Limits of 3D Detectability and Resolution of LWD Deep-Sensing Borehole Electromagnetic Measurements Acquired in the Norwegian Continental Shelf
Authors:   NORCE Norwegian Research Centre, Jahani Nazanin , Torres-Verdin Carlos ,   The University of Texas at Austin, Hou Junsheng ,   The University of Texas at Austin, Tveranger Jan ,   NORCE Norwegian Research Centre
Published: November 2023

Hydrocarbons are often found in three-dimensional (3D) and non-spatially continuous rock formations that exhibit electrical anisotropy. A real-time well geosteering method that adjusts the well trajectory based on the inversion of deep-sensing electromagnetic (EM) measurements can be effective for efficient subsurface resource recovery. Nevertheless, the inversion may lead to uncertain spatial resistivity images of formations around and ahead of the well trajectory, which can cause incorrect geological interpretations and fatal geosteering decisions. Therefore, it becomes imperative to quantify the uncertainty of the inversion results in real time. The limited spatial resolution of borehole EM instruments is a key source of uncertainty. Our primary objective is to quantify (a) the maximum radial distance of detection away from the well trajectory and (b) the spatial resolution of 3D subsurface targets for a commercially available triaxial deep-sensing borehole EM instrument operating in the Norwegian Continental Shelf with respect to (a) measurement acquisition parameters, (b) distance between the well trajectory and the targets, and (c) embedding geological environments. First, we constructed several synthetic cases, including geological targets with varying resistivity contrast, varying radial distances from the borehole EM instrument, and varying measurement acquisition parameters. Next, we adapted our study to a geological formation stemming from actual 3D geosteering conditions present in the Norwegian Continental Shelf. We implemented a finite-volume method to numerically solve Maxwell’s equations for 3D electrically anisotropic heterogeneous rock formations. Measurement noise was assumed zero-mean 2% Gaussian. Magnetic fields were calculated as the percent difference between measurements acquired for formations with and without high-resistivity contrast targets. Additionally, we assumed that the borehole EM instrument could reliably detect targets if the latter percent measurement difference exceeded the threshold for measurement noise. There are several factors that limit the distance to which borehole EM measurements can accurately resolve 3D targets, such as resistivity contrast with the background formation, electrical anisotropy of both background formation and embedded targets, measurement noise, frequency of operation, and distance between transmitters and receivers. We found that low-frequency borehole EM measurements can resolve conductive targets at relatively long radial distances from the wellbore, whereas high-frequency measurements can resolve resistive targets relatively far from the well trajectory. In addition, high-conductivity contrasts between the target and background yield a more accurate definition of the target location away from the well trajectory. Likewise, a higher electrical anisotropy factor for the background formation makes it more difficult to resolve conductive targets. Based on the results obtained from the synthetic study, we interpreted a 3D resistivity image which was generated from the inversion of EM measurements acquired across a turbidite formation on the Norwegian Continental Shelf. We found that the uncertainty of inversion results correlates with both the resolution of the borehole EM instrument and its range of detectability. We also define the geometrical shape and resistivity accurately in locations with reliable EM measurement resolution. Estimation of uncertainty in our study can enhance the accuracy of both geological interpretation and real-time 3D well geosteering. Despite several publications proposing the detectable range of borehole EM instruments, no study has considered the radial range of detectability in a 3D geological environment with electrical anisotropy. We generalized the detectability range and resolution of borehole EM instruments as a function of skin depth and transmitter-receiver spacing. Our findings are used to quantify the uncertainty associated with inverted borehole EM measurements acquired in a complex turbidite formation on the Norwegian Continental Shelf.