What Next After a Decade With Significant Advances in the Application of Ultradeep Azimuthal Resistivity Measurements?

Abstract

Equinor has played an important role in the last decade in the testing and development of ultradeep azimuthal resistivity (UDAR) measurements both for look-ahead and look-around applications. Today, UDAR technology is applied in more than 70% of Equinor’s high-angle or horizontal wells. In this paper, the authors will review the use of UDAR in Equinor over the last decade and highlight both successful use and real-time challenges related to the interpretation of the inversion results. UDAR technology and inversion algorithms have been very powerful for reservoir mapping to geosteer or geostop according to plan. However, we forget far too often the fact that we need a good understanding of the reservoir to interpret and evaluate the uncertainty in the inversion result. The number one mistake in a real-time setting is to interpret a resistivity contrast as a specific layer in the reservoir (for instance, top reservoir) and hold on to that same interpretation, even if we drill away from that contrast and may cross multiple layers as distance to the observed contrast increases. Other challenging real-time UDAR exercises relate to uncertainties in the prediction of resistivity inside the reservoir and reservoir thickness from inversion results when still drilling above the reservoir. A third mistake often seen in real time is the detailed interpretation of one-dimensional (1D) inversion results, even when other indicators are pointing towards two-dimensional (2D)/three-dimensional (3D) complexities in the reservoir. Equinor and other operators have pushed for more and more advanced inversion solutions, leading to 3D mapping capabilities for more complex reservoirs. The UDAR advances over the last few years are important for Equinor’s planned roadmap ahead. However, 1D through 3D inversion results can result in bad decisions if the uncertainty in the inversion result is not managed correctly. We see a need to investigate how to best exploit UDAR technology and inversion results without extending assumptions beyond an acceptable uncertainty level. Better handling of uncertainties in geosteering operations will become increasingly important for the well economy with smaller targets, complex geological settings, and varying sweep efficiencies. How can we best handle the uncertainty in inversion results in real-time operations to avoid inaccurate decisions that can potentially destroy well economy? This is an important question that will be addressed and should be handled in the future if UDAR technology is to continue its important role in well placement in the next decades.

Limits of three‐dimensional target detectability of logging while drilling deep‐sensing electromagnetic measurements from numerical modelling

“…Inversion‐based interpretation results are often subject to great uncertainty due to non‐uniqueness which can cause incorrect geological interpretations (i.e., incorrect target location, electrical resistivity, and shape) and sub‐optimal geosteering decisions. One of the key sources of uncertainty in inversion‐based interpretation results is the limited spatial resolution due to the inherent diffusive nature of borehole EM induction measurements (Antonsen et al., 2022).…”