Patent Application
20090078411
In many types of well related operations, gas influx can be problematic. For example, in coiled tubing operations, such as cleanouts, gas influx into the coiled tubing string is undesirable.
Attempts have been made to detect this influx of fluid, particularly the influx of gas. For example, gas influx or “kick” has been detected by determining the round-trip transit time of mud pump noise. An alarm signal is generated when the rate of change in transit time exceeds a predetermined threshold. In another example, the Doppler frequency shift of a mud pump signal is expressed as a ratio signal and compared with a threshold signal to determine gas influx. However, these relatively complex attempts to determine the presence of gas influx are problematic for a variety of applications.
In general, the present invention provides a method and system for determining gas influx in a fluid transport system, such as a well system. In a well system application, a tubing is deployed in a wellbore, and a pressure signal is transmitted downhole through a fluid in the tubing. Parameters of the pressure signal, e.g. time and attenuation, are measured at an uphole location and a downhole location. Parameter data is evaluated to determine the occurrence of changes indicative of gas influx into the tubing.
Certain embodiments of the invention will hereafter be described with reference to the accompanying drawings, wherein like reference numerals denote like elements, and:
In the following description, numerous details are set forth to provide an understanding of the present invention. However, it will be understood by those of ordinary skill in the art that the present invention may be practiced without these details and that numerous variations or modifications from the described embodiments may be possible.
The present invention relates to a methodology and a system for determining the influx of gas in a variety of fluid handling systems. For example, the methodology and system can be used to determine whether gas influx has occurred within tubing used in a well application. In the well environment, coiled tubing is used in a variety of well related procedures, including production procedures, cleanout procedures, stimulation procedures and other procedures. Often the influx of gas can be detrimental to the operation, and the present system and methodology enable such gas influx to be detected and evaluated. As described in greater detail below, variations in the speed of sound within the tubing are indicative of a gas influx. The variations are monitored via specific parameters, e.g. travel time, of a pressure signal transmitted through fluid within the tubing. Other parameters, such as attenuation of the signal, also can be monitored and utilized in determining gas influx into coiled tubing or other tubing.
Referring generally to
The fluid transfer system 20 can be utilized in well applications in which tubing 22 is deployed in a wellbore 30. By way of specific example, the fluid transfer system 20 can be utilized in a cleanout procedure or other liquid delivery procedure in which a liquid is delivered downhole through an appropriate opening or openings 32. In well related applications, first location 26 may be an uphole location, and second location 28 may comprise a downhole location. Additionally, sensors may be utilized at other locations to detect desired parameters useful in monitoring the well. For example, a pressure sensor can be deployed at an external annulus location 34 to monitor annulus pressure. Similarly, an additional pressure sensor can be deployed at a wellhead location 36 to monitor wellhead pressure. A variety of other instruments can be used at these and other locations to detect parameters related to a specific well application.
Referring generally to
Sensors 44 and 46 measure desired parameters of the pressure signal, such as time and pressure level. By measuring the time at which the pressure signal passes first sensor 44 and second sensor 46, the travel time or time delay between sensors 44 and 46 can be determined for the signal. Additionally, by measuring the intensity of the pressure signal at first sensor 44 and second sensor 46, attenuation of the signal can be determined. Changes in the travel time and/or attenuation are indicative of changes in the fluid through which the pressure signal travels within tubing 22. Determination of the time delay, attenuation or other desired parameters can be performed on a control system 48 based on data output by sensors 44, 46 and pressure modulation device 40.
A variety of control systems 48 can be used with gas influx detection system 38. However, one example is illustrated schematically in
Liquid and gas have very distinct acoustic properties. Incursion of even a small amount of gas in a liquid results in significant changes in its acoustic behavior. For example, the propagation speed of sound dramatically decreases. This sensitivity to the influx of gas causes significant change in the travel time of the pressure signal from first sensor 44 to second sensor 46. The speed of sound in liquid, gas, and liquid-gas mixtures can be characterized mathematically with the following defined terms and equations:
x: gas volumetric fraction;
cl, cg speed of sound in liquid and gas;
ρl, ρg liquid and the gas density; and
Kl, Kg liquid and gas compressibility.
