The document discusses acid fracturing, which involves injecting acid at pressures greater than the reservoir's fracture pressure. This connects the wellbore to the natural fracture system and improves production from low permeability reservoirs. Acid fracturing is mostly used in carbonate reservoirs, where fractures are initiated using a fracturing fluid pad followed by acid. Conductivity is achieved through etching of rock minerals on the fracture faces. Factors like fluid leak-off, reaction rate, and temperature affect the etched fracture length. Simulation tools can be used to design treatment schedules to optimize fracture conductivity and length based on reservoir properties and fluid characteristics. Acid fracturing faces challenges like fracture closure and requires proper additive selection. It is widely applied in deep, hard carbon
2. Enhance production from low permeability reservoirs
Connect wellbore to natural fracture system in reservoir
More uniform depletion in layered reservoir
Reduce sand control problems
Mitigate drilling/cementing induced formation damage
3. What is acid fracturing..?
1. It is a type of acidizing, in
which injecting pressure is
greater then fracture pressure
of reservoir.
2. The success of acid fracturing
depends upon heterogeneous
dissolution among fractures
faces.
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4. Mostly used in carbonate reservoirs.
Fracture is initiated with fracturing fluid (gel) called pad.
Suitable acid solution is injected followed by the pad.
Generally less complicated because of no propping agent is
used.
5.
6. Conductivity achieved by etching of rock minerals on the
fracture faces.
Effective length of an acidized fracture is limited by the
distance travel by the acid along the fracture before
spending.
At high temperature this limit is a greater problem.
7. Etched fracture penetration and conductivity:
Conductivity achieved by fracture acidizing is excessive due to
the high reaction rate.
Conductivity increases considerably when acid forms a deep
channel.
8. Factors effecting fracture conductivity:
Fluid leak-off
Reservoir permeability and porosity
Leak-off filtrate viscosity
Reservoir fluid compressibility
Reservoir temperature
9. Factors effecting etched fracture length:
Fluid leak-off
Reaction rate
Reservoir temperature
Free gas present in the formation
Above bubble point ------ High etched fracture length
Below bubble point ------ Low etched fracture length
10. Aim
To design a treatment schedule for achieving optimum etched
fracture length
A simulator predicts a fracture model based on the
◦ Surface kinetics
◦ Flow and temperature
◦ Fluid loss into fracture faces
11. A design simulator asks for the following data:
◦ Completion details of the well
◦ Reservoir lithology and petrophysical properties
◦ Reservoir pressure and temperature
◦ Reservoir fluid details
◦ Pad type and volume
◦ Acid volume and concentration
◦ Acid Injection rate
◦ Treatment schedule
12. The Challenge
Acids fractures may close
Are they limited to closing stresses lesser than 5000 psi?
Reservoir depletion might accelerate acid fracture collapse
Propped fracturing stimulation is not an easy task in deep
hard carbonates.
Undissolved fines can significantly reduce fracture flow
capacity if not remove with spent acid.
Emulsions can block the etched fracture.
14. Applications
It is widely cited that sixty percent of the world’s oil and forty
percent of the world’s gas reserves are held in carbonate
reservoirs
Many world-class reservoirs are deep (TVD > 10000 ft)
and hard carbonates (E > 4 ×106psi)