Lecture 10 - Well Performance

What is the “Skin” Effect in an Oil Reservoir? What are some examples?

The Skin Effect is anything that distorts the flow lines from perfectly normal to the flow direction. In other words, it is anything that restricts the flow of the fluids in the reservoir (which causes positive skin). Some examples are:

• Mechanical (partial completion, not enough perforations)

• Phase Changes

• Turbulence

• Damage to the reservoir permeability

What is negative skin? How is it different from positive skin? What are some examples?

Negative skin indicates that the fluids are flowing easier/better than what would be naturally possible. This is due to the pressure drop being less than what is naturally expected.

Negative skin means the fluid is flowing better than naturally possible where positive skin is the opposite.

Some examples are:

• Matrix Stimulation

• Hydraulic Fractures

• Highly Inclined Wellbore

What is formation damage? What are the three main SOURCES of formation damage?

Formation damage refers to the decrease in permeability near the wellbore. The three main sources are:

• Migration of foreign fluids during well construction

• Mechanical damage due to well construction

• Damage during production

What are the four TYPES of formation damage?

1) Pore Obstruction

2) Chemical Precipitation

3) Fluid Damage

4) Mechanical Crushing

What is “Pore Obstruction”? How can we fix it?

Pore obstruction refers to the blockage of the pores in the formations. This usually occurs due to particles (usually clay) settling in the throat of the pore which constricts flow.

To fix this, we can match the external fluids to the reservoir fluids, use diavlent ions over monovalent ions, and limit salinity. These help to control particle dispersion and keep them in one place.

What is “Chemical Precipitation”?

Chemical Precipitation is when chemical reactions cause the formation of solid particles which in turn block pores. This occurs due to foreign fluids or changes to temperature/pressure.

What is “Fluid Damage”?

Fluid damage is when flow is restricted due to changes in the fluid itself, not the permeability of the rock. It occurs due to:

• An increase in water content (lowers effective permeability)

• Formation of W/O Emulsions (emulsions are non-newtonian and need a large amount of force to free them up)

• Increase in Free Water Content (changes wetability which changes effective permeability)

What is “Mechanical Crushing”?

Mechanical crushing is the physical compaction of the reservoir rock which effects flowability of the fluid.

Explain “Drilling Fluid”

Drilling fluid is a type of fluid that is injected during well drilling. It serves many purposes such as keeping the formation fluid from escaping, controlling formation fluid properties, and removing cuttings. The drilling fluid flows down through the well and back to the surface where it is then re-injected into the well.

Explain the four methods of perforation.

1) Jet Perforation - Uses high pressure gasses to perforate the rock

2) Bullet Perforation - Uses a projectile to perforate the rock

3) Hydraulic Perforation - Uses a mix of high velocity fluid and solids to perforate the rock

4) Mechanical Cutters - Knives and Cutting tools

What is “Underbalanced Perforation”?

This is when the wellbore pressure is set to less than the reservoir pressure. This reduces the differential pressure and keeps the fluid from flowing. Can be dangerous due to blowout risk. Is advantageous over overbalanced perforation as it reduces shrapnel from entering the reservoir.

What is “Overbalanced Perforation”?

When wellbore pressure is set to be higher than the reservoir pressure. The higher wellbore pressure prevents reservoir fluid from flowing. Main risks are foreign fluids entering the reservoir.