Chemical and mechanical aspects of wellbore stability in shale formations: A literature review

Drilling a shale formation with no borehole problems can be a challenging task. With high content of clays, shale formations are prone to swelling, dispersion, tight hole and other problems. Those problems are well-documented in literature. Different types of clays give different types of problems. For example, when it is exposed to water, a shale sample with a high percentage of smectite tends to swell while another shale sample with a high percentage of kaolinite tends to disintegrate and disperse. The mechanism that destabilizes reactive shale when it comes in contact with water differs from on shale sample to another. It is important to know which mechanism is taking place in order to be proactive and inhibit the drilling fluid with the appropriate inhibitor. It is also important to characterize the shale sample in terms of its geological structure, mineralogical composition and interaction potential. Shale characterization and testing methods include but not limited to the following: X-ray analysis, degradation and reorientation evaluation using scanning electron microscope (SEM), cation exchange capacity (CEC) determination, swelling test, dispersion test, slake durability test, bulk hardness test, accretion test, shale membrane test, inhibition durability test and uniaxial and triaxial compression tests. Different types of shale inhibitors and stabilizers have been used in the industry such as: potassium chloride, calcium chloride, silicates, polyamines, glycol and lignosulfonate. Each of them has a mechanism by which it can inhibit the hydration, disintegration and dispersion of clays in the presence of water. This paper discusses the physio-chemical and chemico-mechanical effects of shale-drilling fluid interactions and the associated drilling problems that are most frequently encountered while drilling shales formations. It also reviews the most common shale testing methods used to evaluate the interaction potential of various shale-drilling fluid systems. Moreover, the paper describes different mechanisms by which shale can be inhibited and wellbore stability problems can be mitigated.