Geomechanics and Pore Pressure Analysis

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Questions and Answers

What is one of the methods used for predicting pore pressure in high porosity/permeability formations?

Laboratory experimental porosity vs pressure correlating with compressive strength

Empirical porosity-to-pressure models

Seismic data analysis

Mudweights (correct)

How is SHmax calculated from wellbore breakouts according to the diagonal line from tensile fractures?

SHmax = 2Shmin + 3Pp + ΔP + σΔT

SHmax = Shmin - Pp + ΔP + σΔT

SHmax = 3Shmin - 2Pp - ΔP - σΔT (correct)

SHmax = 5Shmin - Pp - ΔP

Which lab method is NOT listed for estimating pore pressure in shales?

Hydrostatic compression lab test

Sonic log measurement

Confined compaction lab test

Pressure-volume testing method (correct)

What does effective stress consider in pore pressure measurements?

The difference between applied uniaxial stress and pore pressure Signup and view all the answers

According to the content, how can seismic data contribute to pore pressure assessment?

By pinpointing overpressured zones. Signup and view all the answers

What should be done to calculated pore pressures when possible?

They should be calibrated/verified. Signup and view all the answers

In a strike-slip faulting regime, in which direction does SHmax act as observed in deviated wells?

NW–SE direction. Signup and view all the answers

What does the orientation of wellbore breakouts in deviated wells depend on?

The orientation of the well with respect to the stress field. Signup and view all the answers

What type of fractures typically occur around the wellbore due to drilling in deviated wells?

En-echelon pairs of tensile fractures. Signup and view all the answers

Which factor does NOT contribute to modeling the magnitude and orientation of SHmax?

Pore pressure in connected reservoirs. Signup and view all the answers

What is indicated by the angle of the breakouts with respect to the wellbore axis in deviated wells?

Stress orientation. Signup and view all the answers

How can stress magnitude and orientation be determined around a deviated wellbore?

From compressive and tensile failures. Signup and view all the answers

What equation is used to assess wellbore stability in a deviated well?

Eaton's Method. Signup and view all the answers

What does the initial porosity (φ0) represent in the context of pore pressure prediction?

Porosity at zero effective pressure Signup and view all the answers

What is inferred when geophysical data shows abnormally high porosity at a given depth?

The development of overpressure Signup and view all the answers

What does the ratio method rely on for pore pressure predictions?

Normal values of each parameter Signup and view all the answers

Which statement accurately describes the behavior of effective overburden stress with depth in hydrostatic conditions?

It increases linearly with depth Signup and view all the answers

Why might the ratio method lead to unphysical pore pressure calculations?

It does not consider overburden or effective stress Signup and view all the answers

In Eaton's method, which parameter typically corresponds to the stress in pore pressure calculations?

Vertical stress (Sv) Signup and view all the answers

What can lead to the need for adjusting the exponents in Eaton's method?

Differences in geological regions Signup and view all the answers

What is the acoustic formation factor represented by in the porosity equation?

tma Signup and view all the answers

What does the color red indicate regarding wellbore stability?

An unstable well requiring high rock strength to prevent failure Signup and view all the answers

What is the significance of different strength scales in the wellbore breakout initiation figure?

They show how different stress magnitudes influence rock strength requirements. Signup and view all the answers

Which statement about tensile fracture initiation is true?

Tensile failure requires specific mud pressure, stress, and pore pressure values for initiation. Signup and view all the answers

In the context of wellbore breakout initiation, what does a blue color signify?

A relatively stable well that is less susceptible to failure. Signup and view all the answers

How do the calculations for wellbore breakout initiation relate to wellbore stability?

They represent initiation conditions but not stability considerations. Signup and view all the answers

What factor is specifically mentioned as impacting tensile fracture initiation?

Mud pressure along with stress and pore pressure. Signup and view all the answers

What does red indicate in the context of tensile fractures?

Tensile fractures are likely to form. Signup and view all the answers

What is a key difference between wellbore breakout initiation and tensile fracture initiation?

Breakouts require different mud pressures than tensile fractures. Signup and view all the answers

How does increasing delta p affect drilling-induced tensile fractures (DITFs)?

It increases the lengths of DITFs. Signup and view all the answers

Which of the following is true regarding stress magnitudes in wellbore breakout initiation?

