Communications – electrical: acoustic wave systems and devices – Wellbore telemetering – Through drill string or casing
Reexamination Certificate
1999-12-09
2002-06-04
Edwards, Timothy (Department: 2635)
Communications, electrical: acoustic wave systems and devices
Wellbore telemetering
Through drill string or casing
C367S083000, C367S079000, C368S046000
Reexamination Certificate
active
06400646
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention generally relates to oilfield telemetry systems, and more specifically relates to a method for compensating for offset between a downhole clock and a clock in a surface installation.
2. Description of Related Art
Modem petroleum drilling and production operations demand a great quantity of information relating to parameters and conditions downhole. Such information typically includes characteristics of the earth formations traversed by the wellbore, along with data relating to the size and configuration of the borehole itself. The collection of information relating to conditions downhole, which commonly is referred to as “logging”, can be performed by several methods.
In conventional oil well wireline logging, a probe or “sonde” housing formation sensors is lowered into the borehole after some or all of the well has been drilled, and is used to determine certain characteristics of the formations traversed by the borehole. The upper end of the sonde is attached to a conductive wireline that suspends the sonde in the borehole. Power is transmitted to the sensors and instrumentation in the sonde through the conductive wireline. Similarly, the instrumentation in the sonde communicates information to the surface by electrical signals transmitted through the wireline.
Since the sonde is in direct electrical contact with the surface installation, the communications delay is negligible. Accordingly, measurements can be made and communicated in “real time”. If it should be deemed necessary, a downhole clock in the sonde can be easily synchronized with a surface clock. A computer on the surface can reset a surface clock while simultaneously transmitting a reset command to the downhole clock. Any offset due to the communications delay is for all practical purposes insignificant.
The problem with obtaining downhole measurements via wireline is that the drilling assembly must be removed or “tripped” from the drilled borehole before the desired borehole information can be obtained. This can be both time-consuming and extremely costly, especially in situations where a substantial portion of the well has been drilled. In this situation, thousands of feet of tubing may need to be removed and stacked on the platform (if offshore). Typically, drilling rigs are rented by the day at a substantial cost. Consequently, the cost of drilling a well is directly proportional to the time required to complete the drilling process. Removing thousands of feet of tubing to insert a wireline logging tool can be an expensive proposition.
As a result, there has been an increased emphasis on the collection of data during the drilling process. Collecting and processing data during the drilling process eliminates the necessity of removing or tripping the drilling assembly to insert a wireline logging tool. It consequently allows the driller to make accurate modifications or corrections as needed to optimize performance while minimizing down time. Designs for measuring conditions downhole including the movement and location of the drilling assembly contemporaneously with the drilling of the well have come to be known as “measurement-while-drilling” techniques, or “MWD”. Similar techniques, concentrating more on the measurement of formation parameters, commonly have been referred to as “logging while drilling” techniques, or “LWD”. While distinctions between MWD and LWD may exist, the terms MWD and LWD often are used interchangeably. For the purposes of this disclosure, the term LWD will be used with the understanding that this term encompasses both the collection of formation parameters and the collection of information relating to the movement and position of the drilling assembly.
A number of techniques have been used to transmit data obtained from LWD measurements to the surface. These include mud pulse telemetry, electronic telemetry, acoustic telemetry, and the like, with the system chosen to accommodate the particular conditions and measurements under consideration. For example, it is impractical to run an electrical cable downhole during drilling operations. Consequently, measured data are communicated by other means such as mud pulse telemetry. In mud pulse telemetry, the flow of drilling mud through the drillstring is modulated by periodically obstructing the flow. The resulting pressure waves propagate upstream and can be sensed at the surface. As another example, when it is desired to detect formation boundaries and to map the structure of earth formations, it is useful to conduct seismic profiling. In seismic profiling, measurements are obtained using sound waves, also called acoustic waves or seismic waves. It is well known that mechanical disturbances can be used to cause acoustic waves in earth formations and that the properties of these waves can be measured to obtain important information about the formations through which the waves have propagated. As it is known in art, the arrival time of these waves via the formation provides very useful information regarding the type of formation.
In these examples and in other LWD systems, clocks are often employed to provide timing information at more than one location. If different locations are subjected to varying conditions, such as temperature and pressure, this may result in clock desynchronization. For LWD techniques where it is desirable to compensate for clock error, this issue has not been adequately addressed. Seismic profiling is one such technique.
In a basic version of seismic reflection profiling, an acoustic source is used to send a sound signal from the earth's surface, at an initial time. The signal travels down through the earth, reflecting off boundaries between different formation features. A portion of the reflected signal travels back to a receiver, which registers the intensity of the signal as a function of the time elapsed from the initial time. This allows the time to travel to and from a formation feature to be measured. If the speed of the signal is known, then the travel time can be converted to the distance from the surface, or depth, of the feature. Time measurements are typically made with reference to one or more clocks.
Variations on this basic method of seismic reflection profiling are known. In particular, in vertical seismic profiling, a plurality of seismic receivers are placed in the borehole, with each receiver being at a different depth in the borehole. These receivers are used in conjunction with seismic sources placed either on the surface or inside another well. In reverse seismic profiling a downhole source is used. The source may be the drill bit itself or an alternate source placed downhole. The downhole source is used in conjunction with a plurality of seismic receivers placed at different points on the surface. Combinations of these techniques of vertical seismic profiling and reverse seismic profiling are also known, including three and four dimensional seismic profiling.
The speed of the acoustic signal used in seismic profiling varies with the material through which the signal travels. Therefore calibration of the seismic profiling measurements must be performed by measuring the acoustic travel time for a known distance. A measurement signal is known as a shot. A calibration signal which is used to obtain the speed of the sound wave is known as a checkshot. Calibration is performed by sending a signal a known distance and measuring the travel time. The time measurement is made with reference to one or more clocks.
Wireline checkshots may be used in conjunction with LWD shot measurements. In a traditional wireline checkshot a clock is associated with a seismic source on the surface. Another clock is associated with a receiver at the end of cable, which is lowered a known depth into the borehole. The industry has common downhole position measurement techniques that are known and may be used with checkshot measurements. The source clock is used to record the initial time of generation of the signal. The receiver clock is used to
Birchak James R.
Gardner Wallace R.
Malloy Robert
Minear John W.
Robbins Carl
Edwards Timothy
Halliburton Energy Service,s Inc.
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