Electrical cased well-logging method
The invention relates to the geophysical examination of wells and can be used for determining the electrical resistivity of rock beds surrounding a well cased in a metal column. The inventive method consists in using a sonde embodied in the form of two current electrodes located on the opposite sides with respect of at least the measuring electrodes, in alternately supplying electric current, in measuring, by means of a measuring devices, during each current supply, a supplied current, a potential on a central measuring electrode and the potential difference between the outer measuring electrodes and the central electrode, in using the specific electrical resistivity of the rock beds surrounding the cased well in the form of the electrical parameter of the electrical logging of the cased wells, in forming a measuring circuit according to the resistivity pattern of the layers of the rock beds surrounding the well and of those sections of a casing string which are provided with the collars, wherein the sonde, whose measuring electrodes are spaced without respecting equidistance requirements and current electrodes are embodied in such a way that they can be used in the form of measuring electrodes is applied. In addition, the inventive method also consists in measuring the potential difference of an electric field between the current electrode, to which the current is not supplied at the present switching, and the central measuring electrode, by means of additional measuring devices, and in determining a specific electrical resistivity according to a formula referred thereto.
1. An electrical cased-well logging method, comprising using a sonde in the form of two current electrodes located on the opposite sides of at least three measuring electrodes; alternately applying electric current; measuring, with the use of a measuring device, during each current application, the current applied, the potential across the central measuring electrode, the potential difference between the two outer electrodes and the central measuring electrode; using the specific electrical resistivity of formation beds surrounding the well as an electrical logging parameter of cased wells, wherein use is made of a sonde having measuring electrodes spaced at distances that are required to be equal; additional measuring devices are used to measure the differences in electrical field potentials between a current electrode that is not supplied with current on connection and the central measuring electrodes, and the specific electrical resistivity is found from the formula:
=
·
[
2
Ia
1
·
(
Ub
3
+
Δ
Ub
13
)
+
Ib
5
·
(
Ua
3
+
Δ
Ua
53
)
]
×
×
[
(
Ua
3
·
Δ
Ub
23
-
Ub
3
·
Δ
Ua
23
)
·
(
Ua
3
·
Δ
Ub
43
-
Ub
3
·
Δ
Ua
43
)
(
Δ
Ua
43
·
Δ
Ub
23
-
Δ
Ua
23
·
Δ
Ub
43
)
]
wherein ρ bed is specific electrical resistivity, Ohm·m;
K sonde is the geometrical coefficient of the sonde, m;
I a1 and I b5 are currents applied on the first connection to the upper current electrode and on the second connection to the lower current electrode, respectively, A;
Ua 3 and Ub 3 are electrical field potentials across the central measuring electrode relative to ground on the first and second connections, respectively, V;
ΔUa 23 , ΔUa 43 , ΔUb 23 , and ΔUb 43 are electrical field potential differences on the first and second connections between the upper measuring electrode and the central measuring electrode, and between the lower measuring electrode and the central measuring electrode, respectively, V; and
ΔUa 53 and ΔUb 13 are electrical field potential differences between the lower current electrode to which current is not applied on this first connection and the central measuring electrode, and between the upper current electrode to which current is not applied on this second connection and the central measuring electrode, respectively, V.