Reservoir Characterization Begins at First Contact With the Drill Bit

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 (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 hole drilled contributes and to ascertain the relative quality of the 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.

Real-time estimation of reservoir influx rate and pore pressure using a simplified transient two-phase flow model

“…This requires a fit-for-purpose hydraulics model and a reservoir model, which correlates flow from the reservoir to the pressure drawdown, and also to a productivity index, a lumped parameter which is affected by the length of exposed zone, reservoir permeability, porosity, skin factor, reservoir fluid viscosity and compressibility (Vefring et al, 2003). For this application, we are more interested in the qualitative relationship between inflow rate, productivity and pore pressure, thus we use a qualitatively correct, linear inflow relationship (Shayegi et al, 2012):…”

“…In addition to minimizing reservoir impairment and maximizing production, UBD enables a better understanding of reservoir properties through comparison between real-time production rates and well logging data (Culen and Killip, 2005). Additional information can be inferred from pressure buildup data and gas and liquid flow metering on surface, leading to more accurate estimates of reservoir pressure and productivity index for different reservoir sections (Suryanarayana et al, 2007;Shayegi et al, 2012).…”

Modelling of Formation Pore Pressure Inversion during Tight Reservoir Drilling

“…The model relies on the characteristics of a pressure-buildup curve during a kick and was verified as being strongly antinoise. Employing the method of rate-integral productivity index (RIPI) analysis, Suryanarayana et al [15] and Shayegi et al [16] evaluated the reservoir pressure and productivity during UBD. Besides, a genetic algorithm in conjunction with a transient two-phase reservoir simulator was developed by Biswas et al [17] to analyse the variation of formation permeability along the wellbore.…”