Data processing: measuring – calibrating – or testing – Measurement system in a specific environment – Earth science
Reexamination Certificate
2002-09-09
2003-09-09
McElheny, Jr., Donald E. (Department: 2857)
Data processing: measuring, calibrating, or testing
Measurement system in a specific environment
Earth science
C073S001340, C073S152120, C073S152180, C073S152330, C703S010000
Reexamination Certificate
active
06618677
ABSTRACT:
This invention relates to a method and apparatus for determining flow rates. In particular, it is concerned with the determination of the rate of flow of fluids in a conduit, using techniques for acquiring a distributed temperature profile in an optical fibre over a period of time. Using this time-dependent temperature data, the mass flow rates of fluids along the conduit can be determined when appropriate constants are known. These constants relate to a number of parameters, of which time is of particular importance, and also include measures of distance and thermal variables such as temperature, conductivity and specific heat.
Mass flow rate information is a very important tool for the efficient management of oil wells and the like. It is of course important to have reliable production data, as soon as possible, not only for its own sake. If flow rate data is promptly available, it may also be actively used to adjust or improve the flow rates, to diagnose immediate or potential problems, or to trigger alarms. Significant variations in flow can be met with an appropriate management response.
It has been known in principle for over 25 years that thermal data can be used to derive mass flow rate information, and that this information is applicable to oil field operations and the like. Reference is made to the paper “Use of the Temperature Log for Determining Flow Rates in Producing Wells”, Curtis M R and Witterholt E J, Society of Petroleum Engineers of AIME Paper No SPE 4637. Curtis and Wdterhoft describe a method for calculating mass flow rate of a fluid up a well bore as a function of the temperature profile based on algorithms developed by Ramey, published as “Well-bore Heat Transmissionr”, Ramey H, J.Pet.Tech., April 1962.
Nevertheless, as a practical matter, in the economically and commercially important field of determining the mass flow rate of produced or injected fluids within a well bore, downhole measurements have typically been made using either spinner or venturi techniques in one or a plurality of locations within the production tubing. The equipment or devices that have been used have been either permanently installed in the well bore or conveyed into a measuring location by wireline.
These currently used devices do however have well known disadvantages. The spinner device is typically run on a wireline. Use of this technique commonly involves shutting in the well for extended periods while setting up the equipment, and then running the sensor and cable in the well, which presents a hazard to the integrity of the well. Surveys of this kind are carried out infrequently, and only provide an instantaneous picture of the flow characteristics of the well.
In order to obtain continuous flow information, it is necessary to use downhole instrumentation that is permanently installed. A particular benefit of permanent instrumentation is that it enables a producing well to be better controlled. Venturi techniques, in which the pressure drop across a known orifice is measured, enable flow rates to be permanently monitored, but do however have limitations. Firstly, the orifice device restricts the internal diameter of the tubing. Secondly, the device relies upon two independent high accuracy pressure sensors, but the output of such devices has a tendency to drift with time. Thirdly, the venturi device must be routinely calibrated to a fixed fluid mix density, to ensure continued accuracy of measurement
For the foregoing reasons, among others, there is and has been for a long time a continuing need to find and develop improved methods for downhole mass flow rate monitoring.
We have developed sensing and measuring equipment based on opto-electronic systems at a surface location operatively connected to fibre optic sensors deployed downhole. Using such systems, it is not necessary to have any electronics downhole and the fibre sensors can provide temperature and pressure information, while being resistant to temperatures up to 250° C. and above.
It has been known for over 15 years that optical fibres can report temperature distributions. See for example GB 2122337 and EP 0213872. We have now found that it is possible to combine, in a useful and practical and advantageous manner, the derivation of mass flow rates from thermal data with the acquisition of thermal data by means of a fibre optic sensor.
Typically, the thermal data is acquired as follows. A laser light pulse is sent down an optical fibre wave guide. As the pulse of light travels along the wave guide, the thermal molecular vibration at each point along the length of the wave guide causes a very weak reflected signal to travel back up the fibre towards the source. An optical coupler splits the reflected light away from the fibre and takes it to a detector. The time lapse between the launch of the light pulse and detection allows the distance of the reflection point down the optical fibre to be calculated, since the speed of light in the fibre is constant and is known. The amplitude of the returned light is a function of the molecular vibration at the reflection point, increasing with increasing temperature. As reflected light is detected over a time period corresponding for the time taken for the light pulse to travel the length of the optical fibre and back, the output of the detector is effectively a distributed temperature profile along the whole length of the fibre.
The present invention addresses the deficiencies of the prior art methods of determining downhole flow rates and provides methods and apparatus utilising a distributed fibre optic sensor. We have found that a single optical sensor system can provide sufficient thermal information to determine the mass flow rates of produced fluids within a well bore, using an optical fibre placed within or adjacent to the well bore, almost instantaneously, at any time, substantially continuously if required, without interference with production or prejudicing the integrity of the well.
The present invention concerns aspects of the method and apparatus described below. The scope of the invention extends to all novel aspects thereof whether individually or in combination with any of the other features disclosed herein.
More specifically, in one aspect of the invention a method of determining mass flow rates of fluid in a conduit located in a heat sink differing in temperature from the fluid may comprise obtaining a distributed temperature profile of fluid flowing along a length of conduit by means of optical data obtained from a length of optical fibre in thermal contact therewith, obtaining a profile of the heat sink temperature external to the conduit, and deriving mass flow rates of fluids in the conduit from the said profiles and from measured thermal transfer parameters.
Correspondingly, apparatus for determining mass flow rates of fluid in a conduit located in a heat sink differing in temperature from the fluid may comprise a length of optical fibre in thermal contact with the fluid, means for obtaining a distributed temperature profile of fluid flowing along a length of conduit by means of optical data obtained from said length of optical fibre, and means for deriving mass flow rates of fluids in the conduit from the said distributed temperature profile, from a profile of the heat sink temperature external to the conduit, and from measured thermal transfer parameters.
In a further aspect of the invention there is provided a method of monitoring the mass flow rates of fluids flowing in variable quantities along a length of underground conduit, including monitoring the said rates during both a calibration period and an observation period (which may include some or all of the calibration period); which method comprises:
(a) establishing distributed temperature measuring apparatus comprising an optical fibre extending along the said length of conduit in thermal contact with the fluid and/or with the conduit, together with means for passing light along the optical fibre in the said length, receiving light emergent therefrom, and interpreting temperature- and location-
McElheny Jr. Donald E.
Sensor Highway Ltd
Usher Robert W. J.
LandOfFree
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