Oil
wells require a cement sheath between the casing and the formation to
isolate different geologic formations and prevent the migration of oil,
water, or gas from one layer to another, which is called the zonal
isolation. To fulfill its intended purposes, while meeting pumpability
and setting requirements, the cement has to meet a number of very
serious performance requirements, such as elevated temperatures as high
as 500 F°, high pressure up to 20,000 psi, and heat and moisture
exchange within the cement sheath and with the formation. However, even
if the cement is properly placed and initially provides a good
hydraulic seal, the loss of zonal isolation with time can still occur.
For example, the shrinkage of cement during hydration may lead to
leakage in the annulus. To study the behavior of cement sheaths under
such conditions, a constitutive model is being developed that is based
on the theories of thermodynamics, poromechanics, mechanics of
composite materials as well as experimental results. The primary
purpose of this model is to simulate the cement shrinkage behavior in
deep oil wells. It is expected to have the following characteristics,
simple to use, with a minimum number of input parameters;
takes
into account the effects of high pressure and elevated curing
temperature on the corresponding thermo-chemo-mechanical properties;
capable of predicting shrinkage during the full service life of the cement, starting from the time of placement;
easy to incorporate into a coupled finite element analysis.
