Abstract
Deepwater operations (with high rig spread rates) like those offshore Brazil require an operator to very carefully consider the benefit-versus-cost of a full coring acquisition program. Yet, core-based information is invaluable in the exploration phase of a field's potential, and the basis of thoroughly understanding reservoir rock potential (petrophysics) is founded on these core measurements. With the drive for cost efficiency, conventional full-core acquisition is often curtailed and subsurface teams must rely on percussion or rotary sidewall core samples. This choice is made even though it is known that for many core analysis measurements, the accuracy of the measurement is directly proportional to the pore volume.
A new rotary sidewall coring technology applicable for deepwater environments has been applied offshore Brazil that enables the recovery of more than three times the core volume when compared to cores recovered with standard rotary coring tools. The resulting increase in pore volume positively impacts the accuracy of the core porosity measurement and other measurements that depend on pore volume (e.g., capillary pressure, water saturation, and SCAL measurements, in general). Recovery of up to 60 samples during a single trip in the well is achievable, enabling a good lithology sample spread in one acquisition. Acquisition time per core is more than three times faster that of the previous-generation rotary coring tools, resulting in significant reduction of coring time, especially for offshore operations. Coupled with a new bit design, optimized to take advantage of the increased rotational speed, cores can be cut at even higher levels of overbalance pressure (> 1,000 psi [6.89 MPa], depending on porosity and permeability). The ability to take up to 60 larger-volume cores in one run provides operators, like those in deepwater operations of Brazil, the flexibility to continue to vary the design of core acquisition programs to meet the benefit-versus-cost dilemma of full-core acquisition. They also enable much better contingency data gathering if full-core programs are not successful.
