Accurate Forecasting Of Gas-Condensate-Well Productivity

Summary
Accurate forecasting of gas-condensate-well productivity usually requires fine-grid numerical simulation to model the formation of the condensate bank and to account for high-velocity phenomena such as non-Darcy flow and changes in relative permeability at high capillary number. This paper presents a new technique for forecasting the performance of gas-condensate wells, using simplertechniques that can be used in a spreadsheet. The calculation uses a material-balance model for reservoir depletion and a two-phase pseudopressure integral for well inflow performance. The pseudopressure-integral technique has been extended to include high-velocity effects and to allow for the change in produced-fluid composition caused by the formation of the condensate bank. The new technique hasbeen tested by comparison with there sults of fine-grid compositional simulation, and the results are in good agreement for a wide range of cases covering vertical, horizontal, and hydraulically fractured wells. The spreadsheet-model approach provides a useful tool for rapid forecasts of condensate-well performance, for examining the effects of condensate blockage in different well types, or forstudying sensitivities. It is also valuable where simple models of condensate-reservoir performance are required for use in integrated studies involving issues such as surface facilities, drilling schedules, and gas sales contracts.

Introduction
Well productivity is an important issue in the development of most low- and medium-permeability gas-condensate reservoirs. However, accurate forecastsof productivity can be difficult because of the need to understand and account for the complex processes that occur in the near-well region. When the well pressure falls below the dewpoint, a region of high liquid saturation builds up around the well, impairing the flow of gas and reducing productivity. It is essential to take account of this condensate-blockage effect when calculating wellproductivity.
Most of the drawdown to a condensate well occurs close to the wellbore, where gas velocities may be very high and the relationship between flow rate and pressure drop may be complicated by two additional phenomena—the increase in mobility at high capillary number 1–4 (sometimes referred to as positive coupling or viscous stripping) and the non-Darcy (or inertial) flow.
In manygas-condensate wells, the net effect of the two high velocity phenomena is to improve productivity, reducing the impairment caused by condensate blockage. The importance of high velocity effects has been demonstrated by history matching results for a number of wells in which it was possible to obtain a satisfactory match only when high-velocity effects were included in the simulation model.5,6It is important to include these high-velocity effects when simulating gas-condensate-well performance

Well-Productivity Calculations
The most accurate way to calculate gas-condensate-well productivity is by fine-grid numerical simulation, either in single-well models with a fine grid near the well or in full-field models using local grid refinement. A fine-grid model will allowhigh-velocity effects to be modeled, and most commercial simulators now include options to account for non-Darcy flow and the increase in mobility at high capillary number.
While numerical simulation is suitable for detailed forecasting of reservoir behavior, there are many applications for which this level of modeling is not justified and simpler engineering calculations are more appropriate.Simpler calculations are particularly useful to provide rapid forecasts of well deliverability, for sensitivity studies to assess the impact of parameters such as relative permeability or pressure/volume/temperature (PVT) properties, or to estimate the benefits of fractured or horizontal wells. They may also be more appropriate when accurate data on reservoir-fluid or -rock properties are not...