OVERVIEW

In many oil and gas wells, the observed flow rate is different than that calculated theoretically. As noted in Chapter 2, the concept of skin factor was developed to account for the deviation from the theoretical rate. For example, for an oil well located centrally in the drainage plane, during pseudo-steady state flow, the oil flow rate can be calculated as

All parameters in Equation D-1 are in conventional U.S. oil field units and s _t is total skin factor, which includes the effects of partial penetration, perforation density, well stimulation, mechanical skin damage due to drilling and completion, etc. A positive value of s _t would result in a reduction of flow rate while a negative value of s _t would result in a flow enhancement. Damaged wells have positive skin factors and stimulated wells have negative skin factors.

The mechanical skin factor s _m represents well damage caused by drilling and completion fluids. The majority of the drilled wells when completed show mechanical skin damage, and hence, the well is normally acidized before it is put on production. The mechanical damage, denoted as a positive skin factor, would cause a loss in well productivity if it is not removed. In addition to mechanical skin damage, many other parameters may cause either loss or gain in well productivity. These parameters include (1) wells completed in part of the pay zone, i.e., partially penetrating wells; (2) near-wellbore turbulence; (3) perforation density; (4) slant wells, etc.