SECONDARY RECOVERY - ACID STIMULATION
The purpose of doing acid stimulation on a well is to increase the production rate of the well. Production rate will increased by increasing the permeability of the formation rock near to the well bore.
The permeability of a rock is the ability of that rock to allow fluids to pass through that rock. The higher permeability, the easier it is for fluids to pass through the rock. The lower permeability, the more difficult it is for the fluids 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 - is mainly flooding the wellbore adjacent to the perforations to remove scale and drilling mud solids.
- Matrix acidising - consist 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 - involve 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 dissolve extended pits and channels through the rock face.
|Type of Formation Damage|
TYPES OF ACID NORMALLY USED
The two acids most commonly used for oil well stimulation are:
- Hydrochloric Acid (HCl)
- Hydroflouric Acid 9 (HF)
After these acids have "dissolve" the formation rocks and contaminants the products of that chemicals 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 permeability. The mixture of acids and additives are specifically engineered for each well and each type of formation. The pumping pressures and volumes are also spesifically 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 formations. The acid must also be easy to pump. The main types of additives are:
- Retardant - 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 reducer - which make the acid mixture more slippery and easier to pump.
- Surfactants - which make it also easier to pump the acid. They also help to prevent emultions of the formation 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 bouyancy. 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 then the 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 go through the more difficult or smaller perforations.
When the acid stimulations is complete and pumping has stopped, the Perf-Pac balls will fall to the
bottom of the well.
How far the acid penetrates into the formation is very dependent on how fast we pump the acid. More fast we pump the acid, the less time it is contact with rock as it enters the formation. Live acid therefore goes further into the formation where it can attack the formation rock or contaminants.
|Effect of Injection Rate on Perforation|
|Effect of Damage Zone on Productivity|
Formation damage occurs during the drilling of the well. The rock is crushed by the drilling action and driling fluids enter the formation. The formation is therefore damaged close to the wellbore. Most damage occurs within a radius of one foot of the wellbore. The damage is also called "Skin Effect". Maximum damage occurs within three feet of the wellbore.