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 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.
Muskat derived a method for estimation of the retrograde condensate buildup taking place around the wellbore during gas cycling operations. Muskat stated that the retrograde condensation builds up quickly to the critical value and then flows into the well with the free gas. The region of condensate buildup expands outward from the wellbore at the critical condensate saturation as production continues. Based on this theory, Muskat derived a simple formula for estimating the condensate buildup in the vicinity of the well-bore.
Kniazeff and Naville used a reservoir simulator to demonstrate that liquid buildup occurs in the vicinity of the wellbore. O'Dell and Miller proposed a method for calculating the volume of retrograde liquid around the well-bore. Gondouin et al. observed a similar condensate buildup near the wellbore during isochronal back-pressure testing. Eilerts and Sumner demonstrated the effects of the gas production rates on the condensate accumulation. Fussell discussed the effects of the condensate buildup on the productivity of the well.
The effect of gas or oil saturation buildup in the vicinity of the wellbore during a production or shut-in period can be introduced in the flow equations as a rate and time dependent skin. The "pseudoskin" of Weller is but another name for the rate and time dependent skin.