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SKIN DUE TO TURBULENCE
One of the causes of skin in a gas well is due to turbulence near the wellbore. As the gas flows through the reservoir, it generally flows in a laminar flow regime. However, as the gas approaches the wellbore, the velocity increases which, if high enough, can push the gas into a turbulent flow pattern. There are two reasons for this gas acceleration near the wellbore:
1. Reduced flowing pressure near the wellbore which causes the gas to expand
2. Reduced flow area near the wellbore due to radial flow (Q = vA)
Turbulent flow tends to be less efficient than laminar flow, and thus the pressure drop associated with turbulent flow is much greater than it is with laminar flow. Pressure drop due to friction is proportional to the square of the velocity, and thus turbulence has a greater impact on skin in high flowrate wells. Turbulence in a well tends to increase with the flowrate. Therefore, skin due to turbulence will also increase as the flowrate increases.
“Skin due to turbulence” is sometimes referred to as “skin due to non-Darcy flow”. This is because Darcy’s Law assumes a laminar flow regime. Turbulent flow will lead to a difference in the observed flowrate compared with the predicted flowrate (Darcy’s Law), and this difference shows up in the form of skin.
Identifying Skin Due To Turbulence
Skin can only be quantified with a well test. A single transient analysis only yields “total skin”, or S’, which is the sum of all of the individual components of skin (damage, partial penetration, turbulence, inclination, etc.). Some additional work must be done in order to estimate each individual component of skin. Skin due to turbulence will increase as the flowrate increases, and thus the best way to identify skin due to turbulence is to test a well at multiple different flowrates. All other components of skin should remain constant across all flowrates, and therefore any increases in skin with flowrate can be attributed to turbulence.
Skin due to turbulence can be calculated very simply with the following equation:
Where:
The turbulence factor (D) can be calculated from a well test analysis. The skin component, S-turb, is calculated as the difference in total skin across different flowrates. From S-turb, the turbulence factor can be easily calculated.
Addressing Skin Due To Turbulence
Skin due to turbulence can be difficult to fix, as the only guaranteed solution is to reduce the flowrate, which is usually not desirable. With that said, underreaming and hydraulic fracturing can have a beneficial impact on turbulence as both techniques increase the effective wellbore radius. If nothing else, simply identifying and quantifying skin due to turbulence is extremely helpful when modeling a well’s performance.
Skin due to turbulence can be a significant factor in total skin. The only way to quantify skin due to turbulence is through multiple transient analyses on a single well. By identifying skin due to turbulence, you can have a much better understanding of other skin components, such as damage, which will better inform you decisions regarding workover treatments and deliverability improvements.