A horizontal well, due to its large contact with the formation, may be several times more productive than a vertical well draining the same volume. However, this large contact of a horizontal well with the formation will cause a more severe formation damage than the one obtained from a vertical well. Also, the time that the formation is exposed to the drilling fluid is much longer for a horizontal well compared to that of a vertical well. In addition, the design of a truly non-damaging drilling fluid may be very difficult for some formations. This becomes more complicated as the formation characteristics, such as the fraction of various types of clays, change with the horizontal distance being drilled. Therefore, during the production, some sections of the horizontal well which have severe damage will not contribute to the production. This paper presents the development of a new technique for determining the horizontal well length contributing to production, and the corresponding calculation of the correct mechanical skin damage caused by the horizontal drilling operations.

The method developed in this paper indicates that the response of a horizontal well with mechanical skin and of length Xf1 can be substituted by a horizontal well of length Xf2 with no mechanical skin. A relationship between the mechanical skin and (Xf1,Xf2) is obtained which is applicable for the case of long horizontal wells, thin formations, and formations with high vertical permeability. These are the cases which are most often present in horizontal well drilling and applications. The method developed here is also applicable to hydraulically fractured vertical wells with mechanical skin. For a horizontal well with mechanical skin damage, Xf2 is smaller than Xf1 while for a stimulated horizontal well Xf2 is greater than Xf1.

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