Long-Term Testing of Vertically Fractured Gas Condensate Wells

Abstract

ABSTRACT This paper examines the effect of retrograde condensate blockage on long-term well performance of vertically fractured gas condensate wells. A method is presented to correct the effect of condensate blockage using the concept of time-dependent skin factor. Also documented is important differences in productivity loss due to condensate blockage for vertically-fractured and radial, unstimulated wells. The effects of fluid properties and relative permeabilities are considered, as is the effect of production mode — i.e., constant-rate versus constant-sandface-pressure production. Results are based on numerical simulations using a fine-grid, vertical-fracture geometry model with a modified black-oil PVT formulation. The stimulus for the work has been the historical performance of several North Sea condensate fields producing from chalk formations. Well performance history from these wells, which have been stimulated by large acid treatments, indicates no appreciable deterioration of well performance (i.e., skin) during the first ten years of depletion. Also, the author has not found any publications on productivity loss due to condensate blockage in vertically fractured wells, despite the common practice of stimulation in low-permeability gas condensate reservoirs. The results show considerable differences in long-term productivity between stimulated and non-stimulated gas condensate wells. The argument is made that moderate-permeability wells, which otherwise would not require stimulation, may be prime candidates for stimulations to reduce or eliminate the effect of condensate blockage. Stimulation of such wells would be justified mainly on an analysis of the percentage of well pressure losses that are caused by reservoir and near-wellbore effects, relative to total pressure losses including flow through tubing.