Estimation of Fluid-Fluid Contact and the Transition Zone: A Case Study of Low Contrast Resistivity Zone

Abstract

Abstract Determination of a fluid-fluid contact and the transition zone is a crucial problem in formations having a low contrast resistivity (LCR) characteristic. High reservoir heterogeneity even makes a serious problem when the contact is not encountered in the existing wells drilled. Improper use of capillary pressure data may lead to severe errors in estimating both the contact and the transition zone. This paper presents a technique to determine a fluid-fluid contact by maximizing and integrating geological and engineering data available such as well logs, routine and special core analysis (SCAL), and flow test data. Integration of all the data leads to establishing rock types along the interval of interest and developing the corresponding capillary pressure data needed. Establishment of rock types is shown to be of importance because SCAL data are rock type dependent. Flow test data and relative permeability ratio data are used to determine water saturation at the tested interval. Two well cases of a LCR zone are considered in this study. All the data needed are available, except that capillary pressure data are limited. The rock type model based on the core analysis data available enables us to develop additional capillary pressure curves as needed. Combination of log responses and routine core data are used to predict permeability. The rock types are then distributed in the well within the tested zone. The oil-water contact (OWC) estimated for each of the zones by applying the technique proposed is significantly deeper than that previously determined using resistivity logs. The water saturation distribution within the transition zone is controlled by the distribution of rock types. When the previous corresponding OWC in each zone is used, it is shown that the calculated Water Cut much higher than the actual data. This demonstrates that the electric log-based OWC is too pessimistic. In conclusion, the LCR zone under the study needs an integrated solution. The proposed technique offers a theoretically sound solution for estimating a fluid-fluid contact and water saturation distribution in the transition zone. It can be applied to common problems of fluid-fluid contact estimation. A procedure to overcome limited SCAL data is provided.