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The potential for severe formation damage around horizontal wells exists due to the increased time of formation exposure to the drilling fluids as compared to the time that a vertical well is exposed to these fluids. This paper explains that the combined effect of the formation damage and the presence of impermeable barriers and low permeability regions in the vicinity of some sections of the horizontal wellbore will manifest itself as skin factor in well test analysis if the total drilled length of the horizontal section is used in the calculations. The purpose of this paper is to present practical new equations, based on a recently introduced concept in well performance analysis, for evaluating the actual length of the horizontal well contributing to production. It is determined that the performance of a horizontal well with mechanical skin and of length Xf1 can be substituted by the performance of a horizontal well of length Xf2 with no skin. Xf2 is smaller than Xf1 for a damaged well and is greater than Xf1 for a stimulated well. By combining the concepts of equivalent horizontal well length and equivalent wellbore radius, one will be able to evaluate the performance of a horizontal well in comparison to its expected performance and to subsequently relate it to an equivalent stimulated vertical well. The new equations and guidelines can be used to determine the magnitude of formation damage around horizontal wells and to evaluate the success of a stimulation treatment.

The concept of effective horizontal well length developed in this paper was applied to the pressure drawdown analysis of well test data obtained on a horizontal well. A good match was obtained which were used to calculate the parameters of interest.

Furthermore, in the analysis of interference tests with mechanical skin damage at the active horizontal well, the observation well will seem as if it is closer to the active horizontal well than it actually is. The effective horizontal well length is obtained from type curve matching which will then yield the mechanical skin factor.

It is expected that the concept of effective horizontal well length and effective hydraulic fracture length will gradually be adopted and implemented by the petroleum engineers and by groundwater hydrologists in the environmental engineering discipline.


A horizontal well offers a large contact area with the formation, and a permanent infinite-conductivity fluid flow path. In addition, a horizontal well provides an increased chance of intercepting the formation's in-situ natural fractures. Also, in reservoirs where the bottom water or gas cap renders fracturing undesirable, a horizontal well offers an alternative to obtain high production rates without gas and/or water coning.

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