Mengjiao Yu's dissertation
by
Mengjiao Yu, Ph.D.
University of Texas at Austin, 2002
Supervisors: Mukul M. Sharma and Martin E. Chenevert
A new three-dimensional wellbore stability model is presented that takes into account
thermal stresses and the flux of both water and solutes from drilling fluids (muds) into
and out of shale formations. Mechanical stresses around a wellbore placed at any arbitrary
orientation in a 3-dimensional stress field are coupled with changes in temperature and
pore pressure due to water and solute fluxes. The radial and azimuthal variation in the
stress distribution and the “failure index” are computed to check for wellbore failure.
This model accounts for the hindered diffusion of solutes as well as the osmotically
driven flow of water into the shale. The model for the first time allows a user to study
the role of solute properties on wellbore stability.
Results from the model show that a maximum or minimum in pore pressure can be obtained
within a shale. This leads to wellbore failure not always at the wellbore wall as is most
commonly assumed but to failure at some distance inside the shale. Since the fluxes of
water and solute, and temperature, are time dependent, a clearly time dependent wellbore
failure is observed. The time to wellbore failure is shown to be related to the rate of
solute and water invasion. Comparisons with experiments conducted with a variety of
solutes on different shales show excellent agreement with model results.
It is shown in this study that the solutes present in the mud play an important role in
determining not only the water activity but also in controlling the alteration of pore
pressures in shales. To account for this phenomenon a model is presented to compute the
flux of both water and solutes into or out of shales. The relative magnitudes of these
fluxes control the changes in pore pressure in the shale when it is exposed to the mud.
The effect of the molecular size of the solute, the permeability of the shale and its
membrane efficiency are some of the key parameters that are shown to determine the
magnitude of the osmotic contribution to pore pressure. A range of behavior is observed
if the solute is changed while the water activity is maintained constant. This clearly
indicates the importance of the solute flux in controlling the pore pressure in shales.
Critical mud weights are obtained by inspecting the stability of the wellbore wall and the
entire near wellbore region. Pore pressures at different time and position are investigated
and presented to explain the model results. It is shown in this study that the critical
mud weights are strongly time dependent. The effects of permeability, membrane efficiency
of shale, solute diffusion coefficient, mud activity and temperature changes are presented
in this work. The collapse and fracture effects of cooling and heating the formations are
also presented.
A powerful simulation tool has been developed which can be used to perform thorough
investigations of the wellbore stability problem. A user-friendly interface has been
developed to ease usage.
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