After drilling resistivity log conductivity log sonic log density log kfupm

Review
Definition:
Formation pressure (or pore pressure) is the fluid pressure found within the pore spaces of the formation.

It can be expressed as an average vertical pressure or equivalent mud weight.

Estimation of Formation Pore Pressure Cont.

Prediction and Detection of Abnormal Pressure Zones

3. After drilling
4 Resistivity log
4 Conductivity log
4 Sonic log
4 Density log

estimate the abnormal formation pore pressure. The followings

are the categories:

© KFUPM: Dr. Abdulaziz Al-Majed; dept of petroleum engineering, KFUPM, Dhahran, Saudi ArabiaD 55

Prediction and Detection of Abnormal Pressures

Instructor: Dr. Abdulaziz Al-Majed; Dept of Petroleum Engineering, KFUPM, Dhahran, Saudi Arabia

6

i. Drilling parameters – observing drilling parameters (e.g. ROP) and applying empirical equations to produce a term which is dependent on pore pressure
 Drilling rate, gas in mud, etc.

 D - Exponent

Prediction and Detection of Abnormal Pressures Cont..

iii. Drilling cuttings – Examining cuttings, trying to identify cuttings from the sealing zone. Since overpressured zones are associated with under-compacted shales with high fluid content, these detection methods are aimed at determining the degree of compaction as measured from the cuttings. The methods commonly used are

3- Confirmation techniques (After drilling)

i) Predictive Techniques:

The predictive techniques for formation fluid pressure estimation are applied before drilling. The predictive techniques are based on measurements that can be made by:

v) Wire line logs or mud logging information which is also valuable when attempting to predict overpressures.

Estimation using Correlations

In 1971, Matthews was the first person who showed how to calculate pore pressure from well log data. This strategy utilizes a geologic age specific overlay which indicates the normally pressured compaction trendline for the appropriate geologic age. After plotting the observed resistivity/conductivity data on the geologic age specific overlay, formation pore pressures can be predicted. A simple calibration of the data is required to implement this method. The second pore pressure prediction was developed by Ben Eaton. Eaton developed a simple relationship that predicts the formation pore pressure knowing the normally pressured compaction trendline, the observed resistivity/conductivity data and a relationship for formation overburden stress. The two pore pressure prediction techniques require petrophysical data, specifically formation resistivity or conductivity, to predict pore pressures.

i) Predictive Techniques Cont.:

© KFUPM: Dr. Abdulaziz Al-Majed; dept of petroleum engineering, KFUPM, Dhahran, Saudi ArabiaD 12

Estimation of Formation Pore Pressure Cont.

4 D- Exponent

4 DC - Exponent
4 MWD - LWD
4 Density of shale (cuttings)

Estimation of Formation Pore Pressure Cont.

© KFUPM: Dr. Abdulaziz Al-Majed; dept of petroleum engineering, KFUPM, Dhahran, Saudi ArabiaD 14

ii) Jordan and Shirley Model: Jordan and Shirley (1966) re-organised Eq. (14.2) for . He simplified this equation by assuming that the rock matrix strength constant did not change (A = 1) and the rotary speed exponent was equal to one (i.e. E = 1). The rotary speed exponent has been found experimentally to be very close to one. These simplifications helped to remove the variables which were dependent on lithology and rotary speed. the following equation is produced using Eq. (14.2) as:

(14.3)

Estimation of Formation Pore Pressure Cont.

Estimation of Formation Pore Pressure Cont.

© KFUPM: Dr. Abdulaziz Al-Majed; dept of petroleum engineering, KFUPM, Dhahran, Saudi ArabiaD 17

Solution Cont.:

The d-exponent can be calculated by using the Eq. (14.5) as:

iii) Rehm and McClendon Model: It can be observed that mud weight consideration is not taken care by the “d-exponent” Eq. (14.5). Since mud weight determines the pressure on the bottom of the hole, an increase of the mud weight will increase the chip hold-down effect which results a decrease in ROP. As a result, Rehm and McClendon (1971) proposed modifying the d-exponent to correct for the effect of mud density changes as well as changes in weight on bit, bit diameter, and rotary speed. After an empirical study, they computed a modified d-exponent correlation as:

(14.6)