THE 3
rd
INTERNATIONAL SCIENTIFIC CONFERENCES OF STUDENTS AND YOUNG RESEARCHERS
dedicated to the 99
th
anniversary of the National Leader of Azerbaijan Heydar Aliyev
80
coupling model, in which the chemical potential is the linear graph of solute
mass proportion. After, the linear chemistry-pore-thermoelasticity coupling
model is created by Ghassemi, et al., which assumes the effect of chemical
potential
and temperature, but does not assume the influence of diffusion
potential of drilling mud on hydration osmosis. According to the pore elasticity
theory, a nonlinear fluid-solid-chemical coupling model was established, In
this model, three aspects were improved from the previous models. The first
aspect is the reaction of expansible shale, which is full of saturated fluid, is
altered by hydration osmosis. The model presumes the influx of fluid flow and
ion diffusion on solid deformation. Second, the model considers the influence
of diffusion potential on hydration osmosis.
Negative charges on clay
surfaces can suspend negative ions to diffuse through the medium in most
shales. But the positive ions would be attracted by the negative charge wall,
which makes the shale have the ion-selective membrane ability. Thus, the
formed diffusion potential becomes one of the
driving forces of hydration
osmosis. The third model assumes the nonlinearity of ion diffusion. Because
the chemical potential is not a linear function of the mass fraction of solute,
the linear model of ion diffusion alleviates the hydration osmosis process. A
new nonlinear fluid-solid-chemical coupling model was applied in
determining the borehole instability problems of special wells to discover and
compare the validity of the new model.
Yew and Lew (1992) proposed a theory of pore elasticity in the wellbore
instability model to study the effects of porous fluid influx on wellbore stability.
The influx of pore fluid causes additional normal stresses, which in some
cases disrupt the stability of the wellbore.
After studying the local stresses and rock strength, the stability of the
wellbore is estimated according to the selected deformation criteria. Tension
and strength change when clay gets in contact with a fluid. Therefore,
chemical effects must be added to the mechanical effects.
Chenevert (1970) found that reactive clays tend to adsorb water from
the drilling mud, and if their chemical potential (water activity) is less than the
drilling mud, then the clay will swell [3].
With this concept, Chenevert
developed the theory of "balance of activity" in 1970. According to this theory,
drilling with oil-based drilling mud is more efficient [4].
Due to the water
activity of clays drilled with oil-based drilling mud is the same or slightly lower
than the solution, the wellbore is stable. However, this theory does not show
the effectiveness of WBM. The reason for this is the process of ion flow in
and out of the clay due to the difference in concentration. Bord (1984) studied
that the adsorption of water on reactive clays such as smectite causes a
reversible increase in the volume of the mineralogical structure. In this case,
a gap of 10-18 *10
-10
m is formed between adjacent clay particles.