When drilling wells, drilling mud p

When drilling wells, drilling mud prevents collapse of the wellbore and entrance of formation fluids into the wellbore. High-density additives increase the density of the drilling mud, thus improving its function. On the other hand, high-density mud can fracture the formation. According to the Kirsch solution for vertical wells [1], fracturing occurs, in an impermeable formation with no tensile strength and equal horizontal principal stresses, when the mud is more than twice the least horizontal stress (from which pore pressure has been subtracted). Among the earliest industrial approaches to this problem were conventional loss circulation remedies such as pumping pills. More recently, however, the use of specially designed particles in the mud has strengthened the wellbore, helping the industry to cut the drilling non-productive time (NPT) [2]. Although some successful field applications for wellbore strengthening have been reported [2-8], the extents to which the fracturing pressure in a wellbore can be enhanced and with which materials we can perform remain uncertain. For instance, different materials system, from gels, including cross-linked polymers [4], calcium carbonates [3,8], deformable, viscous, and cohesive sealant (DVCS) [9,10], drill and stress fluid (DSF) water-based systems [11], to materials with higher mechanical strength [12]; have been used in wellbore strengthening applications (Table 1). Although some authors [12] have reported poor experimental results using calcium carbonate and polymer-based mud systems, others [4,8] have applied these materials in the field and observed a significant increase of fracture gradient as a result. The field and experimental data have yielded contradictory information regarding which parameters affect this phenomenon and which physical model describes it. Important questions related to wellbore fracturing remain open: First; are fractures merely being healed? Or are the rock stresses near the wellbore actually being changed? Second, how important are mud properties and mud additive properties such as material size type and strength? Some studies have indicated that these techniques are successful only when specific material sizes are used; others report successful field applications, regardless of material properties. The present study cannot fully address all these questions. Therefore, it provides the foundation for a numerical model of fracture formation and propagation.