The fluid in the annulus is in a variable mass flow state
under
the lost circulation condition, significantly affecting wellbore pressure
distribution and the heat transfer efficiency between wellbore and
formation. Therefore, based on the hydrodynamics and energy conservation
law, a coupling model of transient wellbore temperature and pressure
field under lost circulation conditions is established, which fully
considers the casing program, bottom hole assembly, and heat source
generated in drilling, as well as the influence of the temperature
and pressure coupling in wellbore on the physical parameters of drilling
fluid. The calculation results of the model in this paper are in good
agreement with the field measured data and the previous research results,
which verifies the rationality and accuracy of the model in this paper.
The effects of the loss rate and the lost circulation zone location
on the wellbore temperature and pressure distribution are analyzed,
and the variation rule of the physical parameters of drilling fluid
under the lost circulation condition is studied. The numerical simulation
results show that the density and viscosity of drilling fluid in the
annulus and bottom hole pressure increase with the increase in the
circulation time; the annulus temperature decreases gradually with
the increase in cycle time and tends to become stable after 8 h of
the cycle. The annulus temperature and bottom hole pressure decrease
with the increase in loss rate and closeness of the loss zone to the
well bottom. When the loss zone is in the upper open-hole section,
an inflection point consistent with the location of the loss zone
appears on the annular temperature difference curve between the loss
circulation and the nonloss circulation, and the position of the loss
zone can be judged according to the inflection point.