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Detection of Deep Formation "Damage"
The characteristics of deep formation "damage" and low permeability appear quite similar on a formation test conducted in a normal manner. In the past this has led to the abandonment of potential producers on one hand or to the unnecessary expenditures for heavy stimulation in other wells that offered no real prospects. It was, therefore, necessary to develop a technique removing this weak area of test interpretation. A method is described whereby positive identification can be made between deep "damage" and low permeability. This method takes advantage of a deviation from the Horner assumption in his theory of superposition.
The identification of formation parameters from drill stem test data is normally confined to the radius of investigation created during the flow period. Under drill stem test conditions where the flow rate is very low, an expanding dominant transient is created by the afterflow extending the radius of investigation during the shut-in period. An extended shut-in, in many cases, will thus allow an inspection of the permeability at a great distance from the well bore. Data received at the surface by the "Tele-Pressure" method during the course of a test can be utilized to take advantage of this new technique. Also, a re-test designed with data obtained from an original test of a formation has been successfully used to obtain the desired information.
Field results are given to support this new evaluation technique. Mathematical support for the technique is presented in the appendix.
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