¶ Bumping Cement Plug
Bumping cement plug load is a pre-installation load similar to green cement pressure test load case. The only difference is in input pressure as a differential pressure across the bump plug instead of entering test pressure at the surface. Following are the summary of this load case -
- Pre-installation burst load.
- The default plug is assumed to be at the current shoe.
- Internal pressure assumes a bump plug pressure across the plug. p(zplug|a) − p(zplug|b) = pbump where, p(zplug|a) is the pressure above the plug and p(zplug|b) is the pressure below the plug. The default pbump is assumed to be 500 psi for conductor and surface casings and 1500 psi for other strings.
- Fluid densities inside the string and above the cement outside the string are assumed to be the same. This is defaulted to the running fluid density (fluid density to drill the current hole section).
- When the plug is set shallower than the shoe depth, the internal pressure below the plug will be equal to the external pressure to the corresponding depth.
¶ Calculation
Calculation starts from the top of the annulus or the top of the string. Step-by-step calculations are described in the following -
- External pressure at the top of the string -
The general equation of the external pressure at the top of the string - pe(ztop) = pe(zRKB) + 0.052 × ρe × (ztop−zRKB) where,
- ρe = external fluid density which is the running fluid density for the string of interest.
- pe(zRKB) = pressure at the top of the annulus at the RKB. The default pressure is zero psi.
For offshore subsea well and casings drilled without riser (currently we are assuming conductor and surface casings are drilled without riser), the pressure at the mudline is - pe(zML) = 0.052 × ρsw × (zML−zRKB) where, (zML−zRKB) = water depth and ρsw is seawater density (8.6 ppg as default).
Therefore, for conductor and surface casings in the offshore subsea wells, the external pressure at the top of the string is - pe(ztop) = pe(zML) = 0.052 × ρsw × (zML−zRKB)
For the strings drilled with riser, the pressure at the mudline will be - pe(zML) = pe(zRKB) + 0.052 × ρe × (zML−zRKB)
Pressure at the top of the liner in all offshore well types - pe(zTOL) = pe(zML) + 0.052 × ρe × (zTOL−zML) pe(zTOL) = pe(zRKB) + 0.052 × ρe × (zML−zRKB) + 0.052 × ρe × (zTOL−zML)
For onshore and offshore platform wells, the pressure at the top of the string is zero or more accurately, it is equal to the pressure at the top of the annulus. pe(ztop) = pe(zRKB) = 0
- External pressure at the top of cement -
External pressure from the top of the annulus to the TOC - pe(z) = pe(ztop) + 0.052 × ρe × (z−ztop)
External pressure at the TOC - pe(zTOC) = pe(ztop) + 0.052 × ρe × (zTOC−ztop)
- External pressure at the shoe -
External pressure below the top of cement - pe(z) = pe(zTOC) + 0.052 × ρc × (z−zTOC)
External pressure at the shoe - pe(zshoe) = pe(zTOC) + 0.052 × ρc × (zTOC−zshoe)
where, ρc is the cement fluid density.
If the cement column is composed of lead and tail cements then the external pressure below the cement top can be calculated as following -
- From TOC to the top of the tail slurry - pe(z) = pe(zTOC) + 0.052 × ρlead × (z−zTOC)
- From the top of the tail slurry to the the string shoe - pe(ztail top) = pe(zTOC) + 0.052 × ρlead × (ztail top−zTOC) pe(z) = pe(ztail top) + 0.052 × ρtail × (z−ztail top)
- Hence, the external pressure at the shoe - pe(zshoe) = pe(ztail top) + 0.052 × ρtail × (zshoe−ztail top)
where,
- ρe = external fluid density above the TOC (running mud, mud density used to drill the hole section), ppg
- ρlead = lead cement density, ppg
- ρtail = tail cement density, ppg
- Internal pressure at the shoe - The default plug depth is at the shoe.
- Pressure below the plug - Below the plug the internal and external pressures are same. If the plug is shallower than the shoe depth (see Figure 2) the internal pressure at the shoe and the shoe to the plug will be - pi(zshoe) = pe(zshoe) pi(zplug|b) = pe(zshoe) − 0.052 × ρtail × (zshoe−zplug|b) when zshoe = zplug|b then pizshoe = pi(zplug|b) = pe(zshoe).
- Pressure above the plug - The pressure difference between above and below the plug is the bump plug pressure (pBump). pi(zplug|a) = pi(zplug|b) + pBump
- Internal pressure at the top of the string
Internal pressure above the plug can be calculated as following - pi(z) = pi(zplug|a) − 0.052 × ρi × (zplug|a−z)
where zplug|a = zplug|b. Therefore, the internal pressure at the top of the string will be - pi(ztop) = pi(zplug|a) − 0.052 × ρi × (zplug|a−ztop)
The pressure at the rig floor (zRKB) the internal pressure will be - pi(zRKB) = pi(ztop) − 0.052 × ρi × (ztop−zRKB)
¶ Bump Plug at Shoe
¶ Bump Plug Shallower than Shoe
¶ Lead and Tail Cement Densities
