The increasing number of horizontal wells being drilled, together with die continuing development and use of screen completions has resulted in increasing reliance on formation damage testing to select die appropriate drilling fluid and/or clean up technique. The selection of fluids for drilling and completing such wells is critical as damage, either to the formation or to the completion can result in impaired production and substantial loss of revenue to the operator. Mechanisms of damage are not well understood and methods for determining die type and extent of damage potential are not standardised. In a previous paper (SPE 38154)1 we reported on the initial phases of the development of a recommended practice for formation damage testing. Laboratory evaluation of the methodology developed was reported and the results of this evaluation confirmed that tighter controls of the testing variables would be required if acceptable levels of repeatability and reproducibility were to be obtained. In this paper, the results of a more extensive laboratory study, based on a refined recommended practice for the determination of return permeability are reported. Several key areas have been identified in which strict control is required if results obtained are to be compared with others. This is believed to be the largest single comparative study of return permeability testing ever conducted in terms of the number of different laboratories involved. By standardising file test materials and the testing procedures as far as possible, indications of repeatability and reproducibility of results have been obtained and, where differences have been observed, an attempt has been made to determine whether these are due to inadequacies of the procedures or to differences in the equipment being used.

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