A new approach to estimating recovery factor for extra-low permeability water-flooding sandstone reservoirs

The methods of estimating recovery factor of newly added measured reserves in existing petroleum reserves standardization of China are unsuitable for sandstone oil reservoirs of extra-low permeability with water flooding, so new empirical formula method and analogous method for estimating recovery factor of the reservoirs are studied. The key parameters influencing recovery factor are permeability, formation oil viscosity, porosity, mobility and well spacing density. Based on the requirements such as longer development time, reliable reserves and available reservoir parameters, 37 sandstone oil reservoirs of extra-low permeability were selected, and 19 recovery factor models were summarized. With consideration of high multiple regression coefficient and low standard error, 4 empirical formulae suitable for reservoir of water-flood development with vertical well were established. Comparison of calculated results from the empirical formulae and calibrated recovery factors of PetroChina recently shows that the formulae match well. It is important to choose reservoir with successful pilots and/or developed in large scale as benchmark reservoirs in the analogy estimation. Based on the principle of keeping consistent of analogous method with production performance, a practical example of Chang 8 reservoir, Triassic, FX oilfield was given to illustrate the determination of recovery factor of extra-low permeability reservoir. Empirical formula method and analogous method have their advantages and disadvantages, reserves estimators should pay great attention to the reliability of data, to application condition for empirical formula method and to the avoidance of single reservoirs in analogous method. Key words: extra-low permeability, sandstone reservoir, waterflood development, recovery factor calibration, empirical formula method, analogous method, measured reserves, proved reserves