A Simplified Predictive Model for Steamdrive Performance
Authors: Aydelotte S. Robert , Pope Gary A. 
Published: December 2007

To facilitate screening and evaluation of reservoirs for application of steamdrive as an EOR process, a simplified predictive model for steamdrive performance has been constructed. This model extends existing analytical heat-balance models to include fractional-flow characteristics, vertical and areal sweep effects, steam injectivity, and heat losses in wellbores and surface pipes in a single, easy-to-use computer program.

The predictive model assumes that the steamdrive process can be represented by a series of shock fronts created by the injection of steam into a reservoir. The reservoir is divided into six regions: undisturbed zone, cold-liquid zone, hot-liquid zone, steam zone, hot-water drive, and cold-water drive. The size, saturation, and movement of each zone, or bank, is calculated from a fractional-flow characterization of the reservoir and the growth of the steam zone. Vertical and areal sweeps of the steam zone are estimated from empirical relationships, and the sweeps of the other banks are related to their mobilities. The injectivity calculations yield sandface pressure at a specified flow rate or the flow rate at a specified pressure. Wellbore and surface heat-loss calculations give the enthalpy and steam quality at the sandface. An economic package interprets the results of these calculations and provides a cash-flow analysis from which risk and profitability are determined.

Introduction

The injection of steam into some oil reservoirs has been a successful EOR process for 30 years. Unfortunately, not all reservoirs have responded equally well to steam injection, necessitating decisions from reservoir operators on when, where, and how to inject steam. As only economically successful projects appeal to most operators, reliable methods of predicting the performance and profitability of steamdrive candidates are of benefit. This paper describes a steamflood predictive model (SFPM) that quickly estimates the performance and profitability of the steamdrive process. Such estimations provide operators with information on the costs and rewards of steamflooding a specific reservoir. With this information, an operator may choose to seek another project or to investigate further the potential of the reservoir with more rigorous tools.

The basic characterization of steamdrive performance was initially the analytical estimation of steam-zone size.1,2 Many studies refining and redefining the basic equations of steam-zone size enable this quantity to be calculated with relative confidence.3,4 Other facets of steamdrive performance such as surface5 and wellbore6 heat loss, distillation,7 steam overlay,8 and changes in rock properties9 have been studied. Several analytical and numerical models have been written to describe steamflooding behavior more rigorously,10-12 and scaled physical models are able to represent steamflooding characteristics accurately.13 Most of these investigations report remarkably good agreement with measured field performance. This model was written to provide more deterministic predictions of steamflood behavior than did previous analytical models; it incorporates the combined effects of heat and pressure losses, fluid fractional-flow behavior, reservoir sweep effects, and steam injectivity.