Electricity: measuring and testing – Of geophysical surface or subsurface in situ – Using electrode arrays – circuits – structure – or supports
Reexamination Certificate
2002-12-20
2004-07-20
Le, N. (Department: 2862)
Electricity: measuring and testing
Of geophysical surface or subsurface in situ
Using electrode arrays, circuits, structure, or supports
C324S366000
Reexamination Certificate
active
06765387
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to a system and method for logging geological formation traversed by a borehole and more particularly to measuring formation resistivity through a cased borehole, wherein resistivity measurement is performed in a continuous, non-stop fashion.
BACKGROUND OF THE INVENTION
Measurement of the formation resistivity has been a well-known method to determine presence of hydrocarbons in a formation traversed by boreholes. Typically, however, a borehole is cased shortly after drilling to provide structural integrity of the well. Consequently, the technique of resistivity-through-casing (RTC) was developed to measure the formation resistivity from within the cased well. A general problem of formation RTC measurements is the high electromagnetic attenuation due to the high conductivity of the casing material, since such material is typically a single-wall mild-steel pipe with a conductivity of the order of 10
6
S/m (resistivity is about 10
−6
&OHgr;m). Galvanic RTC measurements provide one method capable of overcoming this problem due to electromagnetic attenuation.
There are several galvanic measurement methods, all of which have a common measurement principle: a known current is injected downhole into the casing and is returned through a surface electrode far from the wellhead. The casing leaks current into the formation, and the corresponding loss of current, being proportional to the local formation conductivity, can be determined by comparing the voltage drop across adjacent sections of the casing.
Important parameters of the galvanic method include, for example the characteristic length &lgr;
L
, which determines on what length scale most of the current has leaked into the formation. This parameter is approximately given by
ρ
R
,
where &rgr; is the average (global) formation resistivity and R is the casing resistance per meter. For typical values of &rgr;=10 &OHgr;m and R=40 &mgr;&OHgr;/m, the characterisitic length &lgr;
L
≈500 m. The characteristic impedance Q, is yet another important parameter, given by Q=Rx&lgr;
L
/2. Q also is equal to the potential developed at the injection point for a current of 1A. For example, R=40 &mgr;&OHgr;/m and &lgr;
L
≈500 m, then Q=10 m&OHgr;; that is a current of 10A develops a voltage drop V
0
of 100 mV between the injection point and a (infinitely) remote return electrode.
Generally, galvanic RTC measurement is performed in the following fashion. A current (typically I
0
=10A) is injected into the casing; this current splits evenly in all directions. Close to the injection point, assuming a local formation resistivity of &rgr;=10 &OHgr;m, the pipe leaks current at a rate of dI=I
0
/2/&lgr;
L
=10 mA/m. This loss of current is proportional to the local formation conductivity and can be determined by comparing the voltage drop across adjacent sections of the pipe. Over the first 1 m section, for example, a voltage drop of 5A×40 &mgr;&OHgr;/m×1 m=200 &mgr;V can be measured. The next 1 m section sees 4.99A×40&mgr;&OHgr;Q/m×1 m=199.6 &mgr;V, the difference being 400 nV. Assuming that the exact casing resistance in both the intervals is known, one can determine the leakage current &Dgr;I over an interval &Dgr;z=1 m. The apparent local formation resistivity reading is approximately &rgr;
a
=k&Dgr;z V
0
/&Dgr;I, where k is a geometric parameter on the order of 1, which depends weakly on the average formation resistivity, the casing resistance and the casing radius.
The problem with this method is that the resistance of the casing is variable. In order to resolve 100 &OHgr;m in &rgr;
a
, one needs to be accurate down to at least 40 nV in the difference voltage. With 5A passing through the casing, 40 nV are added or subtracted by a change of 8 n&OHgr; or 0.02%. The pipe is typically corroded and its diameter and resistance vary much more than that. As a result, the measurement of the resistivity must be done in a stop-and-go fashion. At every station the tool has to initially stop to determine the resistivity of casing, then it can determine the resistivity of the formation, and finally it can move to the next station.
Stop-and-go RTC measurement methods have been implemented in a tool developed by Baker-Atlas, a division of Baker Hughes Inc. Various realizations of the Baker-Atlas tool have been adapted in the industry. The underlining measurement principle of stop-and-go RTC tools is typically that shown in
FIG. 1. A
known current I
0
is passed along the casing from an electrode A to an electrode B (the remote electrode at the surface). I
0
is typically in the range of 5 to 10 amperes. The current has to leave the casing and traverse the formation in order to arrive at the surface electrode B. One-half of this current flows past the electrodes C, D, and E. These electrodes and the connected differential amplifiers register the voltage drop due to the casing resistance. If no formation current is present, the voltage drops C-D and D-E are equal, assuming equal pipe resistance in the intervals C-D and D-E. Current leakage, i.e., formation conductivity, is indicated by an imbalance between the voltage drops, which result in a net difference voltage V
out
.
In practice, the pipe resistivities are unbalanced and a nulling cycle, shown in
FIG. 2
, is required to determine the pipe resistivity at the measurement point and to compensate for any offset voltages and gain differences in the amplifiers. During nulling, the current I
0
is passed between electrodes A and F, a mode in which very little formation leakage occurs. The gain of one differential amplifier is adjusted until V
out
becomes zero. This nulling operation is done at every new station. Once V
out
, has been nulled, the tool must not move before the measurement mode, shown in
FIG. 1
, is completed. This in turn necessitates a stop-and-go operation between measurements.
In addition, the Baker-Atlas type tools generally exhibit strong boundary effects in the presence of inhomogeneities that approach the length scale &lgr;
L
. Under these conditions, the injected current no longer splits up evenly and the current portion that flows under the sensing electrodes C, D, E becomes unknown. For example, approaching an oil-water contact, the injected current would preferably flow into the direction of the water. Depending on the orientation of the tool, this increases or decreases the sensed voltage differences, causing a gross misreading of the local resistivity due to distant changes in large-scale conductivity.
Some of the shortcomings of the Baker-Atlas design have been recognized in the U.S. Pat. No. 5,075,626 to Vail (the “Vail patent”). The Vail patent proposes to use two different frequencies: a lower one for the current traversing the formation and a higher one to only sense the casing resistance. The problem is that these two currents penetrate the casing to different skin depths (due to difference in frequencies) and experience different resistance.
Another problem of the Baker-Atlas design—supplying a large current over the wireline—is addressed in the U.S. Pat. No. 5,510,712 to Sezginer (“Sezginer”). In accordance with Sezginer, the RTC tool may optionally be powered efficiently to replace the surface based current supply with current sources in the tool. Sezginer eliminates the use of surface electrodes by deploying two opposing current loops, each extending over about 10 m of the casing. Between the two current loops, several voltage electrodes monitor the voltage drop due to current leakage into formation. This approach requires that, the tool be very long (at least 22 m). In addition, the sensed voltages are smaller by two orders of magnitude than in the tool-to-surface configuration because most of the current simply circulates on the casing and does not contribute to the measurement.
An alternative solution is proposed in U.S. Pat. No. 5,563,514 to Moulin, in which a Wheatstone Bridge is used as a sensin
Aurora Reena
Day Jones
Le N.
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