Integration of Formation Evaluation Data Overcomes Offshore Coring Challenges
Authors: Carpenter Chris 
Published: August 2020

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 195866, “Overcoming Coring Challenges in a New Unconventional Play Offshore by Integration of Formation Evaluation Data,” by Thomas Bradley, SPE, Baker Hughes, and Per Henrik Fjeld and Jonathan Scott, Aker BP, et al., prepared for the 2019 SPE Annual Technical Conference and Exhibition, Calgary, 30 September-2 October. The paper has not been peer reviewed.

A well was drilled into a prospective unconventional mudstone play offshore Norway. Two of five coring runs were successful while the rest yielded little to no core recovery. Subsequent investigation of the core 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 and is informing casing design.

Introduction

The Valhall field is in Block 2/8 of the Norwegian North Sea. The primary reservoir is Cretaceous chalk, located at approximately 2400 m true vertical depth. Production of this reservoir began in 1982. Above this reservoir are other formations, including a shallower potential unconventional mudstone reservoir.

A production well was drilled targeting the Cretaceous chalk reservoir. As part of the drilling program, logging-while-drilling (LWD) and wireline logs - including LWD density, neutron, gamma ray and acoustic, wireline spectral gamma ray, cross-dipole acoustic, nuclear-magnetic-resonance, and formation-testing and -sampling - were run across the potential mudstone reservoir to appraise the standard petrophysical properties. A conventional core was also attempted; however, recovery was poor, with repeated jams.

Coring Challenges

During coring operations, significant problems were encountered when coring the mudstone intervals. These challenges are illustrated in Fig. 1, which shows a section of the core from xx14 m, demonstrating the typical condition of the recovered core when still in the core barrel. At the time of operation, poor recovery was attributed to the coring practices used and no further coring was attempted.

Formation Evaluation

The mudstone interval was not a primary target for the well. Therefore, limited formation evaluation of these intervals was performed, mainly to estimate the petrophysical properties. However, at the time of initial logging, more-advanced formation evaluation data (e.g., images, full waveform acoustic, and nuclear-magnetic-resonance data) were not evaluated fully.

A few years after initial operations, the log data were revisited - specifically, detailed interpretation was conducted for all recorded log measurements. On examination of the results over much of the interval, the integration of the conventional and advanced volumetric measurements gave considerable extra value in assessing the petrophysical properties of the mudstone. However, detailed discussion of these results is outside the scope of this paper. In addition to the extra information gleaned from the advanced volumetric interpretation, over certain intervals, some unusual log responses and interpretation results were seen that were deemed worthy of further investigation.