Saturation Mapping in the Interwell Reservoir Volume: A New Technology Breakthrough

Abstract A crosswell electromagnetic (EM) tomography survey was acquired between two horizontal wells, a water injector and a producer, using coil tubing conveyance. The survey was conveyed in a swept area of a naturally fractured Middle Eastern carbonate reservoir. This paper presents the work flow that was developed to integrate the EM survey results with other data to successfully map interwell hydrocarbon saturations. A slanted well was later landed in the interwell area based on the crosswell survey interpretation results. This new well targeted an oil column shown to be bypassed by the water injection in the swept area, and the well is currently on production. The survey established a number of world firsts for the EM technology, including horizontal wells deployment, well spacing >1.3 km, full 3D resistivity inversion and 3D apparent water saturation mapping. The field program consisted of an extensive openhole logging sequence in each well prior to the crosswell EM data collection. The crosswell EM survey measured 135 profiles, covering more than an 800 m horizontal section in each of the two wells. High quality data were acquired for almost all the data profiles, even in a 400 m cased section. The field data were interpreted with a 3D inversion code. The fracture corridors were identified as electrical conductive volumes, due to their high water saturation. The less or unswept areas were characterized by higher resistivity, due to their lower water content. A resistivity model cannot be directly used for reservoir evaluation, then the subsequent step was to convert the resistivity model into a saturation volume, based on Archie's law. An innovative workflow of petrophysical simulations was designed to define volumetric distribution of porosity and water salinity based on reservoir simulation scenarios and ad hoc laboratory analysis. Finally, the most probable scenario of apparent water saturation volume in the interwell reservoir volume was produced. This work outlines the critical need for deep characterization in cases of fractured carbonate reservoirs, and in any case where the sweep efficiency is likely to be uneven, due to heterogeneities of reservoir petrophysical properties, and particularly permeability field variations.