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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.

Summary This paper describes the technical basis for the design and implementation of a coiled-tubing (CT) underbalanced drilling (UBD) campaign in the Sajaa field in Sharjah, UAE. The campaign calls for up to three multilaterals per well to be drilled underbalanced from the existing wells' through tubing. The paper describes the analysis, modeling, and result, and their implication in the selection of the operational approach. The key issues associated with the campaign, many of which were unique and challenging within the context of UBD, are discussed, with specific reference to the Sajaa-3 well, which was the first candidate. These includeTemperature modeling and thermal considerations because of the high bottomhole temperature (BHT).Compositional pressure/volume/temperature (PVT) behavior of the produced fluid.Feasibility, underbalanced operability, and controllability of through-tubing UBD operations.Risk to underbalance from near-wellbore depletion of the reservoir caused by long-term production from the motherbore.Estimation and impact of the productivity of the motherbore and open laterals while drilling subsequent laterals.Risk from reservoir parameter uncertainties.Hole-cleaning risks.Designing for zero flare.Consideration of mechanical and hydraulic limitations to the drillable lengths.Equipment limitations. The paper details the analysis approach used, assumptions made, models used, and the results. It should be noted that the Sajaa produced fluid is a retrograde condensate and the inflow performance was nonlinear and, therefore, consideration of these in the basis of design (BOD) becomes critical. The goal of this work was not only to create the technical BOD for the well, Sajaa-3, but also to set the parametric limitations for a wide range of well types that were likely to be encountered in the continuation of the campaign. On the basis of this work, an approach to drilling and monitoring the well to minimize the risks was developed, and it formed the basis of the drilling operation. The ongoing campaign, now well past 30 wells, has been one of the most successful applications of CT UBD. The results of the drilling operation of Sajaa-3 and the subsequent wells and their implications to the design basis are also discussed.

fax 01-972-952-9435. AbstractWe examine in this paper the feasibility of using a concentric jet pump to create an underbalanced condition while drilling subnormally pressured reservoirs, using only single phase injected fluids, eliminating the need for gas. A complete technical basis for the performance and design of jet pumps for underbalanced drilling applications is presented. The problem is handled in four parts: reservoir to jet pump suction, the jet pump itself, annular flow from jet pump exit to surface, and power fluid flow. Multiphase flow theory, viscous flow theory and jet pump flow theory are used to analyze the problem. The design goal is to find the correct combination of jet pump location, geometry, power fluid density and rate such that the jet pump exit energy is adequate to lift the mixed fluids and solids to surface. Other design considerations such as tolerance of varying reservoir conditions and depth, cavitation, sonic velocity limitations, and power fluid injection pressure constraints, are also taken into account in the design procedure. Jet pump design envelopes unique to its application in underbalanced drilling are developed to aid selection and design of the pump. A design program that implements the procedure has been developed. The procedure is illustrated with two examples: an oil well and a gas well. We find that the jet pump can successfully create an underbalanced state for a wide range of conditions. In general, for reservoir pressures in the range of 4 to 7 ppg equivalent, the jet pump appears to enable creation of an underbalanced condition without requiring injected gas, regardless of whether the reservoir is oil bearing or gas bearing. Operational and practical considerations that need to be addressed in proving the design are also discussed. Based on this work, we conclude that the concentric jet pump is a unique device technically suitable for a wide range of uncerbalanced drilling applications.

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.