Effects of Permeability Anisotropy and High Angle Completions on Non-Darcy Skin in Gas Wells

Abstract

Abstract High-rate gas wells can be significantly impacted by the effects of non-Darcy skin. Since the prediction of well deliverability is affected, there are implications for production forecast reliability, reserves bookings, worst-case discharge modelling and relief well planning. In this paper, adjustments are proposed to the inputs of conventional analytical equations for estimating D-factor. These proposals are applicable to gas wells penetrating formations with permeability anisotropy (kx/kz) at high angles. Sensitivity analysis was performed using the Forchheimer pseudopressure inflow equation to evaluate the effects of varying the terms ‘permeability’ and ‘height’ on the predicted sandface drawdown. In textbook examples, the assumption is for a fully penetrating vertical well in a homogenous reservoir, where these two parameters are readily definable. For the more complex case considered here, the appropriate definitions for these two terms are less obvious. Given that turbulence and inertial effects are most pronounced near the wellbore, it was postulated that use of the geometric average permeability and contributing completion length would result in more accurate estimations of D-factor. Spreadsheet calculations were performed to determine the impact of these changes for a range of scenarios. Verifications were subsequently performed using a numerical simulator. It was observed that drawdown pressures are affected more as well angles and the vertical to horizontal permeability ratio increased. Generic examples are presented to help illustrate these dependencies, along with a brief discussion such changes can have on production forecasts and worst-case discharge estimates. These proposed clarifications to the gas inflow equations were found to be undocumented in a survey of the literature, indicating a lack of published research in this area.