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ACID STIMULATION

T
      
    he purpose of doing acid stimulation on a well is to increase the production 
rate of the well.

Production rate is increased by increasing the permeability of the formation rock 
near to the well bore.

SECONDARY RECOVERY ‑ ACID STIMULATION

The permeability of a rock is the ability of that rock to allow fluids to pass through 
that rock.

The higher the permeability, the easier it is for fluids to pass through the rock.

The lower the permeability, the more difficult it is for the fluid to pass through the 
rock.

The pores, or holes, in the formation are known as the 'matrix'.

Acid stimulation is also called 'Matrix Stimulation'.

Most acid treatments fall into one of three general classifications:

·         Wellbore clean‑up,

·         Matrix acidising,

·         Fracturing.

Wellbore clean‑up is mainly flooding the well bore adjacent to the perforations to 
remove scale and drilling mud solids.

Matrix acidising consists of slowly pumping acid into the formation so that the acid
penetrates the pore spaces without fracturing the formation rock. This procedure is 
normally carried out to dissolve mud and drilling contaminants from soft sandstone
formations.

Fracturing involves the injection of acid at rates faster than the natural flow channels can accept it until the formation rock finally fractures or splits. As the treatment continues, the acid moves through the fractures and dissolves extended pits and channels through the rock face.
Types of Formation Damage

TYPES OF ACID NORMALLY USED

The two acids most commonly used for oil well stimulation are:

·      Hydrochloric Acid              (HCI)
·      Hydrofluoric Acid               (HF)

After these acids have 'dissolved' the formation rocks and contaminants the
products of that chemical reaction must be soluble.

If the reaction products were not soluble the solid materials produced would block
and plug the pore spaces in the formation rock and reduce the permeability, not 
increase it.

The mixture of acids and additives are specifically engineered for each well and
each type of formation.

The pumping pressures and volumes are also specifically engineered for each formation

TYPES OF ADDITIVES

Different chemicals are added to the basic acid mixture that is pumped down the
well. There are many different additives used for many different reasons.

Basically we want the acid to attack the formation rock and the contaminants in it.
We do not want the acid to attack the completion equipment nor do we want the
acid to be used up (spent) before it gets into the formation.

The acid must also be easy to pump.

The main types of additives are:

Retardants which delay the action of the acid to give it time to reach the place
where we want it to do it's job.

Friction reducers which make the acid mixture more 'slippery' and easier to pump.

Surfactants which also make it easier to pump the acid. They also help to prevent
the formation of emulsions once the acid has done its job and is mixed with any oil
in the formation. We have to be able to recover all the spent acid and reaction
products once the stimulation job is finished.

PERF‑PAC BALLS

These are hard plastic or hard rubber balls with a negative buoyancy. They do not
float in the acid.

The Perf‑Pac balls are pumped down the well with the acid at different time 
intervals during the acid job.

Some of the larger perforations will accept the acid easier than others. If we are
not careful all the acid will go into the 'easy' or larger perforations. Fluids always
flow the easiest way.

Perf‑Pac balls will travel with the acid being pumped and will plug off the larger
perforations that are accepting the acid. This then forces the acid to go through
the more difficult or smaller perforations.

When the acid stimulation is complete and pumping has stopped, the Perf‑Pac
balls will then fall to the bottom of the well

ACID PENETRATION

How far the acid penetrates into the formation is very dependant on how fast we
pump the acid.

The faster we pump the acid, the less time it is in contact with the rock as it
enters the formation.

Live acid therefore goes further into the formation where it can then attack the
formation rock or contaminants.


Effect of Injection Rate on Penetration
Effect of Damage Zone on Productivity

FORMATION DAMAGE

Formation damage occurs during the drilling of the well.

The rock is crushed by the drilling action and drilling fluids enter the formation.

The formation is therefore damaged close to the wellbore.

Most damage occurs within a radius of one foot of the well bore.

The damage is also called 'Skin Effect'.

Maximum damage occurs within three feet of the wellbore.

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FRACTURING

T
he purpose of doing fracture stimulation on a well is to increase the 
production rate  of  that well. This is done by improving the natural 
drainage conditions around the well bore. This is called improving natural 
conductivity.

Fracture stimulation involves pumping a fluid down the well at high pressure.
This causes the rock in the vicinity of the well bottom to fracture (to split or 
crack). Vertical fractures can be induced ranging in length from a few 
metres to some hundreds of meters.

Fracture widths are in the range of a few millimetres.
The fractures open up drainage channels to the well bore. This increases
the exposed face of the reservoir rock.

Fluids pumped are selected according to the type of rock to be fractured.
Formations with solubilities less than 50% are more often fracture stimulated
with water or oil‑based fracturing fluids. These are normally sandstone
type formations.
PROPPANTSThe function of a 'proppant' is to prevent the fractures created during
fracture stimulat…