Negative Pressure Test

In this section we describe the load case "Negative Pressure Test" available in Oliasoft WellDesign™.

---

Negative pressure test is a collapse load case, where the unknown is the internal pressure profile of the tubing.

NOTE!
Note: In this documentation we denote any tubular as casing or tubing. All calculations however encompass any tubular, such as tubings, casings, liners, tie-backs etc. //: # (end - note)

Summary

Negative pressure test is a collapse load case for production. The initial pressure is set to P$_{n}$ = -34,4 bar at RKB. The pressure profile is given by the water gradient from RKB to $l$ meters below the mud line. From the mud line to the casing shoe, the pressure profile is given by the hydrostatic mud pressure.

Illustrating Pressure Profile Graph

Printable Version

Oliasoft Technical Documentation - Negative Pressure Test

Inputs

The following inputs define the negative pressure test load case

1. The true vertical depth (TVD) along the wellbore as a function of measured depth. Alternatively, the wellbore described by a set of survey stations, with complete information about measured depth, inclination, and azimuth.
2. The true vertical depth/TVD of
   1. The rig RKB, TVD$_{RKB}$.
   2. The wellhead/BOP interface, TVD$_{WH/BOP}$.
   3. The TVD from rotary table to mud line, TVD$_{RT}\ to\ _{ML}$.
   4. The hanger of the tubing, TVD$_{hanger}$.
   5. The shoe of the tubing, TVD$_{shoe}$.
3. Length below mud line to use seawater gradient, $l_m$, default is $l_m$ = 1000ft ($\approxeq$ 300m)
4. Pressure drop, $p_n$, default to $p_n$ = 500 psi ($\approxeq$ 3.4 MPa)
5. The mud weight/density, $\rho_m$
6. The salt water density, $\rho_{sw}$ //: # (end - inputs)

Calculation

The internal pressure profile, parametrized by TVD, of the tubing is then given by

$p_i = \begin{cases} \rho_{sw}\, g\, \text{TVD} - p_n, \quad &\text{TVD} \leq \text{TVD}_{\text{RT to ML}} + l_m, \\ p_{\text{ml+l}_m} + \rho_m\, g\, \left(\text{TVD} - (\text{TVD}_{\text{RT to ML}} + l_m) \right), \quad &\text{else}, \end{cases}$

where $g$ is the gravitational constant, and $p_{ml+l_m}$ is the hydrostatic salt water pressure at the wellhead plus $l_m$ including the pressure drop, i.e.

$p_{\text{ml+l}_m} = \rho_{sw}\, g\, (\text{TVD}_{\text{RT to ML}} + l_m) - p_n$