Pore Pressure and Fracture Gradient 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:

(a) force or stress on matrix increases

(b) bulk volume decreases

(c) pore volume decreases.

Origins of Formation Pressure

Artesian Water system

Reservoir structure

Tectonic activities


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



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