"Integrating Basic Geomechanics into Practical Lost Circulation and Wellbore Stability
Solutions"
Stephen D. Mason
24 th June, 2013
Grand Millennium Hotel
Dubai, United Arab Emirates
Executive Summary
The Seminar was attended by more than 35 professionals representing around 10 international companies working in the hydrocarbon industry including Operators, consultants, service providers, certification bodies and Petroleum engineering students.
The seminar started with the SPE office holders welcoming attendees to the registration, which occurred from 5:30 p.m. till 6:30 p.m.
After that, the chairman of SPE Northern Emirates Section started off the seminar at 6:30 p.m. by a welcome speech and a brief introduction of the speaker, and then yielded the floor to the speaker.
"Integrating Basic Geomechanics into Practical Lost Circulation and Wellbore Stability Solutions
Outline
Wellbore stability and lost circulation are a worldwide problem
Wellbore instability is defined as the decaying or degrading wellbore, which makes it unstable.
Wellbore instability happens due to one or more of the following factors:
1.
Geo-mechanical stresses/instabilities
2.
Chemical forces instabilities
3.
Hydraulic forces instabilities
How does wellbore breakdown happen?
1.
Outright hole collapse
2.
Formation widening
3.
Formation fracturing
How does wellbore deterioration happen?
1.
Breakout/spalling
2.
Washout/enlargement
3.
Sloughing
4.
Swelling; radial closure (by chemical influence)
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Outright hole collapse, formation widening, formation fracturing, and breakout/spalling are all geomechanical problems, and thus occur due to pressure. However, washout/enlargement occurs due to a hydraulic force and sloughing occurs due to a chemical force.
The geomechanical basis of high-risk operations
There is always a struggle between the wellbore pressure and the formation pressure and the strengths of the wellbore. Therefore, there is a need to balance the pressure versus the geological strength.
As well inclination increases, mud pressure or weight should be decreases to protect the wellbore from inflated stresses.
Geomechanics and wellbore stability: Reactions
A high mud weight in a wellbore that has low stresses may cause instabilities only if the forces are not isotropic.
Therefore, a balance of forces between the fracture gradient and the hydrostatic pressure must be achieved. Otherwise, instability and breakdown will happen.
Wellbore strengthening = Increase FBP
Increase formation breakdown pressure by increasing the radial stress locally in the wellbore.
Capabilities of wellbore strengthening
Leak off test: increase the pressure until a small leak in the well bore sample is detected.
The importance of a leak off test is to determine the leak off pressure. Now, increasing this leak off pressure strengthens the wellbore.
The leak off pressure is increased by increasing the local axial stress at a specific weak location in the wellbore.
The leak off pressure can be increased by up to 4500 psi, which allows us to drill many weak wells that cannot be drilled.
The basic geomechanics of lost circulation
1.
The local fluid pressure should be greater than the formation pressure not the fracture gradient pressure.
2.
The wellbore must have a local channel (like cracks or natural caves).
Theories on increasing the formation breakdown pressure
1.
Hoop stress enhancement: adding a stress cage on the local weak area.
2.
Fracture propagation resistance: adding materials that will be trapped in the crack as soon as the fracture happens to stop and prevent any further damage.
3.
Fluid casing (ultra-low fluid loss): a very dense fluid that coats the wellbore, preventing any damage from happening.
4.
Fracture closure stress: add sand or particulates to the mud, which help in blocking cracks to strengthen the wellbore.
Critically sized particulates that are in the mud can greatly help in blocking fractures as they happen.
Particle size distribution is the main key of particulates in the well mud, and is more important than the diameter of the particle and the solid content.
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Required parameters for applying wellbore strengthening solutions
The wellbore pressure at which the fracture occurs.
The width of the fracture created.
Particle sized required to initiate optimally placed initial bridging.
Type of product that will provide the appropriate strength to build effective stress for hoop stress development to work.
The size and concentration of bridging particles necessary to rapidly and effectively seal the fracture against the formation pressure to prevent extension.
Fracture width equation
Fracture width = 4
(1−𝑣) 2
Equation parameters: v: poisson’s ratio
𝐸
(𝑃𝑤 − 𝑆ℎ)√(1 + 𝑅) 2 − 𝑥 2 ; f(Φ, Θ, S
H
, η, P o
)
E: Young modulus ratio
P w
: wellbore pressure
S h
: minimum horizontal stress
S
H
: maximum horizontal stress
Φ: hole orientation
Θ: stress orientation
General requirements for simulating using a monte carlo model
1.
Formation lithology.
2.
Pore pressure and fracture gradient.
3.
Overburden gradient.
4.
Rock properties profile.
5.
Estimated maximum and minimum horizontal stresses.
6.
Wellbore deviation and inclination.
Example: salty sand wellbore in Mexico
Salt layers that are between sand are huge and have low permeability. However, they can have inclusions in them with high permeability, and usually can behave a lot like sandstone.
The areas in the wellbore which possessed low stresses are: 1 above the salt region, 1 in the salt region, and 3 below the salt region.
The result was all of the low stress regions were strengthened with no losses at all from all five regions. The cost was $0.7 million, but $2 million worth of mud loss was saved.
Difference between lost circulation materials and wellbore strengthening materials
Lost circulation materials are any debris or scrap that can be ground and filled in with.
Wellbore strengthening materials are specific materials with high stresses, specific angles, material type, size, granules, and they do not degrade or breakdown easily and seal fractures effectively, thus increasing the wellbore stresses.
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Limitations of wellbore strengthening
Ratio of diameter to length of the fracture must be optimized.
Practical limit to size of material due to limitations in pumps and their filters.
Pre-existing fractures before drilling are hard to be strengthened.
Rock should be permeable to be able to do the leak off test.
Conclusion
Lost circulation and wellbore strengthening are related to:
1.
Wellbore and formation pressure differential.
2.
Horizontal stress differential.
3.
Key formation mechanical properties
4.
Numerous conditionals that will impact design.
5.
To measure, not estimate, the parameters of the fracture width equation, since very small discrepancies will fail the wellbore strengthening design.
6.
Material size, diameter and particle size distribution.
Scribed by:
Mohammed Sami Mahmoud
Student of the American University of Sharjah
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