FORMATION PRODUCIBILITY AND FRACTIONAL FLOW CURVES FROM RADIAL RESISTIVITY VARIATION CAUSED BY DRILLING-FLUID INVASION

In order to characterize conductivity profiles due to drilling fluid i nvasion into a hydrocarbon formation, a model for radial fluid transpo rt is presented. The model assumes a water-based mud and accounts for the convective movement of oil, water, and salt. A mathematical analys is of the model using the method of characteristics is given. An equiv alent graphical construction is also provided. Computations of radial conductivity profiles for specified formation characteristics and tota l filtrate loss are given. For typical fractional flow curves, it is s hown that three qualitatively different profiles may occur depending o n the initial water saturation. These results are compared with numeri cal simulations that include capillary pressure and gravity segregatio n. Next, the important issue of the inverse problem is analyzed. It is shown that from a single snapshot of the conductivity profile, an exa ct calculation of filtrate loss and formation fractional flow curve is possible. The calculation is valid provided there is a resistivity co ntrast between the drilling fluid filtrate and the formation water. Fo r practical application, we show the limits of applicability of this r esult with complete numerical calculations. Thus, if wireline logs Of resistivity variation away from the wellbore are available, formation oil producibility and water cut at native conditions may be estimated. (C) 1997 American Institute of Physics.