spliced into fibers from other wellbores in a well.  

Fiber Optic Technology and DTS Measurements

Principles

When light is transmitted along an optic fiber, transmitted light collides with atoms in the lattice structure of the fiber and causes them to emit bursts of light at frequencies slightly different from the transmitted radiation which propagate back along the fibre and can be detected at its end.  This is known as backscattering.  Optic fiber thermometry depends upon the phenomenon that the frequency shifts occur to bands both less than and greater than the transmitted frequency.  Furthermore, in the case of the Raman shifted bands the intensity of the lower frequency band is only weakly dependent on temperature, while the intensity of the higher frequency band is strongly dependent upon temperature.  The ratio of these intensities is related to the temperature of the optic fiber at the site where the backscattering occurs: i.e., all along the fiber.  The backscatter spectrum is illustrated in Figure 1.  The intensities of backscatter of individual pulses is very weak, necessitating statistical stacking of thousands of backscattered light pulses.

Equipment: Instrument Box and Fiber

Several organisations manufacture and / or operate DTS equipment on a service basis for the E&P industry.  The optic fibers are mass-manufactured by a number of companies with a variety of materials and designs for different applications.  The 'Instrument Boxes' comprise a laser light source, an oven which provides a reference temperature, and photo-electric sensors which capture the backscattered light, along with digital circuitry which both controls the laser transmission and analyses the frequency and intensity of the backscatter.  Instrument boxes and fibers can readily be interchanged. 

The transmission characteristics of optic fibers are dependent upon several factors in addition to temperature, which include their construction, physical deformation and chemical , particularly aqueous, contamination.  For these reasons, calibrations of the equipment are routinely necessary to monitor the transmission characteristics.

Installation in Wellbores

The location of the source of the backscattered light is determined in terms of its distance along the fiber by the two-way transmission time and the velocity of light in the transmitting medium.  It is therefore necessary to know how length along the fiber correlates to measured depth along the wellbore in which the fiber is installed.  The foregoing information includes knowledge of the positions in the well and along the fiber where connections through casing shoes, packers and other features occur. All these can be assumed to remain constant between initial installation and mechanical interventions in the wellbore.

There are several fiber configurations deployed in wellbores,  illustrated in fig 2, which enable various calibrations to be made to the temperature data.

Calibrations

Two kinds of calibration are applied to DTS data. These are:

• factors pertaining to the Instrument Box such as the oven reference temperature, the number of launches being stacked to get a temperature value at the required resolution;

• factors related to the fiber, including the (practically linear) differential attenuation along the fiber, and the refractive index of the fiber.

Where the fiber may be installed in a well bore for several years, it is necessary to recalibrate these factors periodically in order to be able to compare reservoir temperatures over these periods.

The Optical Time Domain Reflectometry (OTDR) technique provides a routine means of examining the condition of the fiber.  OTDR consists of observing the attenuation of backscattered light in the frequency band of the transmitted Rayleigh waves.  An OTDR profile is illustrated in Figure 3.

Figure 3: Optical Time Domain Reflectometry (OTDR) Profile

Measurements

The measurements produced by the Instrument Box for a temperature profile typically comprise a set of records containing contextual data followed by the temperature records.  The contextual data identify the well, the equipment, the date and time, and various calibration data.  Each temperature record contains the length along the fiber of the temperature sample, the computed temperature, and measurements of the Stokes and anti-Stokes intensities.

There are many possible representations of the temperature data, and criteria for selecting among them included: conformance to data produced from DTS instrument boxes; minimal verbosity of the XML transfer files; re-use of existing, implemented data specifications which satisfied DTS requirements; conformance to consuming data stores and applications, and others.  POSC recognised the high degree of similarity of DTS data with other types of well logs, and chose to re-use of the existing WITSML wellLog specification which met all currently known requirements for DTS data.

Requirements

POSC has defined standards for DTS data to be transferred between the oilfield well site and offices of oil and service companies. The DTS data standard should simplify, but does not specify, subsequent storage and access models and processes.   Thus, the standard should improve the overall process of controlling and optimizing production operations.  The standard specifies both the content and the representation of the data, in other words, it covers both the data, and their meaning, that need to be transmitted and what format they should be in.

Scope

• Define XML schemas for DTS  information for transfer from wellsite to office and office to wellsite, including information about the wellbore, the DTS equipment, its installation and calibration, and routine temperature survey data.

• Given the immaturity and rapid development of DTS techniques, the standard is designed so as to be capable of use for other kinds of data such as pressure.

• The DTS data standard re-used, where practicable, existing eXtended Mark-up Language (XML) assets developed for the E&P industry.  In particular, DTS leveraged the WITSML (Wellsite Information Transfer Standard Mark-up Language) being developed and used by the drilling community. 

The following functional requirements are identified but not described are out of scope:

• the analysis, calibration and conversion of intensities of the backscattered light to temperature

• storage in and retrieval from persistent data stores of DTS data

• and non-functional requirements such as packaging, transmitting and verifying transmission of the data over communications networks. 

Use Cases

The specification enables transfer of information for the following use cases, all of which assume the pre-existence of the wellbore and its data:

Installation of DTS fiber in a wellbore.

1. The activity of installing the fiber into the wellbore.

2. The recording of the position in terms of length along the fiber of key points in the wellbore.

Installation of DTS system on existing fiber in a wellbore.

1. Installations of DTS systems where all components are newly installed.
2. Installations where one or more of the components are newly installed with the remainder of the components being previously installed in the wellbore.

Calibration of the DTS system and its components

1. Calibration of DTS system and fiber prior to installation in a wellbore.
2. Calibration of DTS system and fiber subsequent to installation in a wellbore.
3. Calibration of configuration and position of fiber in wellbore in terms of length along fiber and measured depth, and of length along fiber at wellbore datums and at specific equipment items in wellbore.
4. Transfer of Optical Time Domain Reflectometry measurement.

Routine Temperature Survey measurement

1. Wellbore identification [Mandatory]
2. Fiber Instrumentation System as installed in the wellbore [Mandatory]
3. Reference to Calibration data set of DTS system to be used to derive temperature data from Stokes and anti-Stokes data. [Optional]
4. Reference to Computation method used to derive temperature data from Stokes and anti-Stokes data. [Optional].

DTS Schema Concepts

Introduction

The DTS workgroup, a part of the POSC  Integrated Operations Special Interest Group, has defined the requirements outlined above, and POSC staff have produced the accompanying set of schemas which define the XML representation of DTS data, along with example XML files, detailed XML documentation, and this Overview.  These items together comprise the contents of the package and its organisation in both the compressed file  and the WITSML website. 

DTS is modelled as follows:

Well and Wellbore

The DTS specification utilizes the WITSML well and wellbore schemas to provide all of the related well data. This includes information such as operator, geopolitical location, geographic location, well datum, status, etc.

Fiber

The fiber is intended to capture a description of the fiber as a material. Information about the installation and use of the fiber in the wellbore is captured in dtsInstalledSystem.