**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