This paper focuses on practical use of a multiphase flow model for design of underbalanced drilling (UBD) applications. Since most UBD operations involve multiphase flows, UBD operations are synonymous with multiphase flow modeling. Once a candidate well has passed requisite reservoir valuation and screening criteria, the next step is to establish the technical feasibility of the technology and determine the key operational parameters including equipment specifications. Multiphase flow modeling is an essential part of this process, and as will be detailed in the paper, influences all aspects of design and construction of an UBD well.
By definition, the goal in underbalanced drilling is to maintain a pressure below pore pressure along the entire open hole section that is targeted for UBD. While a drawdown is indeed desirable, there is clearly an upper boundary dictated by wellbore stability and/or coning of undesirable phases. The UBD operational envelope is further bound by adequate hole cleaning performance through proper design of the circulation parameters. The conditions that satisfy these underbalanced parameters should also consider limitations of the bottom hole assembly (BHA) components, in particular motors/MWD, and realistic in regards to the injection criteria (rates and pressures). Finally, multiphase flow modeling over a range of conditions, including sensitivities and safety factors, allows the sizing of the surface equipment (with maximum reservoir inflow considerations), and thus implicitly influences the design of the entire UBD operation.
The primary purpose of this paper is to describe the strategy of effective multiphase modeling for UBD including modeling sequence and necessary sensitivities, to ultimately deliver a "robust" design. Reservoir uncertainty is the primary culprite to flow modeling flaws. Moreover, due to the relatively simplistic user interfaces on complex multiphase flow engine software, the results of the model may be misinterpreted. It is through an accepted modeling strategy that this risk is manageable. Although the design example employs a single commercially available simulator, Wellfo 7, the interpretation principles could be adopted with any "acceptable" industry model.
Multiphase flow simulation is an integral element in the preliminary engineering, circulating system design, well controllability analysis and equipment selection process for any underbalanced drilling operation. The primary goals of multiphase flow modeling are to:
Investigate the possibility of achieving a stable underbalanced or near-balanced condition down hole.
Ensure adequate annular velocities and/or transport properties for hole cleaning can be achieved in an underbalanced circulating system.
Confirm that the operating performance of the down hole tools (motor and MWD) is not negatively affected by the underbalanced circulating conditions.
Establish the down hole and surface operating envelope for the planned underbalanced operation.
Establish the expected operational parameters such as injection rates, injection pressure, bottom hole and surface annular pressures, maximum surface return rates, etc.
Identify the technical specifications required and design parameters for all UBD equipment.
The main focus of multiphase flow modeling in the context of UBD is to establish a pressure profile along the entire circulating path. This requires combination of conservation principles with closure relationships such as PVT relationships. In general, a complete UBD flow modeling software program should include, at a minimum, the following features:
A flow regime (or flow-map) model, which predicts the two-phase flow regime at any given location in the flow path,
A liquid hold-up model, to calculate the liquid fraction at any given location, taking into account the slip between phases,