They increase progressively through varying fault systems. Signup and view all the answers

What primarily determines the angle at which the tensile fracture forms relative to the wellbore?

Sigma t min Signup and view all the answers

What happens to the range of angles over which fractures propagate when raising mud weight?

Fractures propagate over a wider range of angles. Signup and view all the answers

What does the heavy black line in Figure 8.10 represent?

The magnitude of SHmax as a function of Shmin. Signup and view all the answers

What is the significance of the angular span θt in the context of en echelon DITFs?

It indicates the range of angles under tension around the wellbore. Signup and view all the answers

What occurs when σtmin is tensile during drilling?

Fractures form at a defined angle. Signup and view all the answers

Which factor is NOT mentioned as influencing drilling-induced tensile fractures in the observed well?

Depth of the well. Signup and view all the answers

Study Notes

SHmax from Wellbore Breakouts

SHmax can be determined from wellbore breakouts, which are elliptical or teardrop-shaped features formed by failure of rock.
The width of the breakout on one side of the wellbore (Wbo) is related to the maximum horizontal stress (SHmax) and the unconfined compressive strength (UCS) of the rock.

Pore Pressure Prediction

Pore pressure is the pressure of the fluids contained within the pores of a rock.
Estimating pore pressure is crucial for drilling operations and formation evaluation.
Pore pressure can be predicted using various methods, depending on the formation type and available data.
Wireline-conveyed and drill stem-conveyed pressure measurement tools directly measure pore pressure.
Mud weights are used to estimate pore pressure by monitoring the drilling fluid density required to balance formation pressure.

Compaction Trends from Lab Measurements

Porosity decreases with increasing effective stress, which is the difference between applied stress and pore pressure.
The relationship between porosity and effective stress can be determined through laboratory experiments.
This relationship can be used to estimate pore pressure from measured porosity values.

Ratio Method (Sonic and Resistivity Logs)

This method uses the ratio of measured sonic and resistivity values to normal (reference) values to estimate pore pressure.
The calculation requires knowledge of normal values for sonic and resistivity for the region.
The ratio method does not explicitly consider overburden or effective stress and can sometimes result in unrealistic pore pressure predictions.

Eaton's Method

This method uses measured resistivity and sonic logs to estimate pore pressure.
It utilizes a relationship between pore pressure, hydrostatic pore pressure, and measured resistivity and sonic travel time.
The exponents in the formula are often adjusted for different regions to improve accuracy.

Stress Around a Deviated Wellbore

The stresses around a deviated wellbore are not uniform, resulting in complex stress conditions.
The orientation of the wellbore with respect to the in-situ stress field influences the stress distribution.
Understanding the stress distribution is critical for wellbore stability and drilling operations.

Observations from Inclined Boreholes

Breakouts in deviated wells can be used to determine the orientation of the maximum horizontal stress (SHmax).
Drilling-induced tensile fractures (DITFs) can also form in deviated wells.
DITFs typically form as en-echelon pairs, which are inclined to the wellbore wall.
The orientation and position of both DITFs and breakouts reveal information about SHmax magnitude and direction.

Wellbore Breakout Initiation Tendencies

Wellbore breakout formation is influenced by the balance between rock strength and applied stress.
The tendency for breakout formation is higher when rock strength is lower and stress magnitudes are higher.
The tendency for breakout formation also depends on the wellbore angle and the direction of SHmax.

Tensile Fracture Initiation Tendencies

Tensile fractures can form in deviated wellbores when there is a tensile stress component on the borehole wall.
The tendency for tensile fracture formation is increased with higher mud pressures.
The orientation and position of tensile fractures provide information about the relative magnitudes of horizontal stresses.

Field Observation on DITFs

DITFs are observed in deviated wells and can be used to constrain the magnitude of SHmax.
The length and orientation of DITFs are related to the difference between mud pressure and the minimum tensile stress on the borehole wall (Δp)
This information is used to build a relationship between SHmax and Shmin.

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Description

Explore the concepts of SHmax, pore pressure prediction, and compaction trends. This quiz covers the relationships between wellbore breakouts, rock strength, and drilling operations. Test your understanding of essential geomechanical principles used in the field.