Reservoir Characterization Begins at First Contact With the Drill Bit

Shayegi, Sara (Shell) | Kabir, C. Shah (Hess Corporation) | If, Flemming (Hess Corporation) | Christensen, Soren (Hess Corporation) | Ken, Kosco (Hess Corporation) | Casasus-Bribian, Jaime (Hess Corporation) | Hasan, ABM K. (Hess Corporation) | Moos, Daniel (Dong E&P)



Underbalanced drilling (UBD) offers a unique opportunity to estimate undamaged, in-situ formation properties upon first contact with the formation while drilling. This paper compares well-testing techniques developed for UBD with conventional methods. The reservoir flow rates in combination with flowing bottomhole pressures (BHPs) acquired while drilling can be used to identify productive intervals and estimate dynamic reservoir properties.

Unlike typical UBD projects where reservoir benefits are the primary focus, the driver for this mature field was overcoming the drilling problems associated with the wide reservoir-pressure variability caused by nearby producers and injectors. UBD was piloted as a means to achieving the requisite lateral lengths for reserves capture and meeting production targets. Minimizing formation damage and characterizing the reservoir while drilling were added benefits.

Several reservoir-characterization methods based on rate-transient analysis (RTA) were used to perform well testing while drilling. Rate-integral-productivity-index (RIPI) analysis uses the rate and pressure data acquired during drilling to determine whether additional holes drilled contribute and to ascertain the relative quality of this rock. In the increasing-boundary method, real-time rate and pressure data during drilling, circulating, and tripping allowed assessment of formation properties through history matching. Pressure-buildup data were also available during trips because the concentric annuli allowed the pressure to be monitored below the downhole isolation valve. These data enabled the estimation of reservoir pressure and productivity index (PI) with a proxy vertical-well model for each productive interval drilled. These interpretation methods show close agreement in results and lend credence to the UBD-derived parameters.