The Basics of a Flow Model for UBD

Step 1 Model the annular pressures and velocities assuming no flow from the reservoir.

Step 2 UBD Operations Window

Upper Pressure Boundary Should be at least 700 kPa less than the reservoir pressure.

Compensates for model errors & overbalanced pressure spikes.

Lower Pressure Boundary Maximum allowable pressure drawdown to prevent:

Wellbore collapse High return surface volumes Fines migration Water/Gas coning

Note: In mature fields, max drawdown allowable is assumed from typical production drawdown.

Upper Fluid Injection Boundary Motor maximum throughput Sometimes restricted by injection or mud equipment.

Lower Fluid Injection Boundary Minimum liquid velocity for adequate hole cleaning. Rare: minimum operating volume of motor or MWD.

Step 3 Modeling Injection Pressure & MWD Operability

Maximum steady state injection occurs at a Bottom Hole Pressure equal to the Reservoir Pressure. For drill pipe gas injection, the difference in pressure varies with torque

Therefore, P off-bottom < P on-bottom Off-bottom case is used for determining motor optimum throughput volumes. On-Bottom case is used for maximum injection pressure determination. From injection pressures & rates, the gas volume fraction (GVF) profile in the drill pipe, as well as maximum GVF, may be determined.

Step 4 Modeling with Reservoir Inflow

Step 5 Well Controllability

Control bottom-hole pressure by varying the choke setting (back pressure) or change injection parameters. Maximum Allowable Working Pressure @ UBD Productivity Index is determined by the choke pressure applied before the well becomes overbalanced. (For both circulating [injection fluid] & production [natural flow.]) Maximum wellhead pressure calculated is used to specify the dynamic rating of the RFD. The static rating of the RFD remains as the Maximum Allowable Surface Pressure, which must always be calculated for a gas gradient to surface for UBD design.

MASP is taken as the lower of the two surface pressure calculations Fracture pressure @ previous shoe minus gas gradient to

surface. Reservoir pressure minus gas gradient to surface.

Step 6 Equipment Specifications

Summary of Practical use of a multiphase flow simulator for UBD applications design The tricks of the trade

SPE/IADC 91958 C.Mykytiw, P.V. Suryanarayana, P.R. Brand, Blade Energy Partners