Saturation modeling in a carbonate reservoir using capillary pressure based saturation height function: a case study of the Svk reservoir in the Y Field

Abstract

Because of the complex pore structure and the strong heterogeneity of the Svk carbonate reservoir in the Y Field, water saturation distribution and petrophysical properties, such as porosity, permeability and capillary pressure are difficult to be characterized. To solve this problem, a new method to interpret water saturation was presented. By this method, the relationships among porosity, permeability and different pore throat radii are fitted and a typical radius R30 (the pore aperture radius corresponding to 30 % of mercury saturation in a mercury penetration test) selected. By fitting the capillary pressure curves with Corey-Brooks function, the threshold pressure, the irreducible water saturation and the Corey exponent ''n'' of each capillary pressure curve can be determined, and the relationship between R30 and the threshold pressure, R30 and the irreducible water saturation, and R30 and the Corey exponent can all be fitted. If the R30 value is known, the threshold pressure, irreducible water saturation and Corey exponent can be calculated by the fitting equations; hence, the water saturation at a given height from FWL can be calculated by the Corey-Brooks function. The water saturation calculated by saturation height function is consistent with that of the well log interpretation and the error is less than by other methods. During the process of three-dimensional saturation modeling, an R30 criterion is presented to define rock types, six petrophysical rock types with different reservoir properties are distinguished and a link is set up between rock types and log responses by the KNN method. Logs are utilized to predict rock types of non-cored wells according to this link, and a three-dimensional rock-type model is established by the Petrel software. R30 value of each grid can be calculated by R30 formula when three-dimensional porosity and permeability models are built with the restraint of rock-type model. Then, on the basis of the saturation height function, a threedimensional water saturation model can be obtained.

Borehole resistivity simulations of oil-water transition zones with a 1.5D numerical solver

“…Determining the water saturation profile in the transition zone in a porous media to predict the recoverable oil saturated in the rock is a demanding task in petrophysics (see, e.g., [27][28][29]). However, sometimes, a saturation profile may be available from previously recorded data.…”

“…Despite the high efficiency of semi-analytic methods, they may sometimes lead to an unrealistic mapping of the reservoir since they have to consider piecewise constant resistivity profiles, and as a result, they often employ a sharp oil-water contact (OWC). However, in many realistic reservoir models, the OWC often appears as an oil-water saturated transition (OWT) zone with a variable resistivity profile (see Figure 2) [27][28][29][30]. To model this with a 1.5D semi-analytical code, it is necessary to approximate the real model in the OWT zone using multiple constant-resistivity layers, which increases the computational cost, implementation, modeling error, and complexity of computing derivatives needed by gradient-based inversion methods.…”