Acoustics – Geophysical or subsurface exploration – Well logging
Reexamination Certificate
2000-05-31
2003-09-09
Lockett, Kimberly (Department: 2837)
Acoustics
Geophysical or subsurface exploration
Well logging
Reexamination Certificate
active
06615949
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention pertains to logging while drilling apparatus and more particularly to acoustic logging while drilling apparatus and attenuation of acoustic pulses that travel parallel to the direction of drilling.
2. Related Prior Art
To obtain hydrocarbons such as oil and gas, wells or wellbores are drilled into the ground through hydrocarbon-bearing subsurface formations. Currently, much current drilling activity involves not only vertical wells but also drilling horizontal wells. In drilling, information from the well itself must be obtained. While seismic data has provided information as to the area to drill and approximate depth of a pay zone, the seismic information can be not totally reliable at great depths. To support the data, information is obtained while drilling through logging while drilling or measuring while drilling (MWD) devices. Logging or measuring while drilling has been a procedure in use for many years. This procedure is preferred by drillers because it can be accomplished without having to stop drilling to log a hole. This is primarily due to the fact that logging an unfinished hole, prior to setting casing if necessary, can lead to washouts, damaging the drilling work that has already been done. This can stall the completion of the well and delay production. Further, this information can be useful while the well is being drilled to make direction changes immediately.
Advances in the MWD measurements and drill bit steering systems placed in the drill string enable drilling of the horizontal boreholes with enhanced efficiency and greater success. Recently, horizontal boreholes, extending several thousand meters (“extended reach” boreholes), have been drilled to access hydrocarbon reserves at reservoir flanks and to develop satellite fields from existing offshore platforms. Even more recently, attempts have been made to drill boreholes corresponding to three-dimensional borehole profiles. Such borehole profiles often include several builds and turns along the drill path. Such three dimensional borehole profiles allow hydrocarbon recovery from multiple formations and allow optimal placement of wellbores in geologically intricate formations.
Hydrocarbon recovery can be maximized by drilling the horizontal and complex wells along optimal locations within the hydrocarbon-producing formations. Crucial to the success of these wells is establishing reliable stratigraphic position control while landing the well into the target formation and properly navigating the drill bit through the formation during drilling. In order to achieve such well profiles, it is important to determine the true location of the drill bit relative to the formation bed boundaries and boundaries between the various fluids, such as the oil, gas and water. Lack of such information can lead to severe “dogleg” paths along the borehole resulting from hole or drill path corrections to find or to reenter the pay zones. Such well profiles usually limit the horizontal reach and the final well length exposed to the reservoir. Optimization of the borehole location within the formation also can have a substantial impact on maximizing production rates and minimizing gas and water coning problems. Steering efficiency and geological positioning are considered in the industry among the greatest limitations of the current drilling systems for drilling horizontal and complex wells. Availability of relatively precise three-dimensional subsurface seismic maps, location of the drilling assembly relative to the bed boundaries of the formation around the drilling assembly can greatly enhance the chances of drilling boreholes for maximum recovery. Prior art down hole devices lack in providing such information during drilling of the boreholes.
Modern directional drilling systems usually employ a drill string having a drill bit at the bottom that is rotated by a drill motor (commonly referred to as the “mud motor”). A plurality of sensors and MWD devices are placed in close proximity to the drill bit to measure certain drilling, borehole and formation evaluation parameters. Such parameters are then utilized to navigate the drill bit along a desired drill path. Typically, sensors for measuring downhole temperature and pressure, azimuth and inclination measuring devices and a formation resistivity measuring device are employed to determine the drill string and borehole-related parameters. The resistivity measurements are used to determine the presence of hydrocarbons against water around and/or a short distance in front of the drill bit. Resistivity measurements are most commonly utilized to navigate the drill bit. However, the depth of investigation of the resistivity devices usually extends only two to three meters and resistivity measurements do not provide bed boundary information relative to the downhole subassembly. Furthermore, the location of the resistivity device is determined by some depth measuring apparatus deployed on the surface which has a margin of error frequently greater than the depth of investigation of the resistivity devices. Thus, it is desirable to have a downhole system which can accurately map the bed boundaries around the downhole subassembly so that the drill string may be steered to obtain optimal borehole trajectories.
The relative position uncertainty of the wellbore being drilled and the critical near-wellbore bed boundary or contact is defined by the accuracy of the MWD directional survey tools and the formation dip uncertainty. MWD tools may be deployed to measure the earth's gravity and magnetic field to determine the inclination and azimuth. Knowledge of the course and position of the wellbore depends entirely on these two angles. Under normal Conditions, the inclination measurement accuracy is approximately plus or minus two tenths of a degree. Such an error translates into a target location uncertainty of about three meters per one thousand meters along the borehole. Additionally, dip rate variations of several degrees are common. The optimal placement of the borehole is thus very difficult to obtain based on the currently available MWD measurements, particularly in thin pay zones, dipping formations and complex wellbore designs.
Until recently, logging while drilling has been limited to resistivity logs, gamma logs, neutron logs and other non-acoustic logs since acoustic noise caused by drilling and acoustic pulses traveling upstring from the transmitter has presented problems in accurate detection and delineation. These problems cannot be easily isolated by arrival time since the acoustic pulses are generated and detected continuously. Recently, the use of acoustic sensors having a relatively short spacing between the receivers and the transmitter to determine the formation bed boundaries around the downhole subassembly has been used. An essential element in determining the bed boundaries is the determination of the travel time of the reflection acoustic signals from the bed boundaries or other interface anomalies. A prior art proposal has been to utilize estimates of the acoustic velocities obtained from prior seismic data or offset wells. Such acoustic velocities are not very precise because they are estimates of actual formation acoustic velocities. Also, since the depth measurements can be off by several meters from the true depth of the downhole subassembly, it is highly desirable to utilize actual acoustic formation velocities determined downhole during the drilling operations to locate bed boundaries relative to the drill bit location in the wellbore.
Additionally, for acoustic or sonic sensor measurements, the most significant noise source is acoustic signals traveling from the source to the receivers via the metallic tool housing and those traveling through the mud column surrounding the downhole subassembly (tube waves and body waves). In some applications acoustic sensor designs are used to achieve a certain amount of directivity of signals. A transmitter coupling scheme with signal processing method
Belov Vladimir
Bolshakov Alexei
Dubinsky Vladimir
Egerev Sergey
Tiutekin Victor
Baker Hughes Incorporated
Lockett Kimberly
Madan Mossman & Sriram P.C.
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