Enhanced LWD Dielectric Processing with Forward Modelling in Horizontal Wells for Improved Geosteering and Formation Evaluation in Fresh Water and Mixed Salinity Environments

Abstract

Abstract To enhance reservoir evaluation in fresh water and mixed salinity environments, we developed a dielectric processing technique for determining relative permittivity and resistivity from measurements acquired by LWD propagation resistivity tools. This enables calculating formation permittivity at 400 kHz and 2 MHz free of shoulder bed and dip effects, and thus estimating saturation in fresh water and mixed salinity environments. This current paper addresses anisotropy effect on the estimation of permittivity and resistivity, and the influence of extremely high dip angles on the performance of LWD electromagnetic (EM) data processing. The most popular formation model for LWD EM data processing is an isotropic model where the permittivity and resistivity are assumed to be uniform in all directions, i.e., the 0D model. However, when the dip is high, the resistivity anisotropy that are especially prevalent in clastic formations begins to kick in. To address this challenge, we have developed a forward model for LWD propagation resistivity tools in high angle and horizontal wells penetrating multilayer anisotropic formations; the 1D model. This model is supportive of all dip angles and allows for users to specify the anisotropy in both resistivity and permittivity in addition to bed thickness. Extensive forward modelling combined with dielectric processing allowed to properly quantify the 1D inversion results at extreme high dip angles. Very good performance is observed, given the ability to correctly depict high contrast and thin layers if resistivity allows a good dielectric signal. Resistivity 1D inversion shows a very strong and solid output, limiting artifacts to the very high relative formation angles and anisotropy. A low ratio between real and apparent components of the currents is found to be a fundamental condition for the applicability of the 1D inversion, especially for anisotropic scenario. The anisotropy ratio is observed to enhance artifact and decrease the capability of the inversion to correctly depict the dielectric constant values in the layers, though in many situation layers could be quantitatively resolved. The 1D inverted resistivity is found to be more robust and tolerant to the high dip angle and anisotropy. Spikes, polarization horns and noise start to appear only at 85° progressively increasing with anisotropy, but layers were rarely obliterated even at the border condition of 85° angle and anisotropy of 5. The use of forward model makes it possible to quantify the anisotropy effect on resistivity and dielectric logs, which can be used to better characterize uncertainty of the two properties where anisotropy effect is non-negligible. It can also assist with understanding the dielectric and resistivity logs and estimating the errors where artifacts are anticipated due to high dips and/or inaccurate dip inputs. The forward model facilitates studying the relationship between formation properties and logs quantitatively. Moreover, when combined with dielectric processing, the model enables an inversion that expands the scope of the problems the dielectric processing can solve.