Overcoming Coring Challenges in a New Unconventional Play Offshore by Integration of Formation Evaluation Data

Abstract

A well was drilled into a prospective new unconventional mudstone play offshore Norway. Two of five coring runs were successful while the rest yielded little to no core recovery. Investigations attributed the poor recovery to sub-optimal coring practices, equipment failure and operational errors. Recently, the accompanying petrophysical logs and seismic data were revisited, and upon detailed investigation several unusual responses were observed to correspond with intervals of poor core recovery. Subsequent investigation of the core itself substantiated that the coring issues largely had natural causes. This understanding is being applied to two imminent coring operations and has driven selection of drilling, coring and wireline technology and procedures, in addition to informing casing design. Wireline nuclear magnetic resonance (NMR) and cross dipole acoustic data, logging whilst drilling (LWD) density (including azimuthal images), neutron porosity and resistivity was acquired over the interval of interest for standard formation evaluation purposes. This interpretation was conducted immediately after the initial drilling and showed the formation to be a series of highly porous oil bearing mudstones. However, no in depth advanced interpretation was conducted at the time. Recently, advanced analysis including high resolution log enhancement, NMR 2D porosity and saturation analysis, acoustic azimuthal anisotropy, near wellbore imaging, fracture interpretation, and borehole image interpretation were performed on the log data, and new and improved 3D seismic data was interpreted. When interpreted in detail it could be observed that unusual responses in the logs showed a close correspondence to the intervals of poor core recovery. In particular, high azimuthal anisotropy was observed, and when this was compared to the near wellbore reflection image a significant planar reflecting feature was identified which is determined to be a fault. Indications of this feature was subsequently found in seismic data. When then compared to the azimuthal density image after resolution enhancement was applied, although the image is still of too low resolution to directly image the fault, disturbed bedding was observed which is commonly associated with faulted intervals. Several core fragments proved to have extensive small-scale fracturing not noticed previously, and slickenlines were found along several larger fractures previously presumed to be drilling induced. The investigations of the log data revealed that a previously unknown sub-seismic fault was present right below the depth where coring problems were encountered. The detailed interpretation was able to determine the precise location of the fault and its extent in the formation. Knowledge of this subsequently explained the coring problems encountered and helps to optimise imminent coring in the same formation. Lessons learned and the methodology likely also applies to similar formations. In this paper we discuss coring issues encountered in a new and unconventional play offshore, present new data and interpretation that sheds light on them and describe the methodology of the detailed integrated interpretation that uncovered the previously unknown root cause. We then discuss how these findings can be (and are) used to optimise both drilling, coring, and logging operations in future wells.

Integrated Fault Seal Analysis: An Introduction

“…Faults that were once a hindrance to reaching (risk of wellbore instability etc., Ogilvie et al 2015) deeper reservoir targets, could, in some geological settings, themselves, become targets for production (fractures around faults) and be an integral part of a field depletion plan. This will likely coincide with an increasing focus upon unconventional reservoirs (e.g., the faulted, oil-bearing mudstones in the overburden section of the Valhall Field, Central North Sea studied by Bradley et al 2019). Fault seal studies will require close integration with geomechanical studies in these types of reservoirs.…”