Physico-chemical analysis of shale-drilling fluid interaction and its application in borehole stability studies
Shale is often the most difficult of all formations to maintain a stable wellbore in when drillincr ::>
for oil and gas. Time and money spent overcoming this problem during drilling, together with
overall reduced profit margins. has led the oil industry to devote considerable time and effort to
solve the problem of unstable boreholes in shales.
It has long been established that the moisture adsorption (or desorption) of shale rocks can be
controlled by the salinity of drilling fluid. When compacted shale (under constant compaction
stress) adsorbs moisture, its total volume increases and swelling strains develop. Developed
swelling strains then become an integral part of the effective radial stress acting on the shale
formation contributing to borehole failure.
A mathematical model has been developed for predicting the swelling behaviour of shale when
placed in contact with water under moderate pressures and the effect of the swelling on borehole
(in)stability. The model is based on thermodynamic theory which suggests that fluid movement
into or out of a shale is driven by an imbalance in the partial molar free energy of the shale and
the contacting fluid. Conversion of the free energy of each system (fluid and shale) into "total
swelling pressure" made it possible to model transient pressures and strains generated in shale.
The analytical solution of the radial diffusivity equation is reduced to a simpler form for the
model. The model was validated using equipment and experimental techniques which allow
continuous monitoring of shale swelling as function of time and distance from the wetting end.
It was found that increasing the compaction stress acting on the shale reduced the rate of
swelling, and increasing the hydraulic pressure of the fluid on the shale's wetted surface
increased the rate of swelling. This behaviour was adequately described by the model which
therefore represents a new method for predicting shale swelling as function of time and radial
distance under different environments. Swelling strains are then used to predict related changes
in shale mechanical properties (failure criteria) and well (in)stability.
Several well-site index tests have been developed to study shale-drilling fluid interaction at wellsite.
These index tests can provide input data for the mathematical model. Drilling fluids can be
screened for their ability to control shale swelling, thus minimising the risk of well bore