Well-history-based prediction of injectivity decline (accounting for oil-water mobility variation during waterflooding)

Abstract

Injectivity decline during sea/produced water injection is a wide spread phenomena in offshore and onshore waterflood projects. It happens due to capture of solid and liquid particles from the injected water by the rock resulting in permeability decline and also due to formation of low permeability external filter cake on the wellbore. Both phenomena result in injectivity impairment. The field injectivity decline history is used for characterisation of the formation damage system and for the consequent well behaviour prediction. The injectivity index increases M times during the damage-free displacement of oil by water (M is the water-oil mobility ratio). It affects the well injectivity prediction during poor quality water injection and changes the results of injectivity decline curve interpretation. The combined effect of injectivity impairment by internal and external cakes and of the total oil-water mobility variation can be described by an analytical model. The model contains four injectivity damage parameters: the filtration and formation damage coefficients, the critical porosity ratio and the filter cake permeability. The reliable modelling-based prediction of injectivity decline from a well injectivity index history requires knowledge of the four above-mentioned parameters. In the current paper we develop a method for determination of the four injectivity damage parameters from the well injectivity history. Seventeen well data from eleven different oilfields have been treated, and the values of the four parameters lie in the same intervals as that obtained from laboratory tests. It validates the proposed method for injectivity decline system characterisation and allows recommending this method for prediction of injectivity decline from a well injectivity history.