Method for a mud cake thickness determination
The method of a mud cake thickness determination provides sending short high-frequency signals into a formation from at least two positions located at different distances from the mud cake and recording arrival times of reflected echo signals. The mud cake thickness is determined based on the time measured.
1. A method for a mud cake thickness determination comprising:
sending short-time high-frequency signals into a formation by at least one transducer from at least two positions located at different distances from the mud cake,
measuring amplitudes of the signals sent from the at least two positions and reflected from a drilling mud/mud cake interface by the at least one transducer,
recording of arrival times of the reflected echo-signals by the at least one transducer,
determining a mud cake impedance Z mc as
Z
mc
=
Z
m
1
+
γ
1
-
γ
,
wherein Z m —a drilling mud impedance, γ is determined as
γ
=
K
mmc
(
d
m
)
exp
(
d
m
d
m
2
-
d
m
1
ln
A
m
1
ln
A
m
2
)
,
d m —a distance between the transducer and the drilling mud/mud cake interface,
K mmc —reduction of the amplitude of signal when a distance between the at least one transducer and the drilling mud/mud cake interface is d m ,
d m1 , d m2 —distances between the positions from which the signals are sent and the drilling mud/mud cake interface,
A m1 —the amplitude of the signal sent from the position located at the distance d m1 from the drilling mud/mud cake interface and reflected from the interface,
A m2 —the amplitude of the signal sent from the position located at the distance d m2 d m1 from the drilling mud/mud cake interface and reflected from the interface,
determining a sound speed in the mud cake as
V mc =Z mc /ρ mc .
where ρ mc —is a density of the mud cake, and
determining the mud cake thickness h mc as
h mc =V mc t/ 2,
where t is a time of the signal propagation in the mud cake.
2. The method of claim 1 wherein short-term high-frequency signals are sent by the at least one transducer moving to different positions against the mud cake.
3. The method of claim 1 wherein a contact transducer is used as the transducer.
4. The method of claim 1 wherein a shear-wave transducer is used as the transducer.
5. The method of claim 1 wherein a refracted ray transducer is used as the transducer.
6. The method of claim 1 wherein the high-frequency signals are sent by at least two transducers installed at different distances from the mud cake.
7. The method of claim 1 wherein a mud cake attenuation is additionally determined.
8. The method of claim 1 wherein echo-signals' spectral analysis is additionally performed.