The length of the annulus or the length of the low-density fluid and mud density of the lost circulation can be calculated based on the annulus capacity behind the drill collar. Suppose the lost circulation volume is smaller than the annulus volume against the drill pipe. In that case, the length of the annulus (i.e., mud loss height) can be expressed in terms of the volume of the low-density fluid pumped to balance the formation pressure and annulus capacity. Here, we will discuss how to calculate the formation pressure of the lost zone while drilling.
Mathematically, If Vl < Van_dp, the length of the low-density fluid required to balance the formation pressure is given by:
Where:
- Can_dc = annulus capacity behind the drill collar, bbl/ft
- Ll = length of the annulus (mud loss height) or the length of the low-density fluid, bbl
- Vl = volume of the low-density fluid pumped to balance the formation pressure, bbl
- Van_dp = the annulus volume against drill pipe, bbl
If Vl > Van_dp, the length of the length of the low-density fluid required to balance the formation pressure is given by:
Where:
- Can_dc = annulus capacity behind the drill pipe, bbl/ft
- Ldc = length of drill collar, ft
- Van_dc = the annulus volume against the drill collar, bbl
Formation Pressure Calculation For The Lost Circulation Zone
Formation pressure is given by
Where:
- Dv = vertical depth of the well where the loss occurred, ft
- Dw = vertical depth of water, ft
- Pff = formation pressure, psi
- ρm = mud density, ppg
- ρw = seawater density, ppg
In addition, improper annular hole cleaning, excessive mud weight, or shutting in a well in high-pressure shallow gas can induce fractures, which can cause lost circulation. Equations (1) and (2) show the conditions that must be maintained to avoid fracturing the formation during drilling and tripping pipe. However, Eq. (4) must also be satisfied to avoid the fracture.
Where:
- ρeq = equivalent circulating density of mud, ppg
- ρmh = static mud weight, ppg
- ρfrac = formation pressure fracture gradient in equivalent mud weight, ppg
- ρs = additional mud caused by surge pressure, ppg
