Pore Pressure Calculations

Effects of Pore Pressure

Pore Pressure

  • Frac Gradient Calculations
  • Fluid Design
  • Cement Design
  • Casing Design
  • Tubing Design
  • Rig Design

Types of Pressures

  • Normal Formation Pore Pressure
  • Abnormal Formation Pressures
  • Subnormal Formation Pressures
  • Causes of Abnormal Formation Pressure
  • Estimation of Formation Pressure
  • Detection of Abnormal Pressure

As fluids are produced from reservoir, pore pressure usually decreases while overburden is constant, and:

  • force or stress on matrix increases
  • bulk volume decreases
  • pore volume decreases.

Origins of Formation Pressure

  • Artesian Water system
  • Reservoir structure
  • Tectonic activities
  • Faults
  • Shale diapirism
  • Sandstone dikes
  • Osmotic phenomena
  • Diagenesis phenomena
  • Salt deposition
  • Permafrost environment

Abnormal Pressure Indication

  • Monitor trends, what is happening and why.
  • No magic bullet.
  • Could be one, or more or none.
  • Change in drilling rate
  • Shale density
  • Resistivity of shale
  • Mud gas
  • Chloride content
  • Mud return temperature
  • Mud properties change

Methods of Estimating Abnormal Pressures

  • Before Drilling
  • Predictive Methods
  • Correlation available from nearby wells
  • Seismic Data
  • Characteristics of the geological basin
  • During Drilling
  • d and dc exponent
  • MWD and LWD
  • Kick
  • Drilling and mud parameters
  • After Drilling
  • Verification Methods

Formation Pressure

Hubert and Willis

Under static conditions, stress due to weight of the formation above must be supported by the matrix stress and fluid pressure in pores

Matrix stress, psi = overburden pressure, psi – pore pressure, psi

Fracture pressure = horizontal stress, psi + minimum horizontal stress, psi

Horizontal Stress is assumed to be ½ and 1/3 of the overall stress

Mathews and Kelly Correlation

Minimum pressure required to create a fracture is at least the formation fluid pressure and any additional pressure may be related to overcome the formation matrix

Eaton’s Method

Horizontal and vertical stress ratio and the matrix stress coefficient are dependent on the Poisson’s ratio of the formation.

Stress in the x direction = stress in the y direction = horizontal stress = Poissions ratio x stress in z direction /( 1 – Poisson’s ratio )

Christman’s Method

Effect of water depth in calculating the overburden gradient is being accounted in this method