During the drilling process of exploratory wells, the formation data is often limited; most of the time drilling engineers must estimate the pore pressure and fracture gradients. This information is critical for designing the drilling fluid, spacers, and cement slurry as to provide the necessary density and rheological properties.
An error in estimating the pore pressure or fracture gradients can induce wellbore instability and lost circulation. If these losses are not controlled, during the cementing job the programmed top of cement (TOC) will not be achieved, zonal isolation will be compromised, and the casing string will not be fully supported. These problems can lead to additional expenses such as remedial jobs, non-productive rig time, use of additional materials, costly logistics, etc.
This paper presents a case history with a critical cementing job in which the operator was drilling an offshore exploratory well in Southern Mexico when an area with constant gas flow with a narrow pore-pressure to fracture-gradient window was encountered. The operator had to increase the density of the drilling fluid, which in turn induced total losses.
The cementing company recommended pumping a unique spacer technology that enabled circulation to be regained while pumping the cementing job. This technology is an ultra-low invasion cementing spacer that creates an impermeable barrier on the face of the formation through differential pressure. This barrier helps to isolate the formation from the total equivalent circulating density (ECD), thus allowing circulation even in situations where the ECD approaches or slightly exceeds the fracture gradient.