Advanced logging and analytical methods unlock producibility potential in fresh formation water — A case study from offshore West Africa

Abstract

The sedimentary basins of West Africa encompass a vast diversity of geologic depositional settings, in and from which hydrocarbons are being explored and produced. The siliciclastic rock units can be structurally heterogeneous, the reservoirs distribution and quality is highly variable, and formation waters have changing salinity values. The fresh to very fresh formation water diminishes the contrast to hydrocarbon complicating any salinity-based measurement technique, such as resistivity. As a result, the reservoir potential might be under- or overestimated by conventional methods; hence, an intelligent petrophysical evaluation and a fit-to-purpose solution become essential to enable the most appropriate development and production strategy. As part of an extended portfolio of wireline logging technology, dielectric dispersion measurement is a critical contribution to the logging programs across the region. The advanced measurement provides dielectric permittivity and conductivity at multiple depths of investigation through the use of multiple frequencies, receiver spacing, and polarizations that are adequately fit into an innovative mandrel design. Robust inversion of all measurements enables solving for salinity, invasion profile, water fraction, shallow zone resistivity, and saturation even when it is hard to distinguish oil from freshwater. We have developed a remarkable example of intelligent logging assessment in highly complex reservoir units where the novel dielectric dispersion measurement is combined with high-resolution magnetic resonance for improved formation evaluation and reservoir management. An integrated data analysis workflow enables fast determination of pay zones and movable fluids independently from and to support saturation equation and input parameters setting. Comparison of the results from well testing confirms the improvement in reservoir description with additional advanced logging measurements (at an early stage of reservoir development) into reservoir models. The information provided is vital to guide perforation and completion designs.