Pore Pressure and Fracture Gradient Calculations
Effects of Pore Pressure
Frac Gradient Calculations
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
Abnormal Pressure Indication
Monitor trends, what is happening and why.
No magic bullet.
Could be one, or more or none.
Change in drilling rate
Resistivity of shale
Mud return temperature
Mud properties change
Methods of Estimating Abnormal Pressures
Correlation available from nearby wells
Characteristics of the geological basin
d and dc exponent
MWD and LWD
Drilling and mud parameters
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
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 )
Effect of water depth in calculating the overburden gradient is being accounted in this method