The economic success of a well construction is contingent upon establishing zonal isolation within the cemented wellbore. The concentration of stress within a wellbore produced by temperature/pressure fluctuations, hydraulic fracturing/perforating operations, dynamic forces imparted during drilling, formation compaction, etc. can result in the mechanical disruption of the cement sheath. These effects are amplified in a high-temperature high-pressure (HTHP) well environment. As such, managing, minimizing, and mitigating zonal isolation loss under HTHP conditions is the focus of considerable research efforts within the ranks of both operators and service companies.
This paper describes cement systems exhibiting very high tensile and flexural strength values in a HTHP curing environment. These stress-resistant systems are produced by simple addition of coarse (100-mesh) silica sand into the slurry design. The coarse silica serves to direct the fracture propagation along a tortuous path within the cement matrix, producing a non-linear fracture path compared to the essentially linear fracture pattern observed in conventional HTHP cement containing fine silica.
This paper introduces the general concept of mechanical property enhancement of a well cement by taking advantage of the interfacial transition zone (ITZ) produced within the set cement matrix. The ITZ serves to direct the fracture path in such a manner as to spend the applied fracture energy over a greater distance within the cement matrix. Such fractures are also wider than those produced in fine silica – cement matrices.