3D Inversion of Ultradeep Electromagnetic Data, the Key to Understanding Complex Subsurface Geology
Authors: Clegg Nigel Mark , Eriksen Endre , Rawsthorne Luke , de Leeuw Vera , Marchant David , Belliveau Patrick 
Published: November 2020

Abstract
In complex reservoirs, the position of geological and fluid boundaries can vary significantly in all directions; for comprehensive reservoir understanding and optimal placement of the wellbore, these boundaries should be mapped and understood in three dimensions (3D). To a degree, this is possible using dip calculations from density and gamma ray images. These are measurements close to the wellbore, in the order of inches, and only represent a very small volume of the reservoir. With ultradeep electromagnetic (EM) technology, resistivity changes can be identified more than 100 ft (30 m) from the borehole. Thus far, the most common inversion run on these data has been one-dimensional (1D), allowing mapping the position of resistivity boundaries only above and below the wellbore. Even with their considerable depth of investigation of approximately 100 ft (30 m), this only provides a snapshot of the reservoir.
To better understand complex subsurface geology and fluid position, particularly in fields with a history of production and fluid injection, a full 3D inversion of ultradeep EM data is necessary. Discussed here are 3D inversion results of synthetic and field EM data representing complex subsurface geology and demonstrating the additional reservoir understanding that a full 3D inversion can provide. The 3D inversion allows the position of resistivity boundaries to be mapped in 3D around the wellbore. By allowing for changes other than just those occurring above and below the wellbore to be considered, this provides a more comprehensive picture than obtained from 1D inversions.
1D inversions for the wells under investigation exhibit a good representation of the position of lithology and fluid boundaries above and below the wellbore; however, it is evident from the azimuthal resistivity images that there are significant changes in the position of resistivity boundaries to the sides, not represented by the 1D inversions. Building up a complete picture of the reservoir from these images and the 1D inversion is difficult because the images do not directly indicate a distance to the resistivity boundaries identified. Qualitatively, these images are an excellent tool to verify the 3D inversion results, which clearly exhibit the distribution of resistivity in all directions in the region of 82 ft (25 m) in these wells.
Complex reservoirs require a 3D inversion to truly map and quantify the distance to resistivity boundaries associated with lithology and fluid variations in all directions around the wellbore. Traditional 1D inversions only provide a simplified representation of the geology and fluid distribution. When applied in real time, 3D visualization of the reservoir can allow improved well placement decisions, with the potential for making well-informed alterations not only to the well's inclination (as in traditional geosteering) but also to its azimuth. Additionally, a 3D inversion provides greater understanding of the reservoir by clearly displaying changes in the resistivity all around the wellbore to a significant distance.