Well Bore Stability Plots

ResInsight can create Well Bore Stability plots for Geomechanical cases. These plots are specialized Well Log Plots to visualize Formations, Well Path Attributes as well as a set of well path derived curves in different tracks.

In the figure above, the first track contains Formations while the second track contains well attributes of Casing Design. The third track shows the following stability gradients (all normalized by mud weight):

  • FG: Fracture Gradient for sands based on Kirsch.
  • OBG: Overburden stress gradient: Stress component S_33.
  • PP: Pore pressure.
  • SFG: Shear Failure Gradient for shale based on Stassi-d’Alia.
  • SH: Minimum horizontal stress.

The fourth track contains curves showing the angular orientation of the well path as azimuth (deviation from vertical) and inclination (deviation from x-axis) in degrees.

Create Well Bore Stability plots

Well Bore Stability plots can be created from the right-click menu for a well path in Project Tree or from the the right-click menu of the Well Log Plots entry in Plot Project Tree. In the former case, the well bore stability plot will be created for the selected Well Path. In the latter case, it will be created for the first well path in the well path list and the well path for the entire plot can be changed with the Change Data Source Feature.

Input requirements

In order to calculate FG and SFG, the following input parameters are required:

Curve Required Parameters
FG Pore Pressure (PP), Poissons’ Ratio
SFG Uniaxial Compressive Strength (UCS)

These parameters may be read in in the following ways.

Change Data Source Feature

The numbering for import is order of preference if multiple sources are found.

Parameter Default Import mechanisms
PP Hydrostatic PP (TVD x 9.81 / 100 bar) 1. Grid (Grid units), 2. LAS-file as mud-weight (Variable: “PP”, Units: kg / m^3), 3. Element Property Table (Variable: “POR”, Units: Pascal)
Poissons’ Ratio 0.25 1. LAS-file (Variable: “POISSON_RATIO”), 2. Element Property Table (Variable: “POISSON_RATIO”)
UCS 100 bar 1. Las-file (Variable: “UCS”, Units: bar), 2. Element Property Table (Variable: “UCS”, Units: MPa)

Equations and calculations

Stresses at the borehole wall - Kirsch equations

The basic input to wellbore stability models is the stresses at the borehole wall given by the Kirsch equations in cylindrical coordinates:

The transformation of stresses from cartesian coordinate system to x’, y’, z’ is performed by pre- and transposed postmultiplication of the stress tensor with a 3x3 transformation matrix M, i.e. .

Fracture gradient calculations based on Kirsch in sand

To estimate the fracture gradient FG, first step is to find the principle effective stresses at the borehole wall:

Next step is to find the well pressure Pw that gives for . Then calculate FG in equivalent mud weight units as FG = Pw / (TVDRKB * g * 1000) where TVDRKB = TVDMSL + RKB and g = 9.81.

Stassi-d’Alia failure criterion in shale

Stassi-d’Alia failure criterion in shale is calculated by finding the well pressure Pw that satisfies the following equation for .

where are the effective principal stresses and UCS is the uniaxial compressive strength.

Shear Failure Gradient is then given as SFG = Pw / (TVDRKB * g * 1000) where TVDRKB = TVDMSL + RKB and g = 9.81.