Reservoir Characterization Begins at First Contact with the Drill Bit
Authors: Shayegi Sara , Kabir C. Shah , If Flemming , Christiansen Soren , Kosco Ken , Casasus-Bribian Jaime , Hasan Abm Khalid , Moos Hasse 
Published: May 2016

Abstract
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 (BHP) 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 owing to 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 contributes and 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 was also available during trips because the concentric annuli allowed the pressure to be monitored below the downhole isolation valve. This 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.

Introduction
In UBD, the wellbore pressure is lower than the reservoir pressure. This wellbore condition allows formation fluids to flow into the wellbore during drilling. Proper instrumentation, data acquisition, and drilling procedures allow acquisition of data that are interpreted and analyzed to extract information about the reservoir.
Reservoir characterization with UBD data is one of the benefits of drilling underbalanced. Unlike conventional overbalanced drilling or even managed-pressure drilling (MPD) with reduced overbalance margin, the UBD environment provides a unique opportunity to gather data that have the potential to provide important information about the reservoirs encountered during drilling. This real-time data provides the flexibility to adjust a drilling program still in progress.
Subsequent well operations have the potential to damage the reservoirs that have been opened to the wellbore during drilling. Evaluating these reservoirs in an undamaged state (or as close to undamaged as possible) requires the capability to evaluate the data acquired during drilling. In essence a UBD operation allows transient-pressure testing and production logging while drilling. Based on rates and annular pressures gathered during drilling and mud circulation periods, the derived reservoir information allows determination of the in-situ undamaged production potential of the formation and fractures encountered.
The early analysis done with rate and pressure data from underbalanced wells was simple PI and PI per unit length calculations in real time using calculators integrated into the data acquisition system (da Silva et al. 2007). More sophisticated methods using the PI concept emerged and one that has proven useful is RIPI by Suryanarayana et al. (2007), which is one of the methods used in this study.