Want to read more? Click here for more Well Insights.
SKIN DUE TO DAMAGE
The most common cause of skin in a well is due to damage to the formation near the wellbore. This occurs when chemicals or debris interact with the sandface and create a reduction in permeability in the formation near the wellbore. Typical causes of formation damage include:
1. Drilling fluids penetrating the sandface and clogging up pore space
2. Fines migration from the reservoir packing into larger pores near the wellbore
3. Debris from pipelines and compressors pushed downhole on injection or storage wells
4. Clay swelling due to water leaking into the formation, causing pores to close off
As we discussed in our previous Well Insights, skin results in an additional pressure drop near the wellbore, beyond the theoretical pressure drop that would be expected based on Darcy’s Law. This additional pressure drop is referred to as “pressure drop due to skin” and occurs when the reservoir fluids have to flow through a damaged section of the formation before reaching the wellbore.
The diagram below illustrates pressure drop due to skin damage. The formation immediately surrounding the wellbore (several inches or feet from the wellface) is damaged. The pressure of the flowing reservoir fluid is steadily decreasing as it approaches the wellbore. But once the fluid reaches the damaged area, the pressure drop becomes more extreme. The ultimate result is a lower flowrate from the well.
The opposite of damage is known as “stimulation”. This occurs when the formation near the wellbore has an increase in permeability and thus the pressure drop near the wellbore is less than expected, resulting in a higher flowrate than anticipated. Typically, stimulation only occurs if the well has been hydraulically fractured, or if there are natural fractures existing in the formation.
Damage to a formation can typically result in skin factors between 0 and +30, however, in extreme situations, skin factors can be greater than +100. Conversely, stimulation can result in skin factors between 0 and -7.
Once formation damage has been identified, the next step is to eliminate it with a workover treatment. Typically, this means either an acid treatment or a frac job. Acid treatments work by dissolving the debris that is clogging up the pore spaces, thus increasing the permeability near the wellbore. A frac treatment on the other hand, works by extending a highly-permeable fracture through the damaged area and into the undamaged formation, thus allowing the formation fluids to bypass the damaged area. Generally speaking, frac treatments are much more effective than acid treatments at creating a negative skin factor because you are introducing a proppant which typically has a much higher permeability than the natural formation.
Formation damage is a common occurrence and the impact on production can be massive. Identifying wells with skin damage is the first step to improving well deliverability. Only once skin damage has been identified with a well test can a proper workover treatment be implemented.
Want to read more? Click here for more Well Insights.
You can also subscribe to receive Well Insights updates directly to your inbox.