Shale is troublesome for drilling. This is true partially due to its swelling characteristics when interacting with water. However, shale swelling is a complicated matter and how shale swells is still not crystal clear to many engineers who deal with wellbore instability issues daily. A simplified shale model has been built with equilibrium between osmotic pressure and normal stress acting on the clay platelets by ignoring insignificant factors under practical drilling conditions. This new model reveals that there are two distinct water bodies in swelling shale. Changing stress on the platelets will change the water distribution and their water activities in these two water bodies. This model can explain observed stress sensitive activity phenomena reported on swelling shale. The model provides a quantitative way to predict water activities for shale and a new and important equation for improving wellbore stability analysis. With the model many other phenomena such as time-dependent wellbore swelling or tight-hole can be better understood.
Some say that 90% of wellbore instability comes from shale, which comprises about 70% of the formations drilled. Many studies have been attempted to address wellbore instability in shale drilling. These studies include shale composition in terms of mineralogy, grain grading, anisotropy of density, petrophysical properties such as permeability, porosity, mechanical properties such as shear modulus and Poisson's ratio, chemical properties such as linear swelling coefficients and pore water activities, etc. Shale is different from other rock types at least partially due to the fact that it tends to be water sensitive and may start to swell when contacting water or water-based drilling fluids. To maintain wellbore stability, one can see that shale water sensitivity is of great importance for engineers to understand. The major difference between shale and sandstone is that swelling shale contains swelling clays and clay swelling is more or less related to water activities. Lab tests on shale have indicated that water activities of some shales are sensitive to confining stress. However, similar tests on sandstone show no such sensitivity. This intriguing phenomenon has not been well explained and further understanding on shale internal structures is needed. Many studies have been focused on shale interactions with its surrounding fluids such as mud by treating shale as a uniform subject with a water activity unchanged at various stresses, possibly generating misleading results and conclusions. Furthermore, it is obvious that knowing shale water activities is the prerequisite for designing a water activity balanced mud for swelling control. However, lab tests normally cannot provide a good number easily. It would be of great value if a mathematical equation could be used to predict the value with acceptable accuracy under downhole conditions. Such an equation may also be of significance for correlating water activities and interpreting shale behaviors for lab tests. It is normal for rock to have its anisotropy. Would shale show swelling anisotropy or swelling with different magnitudes in different directions? Can or should wellbore instability analysis take this into account to better reflect the reality?
