Accurately identifying
the location of loss zone after lost circulation
is the key to subsequent plugging operation. In view of the difficulty
of identifying the location of lost circulation zone, a method of
identifying the location of loss zone by transient pressure wave signal
is proposed. When lost circulation occurs, transient back pressure
is applied to the wellhead at the surface choke manifold to produce
transient pressure wave. The transient pressure wave propagates downward
from the wellhead. The propagation process of transient pressure wave
in an annulus system is analyzed, and the position of loss zone is
determined according to the change of pressure signal at the choke
manifold. Based on the simulation of this method, relevant experiments
are also carried out. Aiming at the problem of excessive noise of
the pressure wave signal collected in the experiment, variational
modal decomposition (VMD) is used to decompose the signal into multiple
band-limited intrinsic mode function (BIMF) components. Combined with
a Hilbert spectrum, the time–frequency characteristics and
energy distribution of each BIMF component are analyzed in turn. The
main frequency component is selected to reconstruct the signal to
achieve the denoising effect. On this basis, a wavelet modulus maxima
method is used to decompose the denoised signal, extract the characteristic
points of the signal, identify the loss circulation information in
the signal, and then identify the thief zone position by a time–domain
method. Through experimental verification, the existence of loss zone
will affect the change trend of pressure wave; a VMD–wavelet
modulus maxima algorithm can effectively remove the noise of the pressure
wave signal and locate the pressure change point. The experimental
recognition error range of this method is 0.10–9.22%, which
has certain guiding significance for field application.