A petrophysical model to distinguish water-wet and oil-wet fractions of unconventional reservoir systems using triple-combo log suites

Abstract

It is commonly recognized that mixed wetting occurs in unconventional oil reservoir systems. Merkel, et al (2016) presents methodology using dielectric log interpretation to recognize oil-wet systems. However, there are no published petrophysical methodologies as to quantitative differentiation between the oil-wet and water-wet fractions using commonly available log suites. This presentation fills that void. This presentation builds on our previous publication that describes the unconventional reservoir petrophysical model we have developed (Holmes 2014). Essentially, we define four porosity components, namely total organic carbon, clay porosity, effective porosity, and ‘free shale porosity.’ This last component is an indirect calculation if the first three components do not sum to total porosity. Porosity/resistivity plots can be constructed for the total porosity and interpreted in a standard fashion. These will mostly indicate a water-wet system where the effective porosity fraction is examined. A second porosity/resistivity plot compares resistivity with ‘free shale porosity,’ and is clearly interpreted to indicate Archie saturation exponents of much larger than 2 — frequently in excess of 3 — indicating the oil-wet fraction of the reservoir system. Examples from the Bakken and Niobrara reservoirs are presented showing quantitative distinction of water-wet vs. oil-wet reservoir components.

AAPG Datapages/Search and Discovery Article #90266 © 2016 AAPG Pacific Section and Rocky Mountain Section Joint Meeting, Las Vegas, Nevada, October 2-5, 2016