Drilling through a shale formation can result in borehole instability problems, and this is known to add substantial costs to the operation. This is because conventional drilling fluids tend to interact with clay minerals in shales, and the mechanical properties of rock are changed by clay swelling. To avoid those problems, drilling fluid design is one of the most important factors that need to be considered. Oil-based muds (OBMs) are traditionally preferred due to their ability to minimize this interaction. However, OBMs are expensive and are subject to environmental constraints, so water-based muds (WBMs) are often preferred.
To reduce the interaction between water-based muds and shales, we need to reduce water invasion into the shale. To reduce this invasion, internal or external filter cakes should be developed. Shale has extremely low permeability and a very small pore throat size, so normal mud cakes fail to stop or even reduce the fluid invasion. We have shown that the addition of nanoparticle additives to water-based drilling fluids can significantly reduce water invasion into shales. In this paper, we report results for shale permeability and pressure penetration though shales using different fluids: brine, base mud and nanoparticle based muds. To better define the effect of nanoparticles, we used different concentrations of nanoparticles in the mud. From the large reduction in permeability and the pressure response results, we clearly show that nanoparticles act as good shale inhibitors to ensure wellbore stability during drilling.
Drilling through a clay rich shale layer often results in borehole instability problems. It has been estimated that shale formations make up more than 75% of all drilled formations; they account for more than 90% of all expenses associated with wellbore instability problems. Among the various factors that produce instability problems, our study will focus on the pressure loss to shale by water invasion. Shale invaded by water raises a host of problems, including hole collapse and wellbore weakening. This is because the clay minerals inside the shale interact with water in the drilling fluid. To prevent water from contacting the shale, oil-based muds (OBMs) are traditionally preferred. OBMs have good wellbore stability and lubricity properties since they do not interact with shales. OBMs are also preferred for their capillary sealing of the shale surface. OBMs, however, are expensive and are subject to environmental constraints. Because of this, water-based muds (WBMs) are often preferred.