Electricity: measuring and testing – Of geophysical surface or subsurface in situ – Using electrode arrays – circuits – structure – or supports
Reexamination Certificate
1998-07-15
2001-02-20
Strecker, Gerard (Department: 2858)
Electricity: measuring and testing
Of geophysical surface or subsurface in situ
Using electrode arrays, circuits, structure, or supports
C324S371000
Reexamination Certificate
active
06191588
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to the investigation of earth formations. More particularly, the invention relates to methods and apparatus for electrically imaging the wall of a borehole.
2. State of the Art
When analyzing a hydrocarbon well, it is desirable to identify earth formation features at various borehole depths. Some of the formation features which are desirable to identify include fine beddings and facies, the heterogeneity of carbonate deposits and the structure of fractures.
The detection of beddings involves detecting shaly-sand sequences where the shales establish a basal contact for each sequence. Facies identification involves identifying the lithology between basal contacts. The analysis of carbonates involves detecting non-homogenous features such as those due to irregular cementation, variations in the pore sizes, small scale lithology changes, etc. Fractures play a major role in the flow characteristics of reservoir rock. Therefore, the measuring or detecting of fractures, determining their orientations, density, height, vertical and lateral continuity is highly desirable.
Co-owned U.S. Pat. No. 4,468,623 to Gianzero et al. (the '623 patent) discloses an earth formation investigating tool which can detect borehole wall features which are only millimeters in size. As shown in prior art
FIGS. 1 and 2
, the tool
10
includes an array
12
of small survey electrodes (buttons)
14
a
-
14
l
mounted on a conductive pad
16
which is pressed against the borehole wall
18
. A constant current source is coupled to each button such that current flows out of each button
14
into the adjoining formation, perpendicular to the borehole wall
18
as illustrated in
FIG. 1
by the arrows E
1
, E
2
. The current returns to an electrode (not shown) which is located at or near the surface, or on another part of the tool
10
. The individual button currents are monitored and recorded (by an uphole processor
20
) as the tool
10
is moved through the borehole. The measured button currents are proportional to the conductivity of the material in front of each button. The conductivities are plotted as a function of depth to form a “wiggle trace” (or log) which can be analyzed to identify formation features at the different borehole depths.
Co-owned U.S. Pat. No. 4,567,759 to Ekstrom et al. (the '759 patent) discloses a method and apparatus for producing a high resolution image from the data collected by the tool described in the '623 patent. According to the methods of the '759 patent, signals from a conductivity measuring tool are processed to compensate for conditions such as variation in tool velocity, variations in borehole environment, etc. This processing enables subsequent signal enhancements with which the signals can be displayed in a manner that approaches the character of a visual image of the borehole wall taken from inside the borehole. Since the human eye is highly perceptive, fine high resolution features of the borehole wall can be visually discerned and interpreted. Such features include minute variations in the borehole wall in both the circumferential as well as vertical directions. Features which can be discerned from the image include vugs, small stratigraphy beds with their circumferential thickness variations, small scale lithology changes, pore sizes, fractures with their density, height, vertical and lateral continuity, etc.
Further enhancements to the methods and apparatus of the '623 patent and the '759 patent are disclosed in co-owned U.S. Pat. No. 4,692,908 to Ekstrom et al.(the '908 patent). The '908 patent discloses an acoustic method and apparatus for measuring the distance between the electrode buttons and the borehole wall. This distance is likely to change as the tool is moved through the borehole. Distance measurements made according to the '908 patent are recorded and used to correct the conductivity measurements if deemed necessary.
As disclosed in the '623 patent, the size and spacing of the electrode buttons is important to obtain good resolution and signal to noise ratio. In particular, the buttons should be closely spaced for high resolution and small in area for good spatial bandwidth. If the buttons are too small, however, the signal to noise ratio (SNR) is adversely affected. This is demonstrated by analyzing the current flow through the buttons into the formation. For example, as shown in prior art
FIG. 1
, buttons
14
a
and
14
b
are located on opposite sides of a bed boundary B which separates beds having different resistivities R
1
and R
2
. Assuming that the pad
16
is in perfect contact with the borehole wall
18
, the electric field near the pad
16
is perpendicular to the pad face or parallel to the bed boundary B. The parallel component of the electric field is continuous across the two different media as shown by
E
1
=E
2
(1)
where E
1
and E
2
are the electric fields on the two sides of the bed boundary B. In an ideal case, with an infinitely long pad, the equipotential surfaces near the center of the pad are all parallel to the pad face and the electric field is constant. The current density j
i
flowing into each bed is proportional to the conductivity &sgr;
i
of that bed as shown according to
j
i
=E&sgr;
i
(2)
If the conductivity &sgr; of the formation is continuously varying, then the current density j may be expressed by
j=E&sgr;
(3)
The electric current I
b
flowing into a button is therefore the integral of the current density over the area of the button according to
I
b
=∫j·da=E·∫&sgr;da
(4)
The electric field E depends on the conductivity distribution away from the pad and is not calibrated. Therefore, the button current is not a quantitative measure of the local conductivity. However, if one computes the ratio of button currents at two nearby points, the unknown Es cancel out. Thus, the ratio of the currents passing through two nearby buttons is a quantitative measurement of the ratio of shallow conductivities. From the foregoing, it will be appreciated that the size and spacing of the electrode buttons will govern resolution and SNR, and that resolution can be increased only at the expense of decreasing SNR.
The methods and apparatus thus far described with reference to the co-owned '759 and '623 patents are known in the art as FMI™, a trademark of Schlumberger and an abbreviation for “formation micro imager”. FMI™ has been widely successful in producing accurate images of boreholes when used in wells which have been drilled with water based mud (WBM). However, the FMI™ produces lower quality images in wells which have been drilled with oil based mud (OBM).
At the present time there are no tools available which can produce borehole images in an OBM well which are comparable in quality to the images produced by FMI™ in a WBM well. Despite this fact, the use of OBM in well drilling is increasingly popular.
It is believed that in an OBM well non-conductive mudcake between the conductor buttons and the borehole wall interferes with the conductivity measurements.
SUMMARY OF THE INVENTION
It is therefore an object of the invention to provide methods and apparatus for accurately imaging a borehole wall in an OBM well.
In accord with this object which will be discussed in detail below, the imaging apparatus of the present invention includes a tool similar to the FMI™ tool having an array of electrode buttons mounted on a pad. However, according to the invention, the pad is non-conductive and the buttons are voltage electrodes rather than current electrodes. In the preferred embodiment, a current source and a current return are located at opposite ends of the non-conductive pad; although the current source and current return may be located off the pad. However, whether located on or off the pad, the locations of the current source and return are designed to force a current to flow in the formation parallel to
Batzer William B.
Gordon David P.
Schlumberger Technology Corporation
Strecker Gerard
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