The IADC's Underbalanced Drilling committee recently wrote up a set of guidelines related to the ideal underbalanced drilling simulator in this document. 

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Our simulator will aim to satisfy exactly these requirements, including:

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1. Bringing in a well with gas starting from an initially overbalanced condition.
 

2. Drilling with a stable system in an underbalanced condition.
 

3. Able to model the dynamic response (drill pipe, bottom hole and choke
pressures) that occurs after changes in the controlled variables (opening or
closing the choke, varying the pump rate, varying the density of the pumped
fluid). This must include a direct link to the reservoir inflow and a delayed
response at the various observation points.
 

4. Able to model various problem situations such as:
a. a wash out (leak in the drill pipe)
b. a plugged nozzle at the bit
c. packing off in the annulus
d. opening part of the reservoir with higher pressure or flow potential
e. lost circulation or cross flow into the formation 
f. a washout or restriction in the surface choke
 

5. The simulator must be able to model the pressure response associated with
making a connection. Specifically:
a. If the well is shut-in during a connection, gravity separation of the fluids
in the annulus and the resulting pressure slugs
b. Conversely, if the well is allowed to flow during a connection, increased
flow from the formation and unloading of fluid from the annulus
 

6. Killing a well from the dynamic well inflow to a static condition.
 

7. Simulator must be able to handle the following well geometries and set-up
information:
- Vertical and directional well profiles (including inclinations > 90°)
- Ability to inject gas into annulus using parasite or concentric
casing strings
- Fully generalized string and casing geometries – no presumption of
‘traditional’ drill pipe/HWDP/drill collars configuration.
- Variable pore pressure, fracture pressure and formation PI’s as a
function of measured depth.
- Temperature profile for all injection paths (string, parasite string,
concentric-casing annulus) and annulus (T as a function of MD).
-Alternatively, and without significant additional computational
burden, simulator could compute steady-state temperature profiles
for injection and return streams 

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