Electricity: measuring and testing – Of geophysical surface or subsurface in situ – With radiant energy or nonconductive-type transmitter
Reexamination Certificate
2002-11-08
2004-08-17
Le, N. (Department: 2862)
Electricity: measuring and testing
Of geophysical surface or subsurface in situ
With radiant energy or nonconductive-type transmitter
C324S335000
Reexamination Certificate
active
06777940
ABSTRACT:
BACKGROUND
1. Field of the Invention
This invention relates to the field of electric logging of “wells” or earthen boreholes. In particular, the invention relates to well logging apparatus and methods for determining formation properties, such as resistivity, at several different distances extending radially from the borehole into the surrounding formation. The invention has general applications in the well logging art, but is particularly useful in measurement while drilling (“MWD”) applications.
2. Description of the Related Art
Resistivity logging is a commonly used technique for evaluating potential hydrocarbon-bearing formations surrounding a borehole drilled into the earth. Porous formations are more resistive to a flow of electric current when they are saturated with hydrocarbons, and less resistive when saturated with water (which contains some amount of salt, rendering it more or less conductive). The formation immediately surrounding the borehole can be altered by invasion of borehole fluids during the drilling of the well, and can therefore exhibit a different resistivity than the formation farther from the borehole—so-called “virgin” formation. In order to determine the true resistivity of the virgin zone, the well logging device must be capable of performing measurements at multiple depths of investigation. The multiple depths permit mathematical correction of the different measured values.
Historically, resistivity logging tools, conveyed by wireline after the borehole has been drilled, have measured resistivity at three depths of investigation (shallow, medium, and deep). Mathematically, the three measurements are used to solve for three unknowns (Rt, Rxo, and Di). The shallow and medium measurements are used to correct the deep measurements to obtain a more accurate measurement of true virgin resistivity (Rt). The medium and deep readings are used to correct the shallow reading to obtain a more accurate reading of flushed zone resistivity (Rxo) (the flushed zone being the formation nearest the borehole, in which the original formation fluids have been at least partially displaced by drilling fluids). The three readings are also used to determine the depth of invasion Di (that is, how far drilling fluids have intruded into the formation), when a simple step invasion profile is assumed.
Large values for depth of invasion indicate zones of high permeability, which suggest potential high fluid flow rates, desirable for producing commercially significant quantities of hydrocarbons. Computed values for Rt and Rxo may be used for estimating water saturation (Sw), under certain favorable conditions. Low values of Sw indicate the presence of hydrocarbons in the formation.
The present invention relates to a type of resistivity well logging known as electromagnetic propagation logging. Propagation logging is well suited for determining resistivity by apparatus designed for use while drilling, so-called MWD tools. The basic principle of such measurement is a transmitter propagating electromagnetic energy into the formation, at a known frequency and strength, and reflections of that transmitted energy are detected by receivers spaced apart from the transmitter. Earlier generation MWD propagation resistivity devices provided only two depths of investigation, from the phase difference and attenuation measurements. By “phase difference” is meant a difference in timing between the transmitted and received signal. By “attenuation” is meant a lessening or decrease in the amplitude of the transmitted signal.
Separation of the curves is used to identify invasion; however, it is mathematically impossible to solve for the three desired unknowns (Rt, Rxo, Di) from only two measurements. Another disadvantage of the earlier generation tools is that the vertical response of the attenuation measurement is not as sharp as the vertical response of the phase difference measurement. As a result, separation of the curves results at bed boundaries (that is, the boundaries between beds of dissimilar rock type within a zone), even when invasion is not present. Also, it is known in the art that dielectric uncertainty can cause the phase difference and attenuation curves to separate even when no fluid invasion is present. In fact, the separation of phase difference and attenuation curves can be used to estimate the dielectric constant in thick beds.
Another disadvantage of the attenuation measurement is reduced dynamic range when compared to the phase difference measurement. As the formation resistivity increases, the attenuation measured between the two receiver antennas approaches a constant value, and the measurement becomes insensitive to changes in resistivity. In contrast, the phase difference measurement retains sensitivity to higher resistivity values and thus has a broader useful range. The limited dynamic range of the attenuation measurement sets an upper resistivity limit on the utility of apparatus employing this method for detecting invasion.
More recent propagation MWD resistivity devices have added measurements at additional depths of investigation. However, these prior art apparatus and method still have various limitations. One group of apparatus achieves the multiple depth resistivity measurements via additional transmitter and receiver antennas, each tuned to transmit or receive at the same frequency but spaced differently, thereby resulting in different depths of investigation. The additional transmitters and receivers, it will be appreciated, added greatly to cost and complexity of the tools.
Yet another group of apparatus employed multiple different frequencies to yield multiple depths of investigation (it being known in the art that different frequencies yield different depths of investigation, the lower frequencies yielding a deeper investigation, while higher frequencies yield a shallower depth of investigation). However, this group of tools still required multiple additional transmitters and receivers, each tuned to transmit or receive only a single frequency. Again, increased cost and complexity of tools resulted. Many of these prior art apparatus exhibit other limitations, such as high electrical power consumption.
The apparatus and method of the present invention provide resistivity measurements at multiple (three or more) depths of investigation while avoiding the disadvantages of related art methods and apparatus. The apparatus and method herein provide multiple resistivity measurements that have nearly equivalent vertical resolution and maximum dynamic range, by using only phase difference measurements. Since attenuation measurements are not used for additional depths of investigation, the attenuation measurements can be combined with the phase difference measurements to solve for the formation dielectric constant at multiple frequencies. The current apparatus minimizes the number of antennas required for either a borehole compensated and electronically compensated measurement (four), or alternatively for an uncompensated measurement (three), as a result minimizing manufacturing and maintenance cost and maximizing reliability. Furthermore, untuned coils are used for the transmitter and receiver antennas, allowing each coil to be used for more than one frequency and eliminating error caused by mutual inductance between adjacent series tuned receiver antennas. The apparatus minimizes electronics required to transmit multiple frequencies by using a switch-mode transmitter circuit, which has the further advantage of generating the desired frequencies simultaneously. The transmitter electronics disclosed are also simpler and more efficient than methods used in the prior art. Transmitter energy is minimized by using low noise electronics and coherent detection in the receiver. Time required to complete a measurement can be minimized by simultaneously detecting multiple frequencies in the receiver.
OBJECTS AND ADVANTAGES
Accordingly one of the objects of this invention is to provide resistivity measurements of a formation surrounding a borehole at multiple (three or more)
Dominque & Waddell PLC
Kinder Darrell
Le N.
Ultima Labs, Inc.
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