Formation water saturation from drilling fluid filtrate invasion: Comparison of displacement modelling and induction well log response

One goal of log measurements is to determine the degree of water saturation in oil- and gas-bearing beds, which can be calculated using resistivity logs and Archie's equation if the formation resistivity (R,) is known. The measurement of R, presents many difficulties: (1) During drilling, the penetration of drilling fluids into the formation will influence the measured resistivity values; this is known as the invasion effect. (2) In oil- and gas-bearing beds having a thickness less than the vertical resolution of the log instrument, the measured resistivity value will also be influenced by the presence of more conductive clayey layers; this is known as the shoulder effect. This paper presents a numerical model that aids in the interpretation of log results from thinly-layered formations, giving water saturation curves for the units of interest. For a series of gas-bearing horizons, the simulated water-content distribution has been compared with the saturation values obtained using a resistivity log instrument; this comparison has allowed an assessment of the shoulder and invasion effects.