Petrophysical Challenges in Unconventional and Tight Source Rocks, Onshore Abu Dhabi | Schlumberger
Tech Paper
Location
Middle East, United Arab Emirates, Asia, Onshore
Byline
S. Steiner, E.R. Monson, B.A. Abu-Snaineh, and A. Alharthi, ADCO; I. Raina, S. Dasgupta, R. Lewis, G.P. Lis, and A. Chertova, Schlumberger
Society
SPE
Paper Number
177614
Presentation Date
9–12 November 2015
Products Used
Premium

Petrophysical Challenges in Unconventional and Tight Source Rocks, Onshore Abu Dhabi



Abstract

ADCO started its unconventional exploration campaign in 2012 targeting the tight carbonate sequences known as Wasia Group, onshore Abu Dhabi. A front-end loaded data gathering strategy was employed to acquire extensive latest generation logging data tailored for unconventional reservoirs. In a number of wells the entire reservoir section was cored, often up to 800 ft per well, leading to more than 3000 ft of core retrieved to date. ADCO applied unconventional core analysis technologies, such as retort analysis, to generate the optimal core results. Key parameters such as effective porosity, pore size distribution, TOC, source rock maturity, mineral compositions and fluid saturations were determined from logs and core data (where available).

This paper will focus on the petrophysical challenges during the evaluation of the Wasia Group. We will demonstrate that conventional core analysis techniques have only limited applicability, whereas core analysis techniques designed specifically for unconventionals provide more relevant results. A log analysis methodology centered on the application and importance of NMR in unconventional liquid plays is presented. Porosity data measured through retort analysis provide an excellent fit to NMR log-based porosity measurements. Conventional core analysis results generated a poor fit to log porosity, and the resulting values exhibited scatter with a large standard deviation. T2 distribution from NMR log data suggests the presence of large pores with good fluid mobility, which requires confirmation through formation testing or production.

Log data-derived rock typing was performed. It is based on principal component analysis of the reservoir section. Rock classification may help in selecting suitable zones for hydraulic fracture initiation.

Lessons learned from the initial wells for core recovery and analysis techniques are summarized below and have been implemented in later wells:

  • Preserve part of the core for robust saturation measurements.
  • Stop acquisition of conventional poro-perm data
  • Focus on unconventional-specific retort-based techniques for core petrophysics
  • Focus on pulse decay permeabilities
  • Use scratch test to aid in core analysis sample selection process, especially for rock mechanics
  • Add core T1/T2 NMR and MICP to future core analysis programs

The complete integration of core and log data has allowed for a thorough assessment of the unconventional hydrocarbon potential within the ADCO concession.

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