Because of the presence of both compressible and incompressible fluids, accurate multiphase flow modeling is necessary for the planning and implementation of most UBD projects.

In underbalanced drilling (UBD), the pressure throughout the well bore is intentionally kept lower than the formation pressure, allowing the well to produce during the drilling operation. This is done for a variety of reasons including reducing formation damage, increasing rates of penetration, eliminating drilling problems such as lost circulation, all with the added benefit of characterizing the reservoir while drilling. To control the bottomhole pressure in an underbalanced condition, a gas-liquid mixture is generally used in place of traditional drilling mud. However, the resulting presence of multiphase flow in every aspect of the UBD operation offers unique challenges.

UBD flow modeling overview

Multiphase flow typically occurs throughout UBD operations and flow modeling is used in all aspects of planning and implementing the UBD program. The applications of flow modeling range from determining the feasibility of UBD to optimizing changes in operating parameters as a result of changing conditions during the actual UBD operation. These include such other goals as:

•Establishing operating parameters such as injection rates and pressures;

•Ensuring adequate hole cleaning and motor performance;

•Evaluating fluids required to achieve UBD;

•Identifying the technical specifications for equipment;

•Defining the operating envelope for UBD;

•Monitoring drilling to determine changes to operating parameters; and

•Developing an understanding of reservoir performance.

Once the fundamental framework for the UBD operation has been defined, one of the most valuable products of flow modeling is the operating envelope. The operating envelope shows the range of conditions for which an underbalanced pressure can be achieved and provides guidance in making appropriate changes to surface conditions and injection rates in order to maintain this condition while satisfying other operational requirements.

Many of these operational requirements become components of the operating envelope plot. The individual components of this plot include pressure limits, flow performance limits, downhole motor limits and hole cleaning limits.

Any modeling tool used for the range of applications such as those previously outlined should meet certain minimum requirements. Most importantly, it must contain multiphase flow technology that predicts three fundamental characteristics of multiphase flow: flow pattern, liquid volume fraction, and frictional pressure loss. Frictional pressure loss is well known; flow pattern refers to how the gas and liquid are spatially distributed in the tubulars and liquid volume fraction is the fraction of the cross-sectional area that is occupied by liquid. Models of such phenomena are complex functions of the flow rates, pressure-temperature conditions, fluid properties and geometry. Furthermore, they must account for how these parameters change as the well is traversed.

To support the multiphase calculations outlined above, detailed fluid and transport property calculations are a necessity. The multiphase model must consider oil and gas volume effects (compressibility, phase change, etc.). This is perhaps the most often overlooked aspect of any flow modeling application.

Other capabilities required of a multiphase model are:

•Support for detailed drilling profiles and well geometries;

•Thermal modeling;

•Hole cleaning efficacy;

•Reservoir inflow performance relationship (IPR) modeling; and

•Commingling of produced and circulated fluids.

Wellflo from Neotechnology Consultants Ltd. satisfies these requirements and is widely used by service companies, engineering firms and operators involved in UBD. It was used to automatically generate operating envelopes and its results will be compared below with measured data from an actual drilling operation.

UBD case study overview

A lateral was drilled underbalanced in a Dolomitic limestone formation to reduce formation damage primarily by preventing drilling fluids from invading the formation. The formation is a sour gas (up to 9% H2S) bearing reservoir with an estimated reservoir pressure of 15,400 kPa. Because of the anticipated pressure variation during connections and tripping, it was planned to drill the well with a bottomhole circulating pressure (BHCP) between 9,800 and 10,500 kPa. A gasified liquid system using synthetic oil and methane was chosen.

Methane circulation rates during drilling ranged from 45 to 55 cu m/min while oil circulation rates ranged from 150 to 200 l/min. Gas production from the reservoir was encountered early in the openhole section and steadily increased throughout drilling of the reservoir.

UBD case study results

The operating envelope was developed for the case study well and is consistent with operations as they occurred at a measured depth of 13,907 ft (4,240 m). The plot indicated that these are viable and fall within a relatively small window in which all of the conditions for effective UBD can be met.

The results of a series of calculations consistent with actual operating conditions at this measured depth were compared with measured BHCPs. The circulation rates for the series of cases were fixed and the inflow from the reservoir varied as the wellhead pressure was varied. The maximum difference between the measured and calculated bottom hole pressures was approximately 90 kPa or about 1% of the measured pressure loss in the annulus.

For this type of well, Wellflo is typically used thoughout UBD planning and implementation (Table 1).

The proposed fluid system had never been used before. This would be the first underbalanced well drilled with natural gas and an oil based fluid into a sour gas reservoir. All gases would be subsequently cleaned and compressed to flow into an existing gas gathering pipeline. Surface configuration options were extensively investigated while considering their effects on the downhole parameters. For example, consideration was given to using a high-pressure separator to eliminate the need for compression, but the flow model showed that the target BHCP could then not be achieved.

(The computer model also proved valuable while planning the casing program. Modeling showed that although 7-in. casing would be ideal from a completions perspective, the required methane injection rates for efficiently cleaning the hole would be unrealistic.) Further analysis showed that the well objectives could be met using 51¼2-in. casing.

Additional sensitivity analyses were conducted during the planning phase. While considering some of the potential well situations, the model identified a possible 22% variation in BHCP when not considering the solubility of the gas within the drilling fluid. If solubility would have been overlooked, inadequate injection rates would have been planned for, most likely resulting in an unsuccessful application of the technology.

Flow modeling proved to be essential for the UBD case study well. Essential decisions related to casing configuration, surface equipment, and pump rates were made in the planning and implementation stage of this "first of its kind" operation. The modeling of multiphase flow has been recognized as a critical tool that influences all elements of the design of an UBD program.