Mixture compressibility: K=x·Kg+(1−x)·Kl
Mixture density: ρ=x·ρg+(1−x)·ρl
Speed of sound in medium i: ci2=1/(ρi·Ki)
Speed of sound in mixture: c2=1([x·ρg+(1−x)·ρl]·[x·Kg+(1−x)·Kl])
The speed of sound, and thus the speed of the pressure signal, varies with both pressure and temperature. However, the pressure dependency for gas is marginal at low pressure. The travel speed of the pressure signal decreases dramatically as the gas fraction within the fluid increases, as illustrated graphically in
In graph 58, the speed of travel is illustrated by a solid line 60, while its derivative is illustrated by the x-line 62. As demonstrated, the speed at which a signal propagates through the fluid has a high sensitivity to the presence of even small amounts of gas in liquid. At higher gas fractions, however, the sensitivity decreases substantially. In the example illustrated in
The properties of liquid-gas mixtures enable the use of gas influx detection system 38. In
Control system 48 can be used to process the data provided by sensors 44, 46 and to display the data to an operator in graphical form or other forms. For example, the control system can be used to display a delay time diagnostic plot, as illustrated in
The data can be processed and displayed in a variety of other forms, including the amplitude ratio versus pressure signal illustrated in the signal attenuation diagnostic plot of
Control system 48 can be programmed to monitor the selected parameters, e.g. time delay and attenuation, to determine changes that indicate gas moving into tubing 22. The control system 48 is then able to output this information to an operator via output device 56. Depending on the source of the pressure signal, control system 48 can be programmed to compensate received/emitted signals due to possible variations in the intensity of the source signal. However, if pressure modulation device 40 can provide sufficient reproducibility of a consistent pressure signal, determination of gas influx can be achieved by comparing each pressure signal received at second sensor 46 with a baseline value. The baseline value can be obtained, for example, through a calibration phase in 100% liquid at the predetermined depth of second sensor 46.
Depending on the application and/or environment in which gas influx detection system 38 is utilized, a variety of pressure modulation devices 40 can be employed. In well applications, for example, a throttle valve or similar equipment may be used to create the pressure signal, either as simple pulses or a continuous wave. If the signal is created as pressure pulses, the time between pulses and the pulse intensity are selected based on a variety of factors, such as travel time to second sensor 46, predicted attenuation, potential reflections of the signal, and other factors. For example, the propagating pressure signal is increasingly attenuated over distance, and the characteristic decay length is a function of pipe diameter, signal frequency, compressibility and viscosity. These factors are taken into consideration when selecting sensors 44, 46 and designing detection system 38 for a given tubing and application.
In some environments, the broadband character of pulses may create difficulty in extracting the signal from environmental noise. For these applications, detection of a continuous wave pressure signal can be more reliable. Generally, environmental noise inside the coiled tubing utilized in well related applications is fairly low, rendering these applications suitable for pressure signals in the form of either pulses or a continuous wave.
Gas influx detection system 38 and its control system 48 also can be designed to detect and monitor a variety of other parameters to ensure accurate detection of gas influx. For example, flow sensors can be utilized to measure fluid velocity, and additional pressure sensors can be used to provide data on the pressure gradient present in the coiled tubing. The pressure gradient and flow velocity can affect detection of the gas fraction, for example, because movement along the pressure gradient causes a change in the volumetric gas fraction within the coiled tubing. However, the properties of the liquids and gases, e.g. density, sound velocity, compressibility, etc., enable the control system 48 to be readily programmed, as necessary, to compensate for these external factors.
The overall gas influx detection system 38 is useful in many types of fluid transfer systems 20 employed in well related applications and other applications. Additionally, the system can incorporate a variety of components; including a variety of first sensors 44 and second sensors 46 as well as a variety of acoustic modulation devices 40 to input suitable pressure signals. Furthermore, additional parameter sensors can be deployed along the tubing 22 or within a plurality of zones along tubing 22 to monitor gas influx at a plurality of locations. Control system 48 also can be programmed to process many types of data provided by sensors 44, 46 and to output related data, predictions, results, and other information related to the influx of gas, gas fraction, or effects of the gas influx.
It is important to note that the above described systems and methods of determining gas influx may be practiced in both a reverse cleanout procedure (wherein a cleanout fluid is pumped down the annulus of a well and returned up the coiled tubing), and a conventional cleanout procedure (wherein a cleanout fluid is pumped down the coiled tubing and returned up the annulus of the well).
Accordingly, although only a few embodiments of the present invention have been described in detail above, those of ordinary skill in the art will readily appreciate that many modifications are possible without materially departing from the teachings of this invention. Accordingly, such modifications are intended to be included within the scope of this invention as defined in the claims.
