Work Flow Determines Petrophysical Properties From As-Received Core Samples
Authors: Feder Judy 
Published: July 2019

This article, written by JPT Technology Editor Judy Feder, contains highlights of paper SPE 191476, “As-Received Core Electrical-Properties Tests for Determining Formation Factor (FF) and Resistivity Index (RI),” by Kent Newsham, SPE, and Roland Chemali, SPE, Occidental Oil and Gas, and Ray Hanna, SPE, Robert Lee, and Craig Whitney, Core Laboratories, prepared for the 2018 SPE Annual Technical Conference and Exhibition, Dallas, 24–26 September. The paper has not been peer reviewed.

This paper introduces a new core-analysis work flow for determining resistivity index (RI), formation factor (FF), and other petrophysical properties directly from an as-received (AR) set of core samples. Unlike common practices that require lengthy core cleaning and wettability restoration, the new work flow does not introduce external liquids or alter the wettability of the matrix. It starts with AR cores in the laboratory. With the new work flow, cleaning, drying, and resaturating the sample are no longer required. The risk of altering the initial wetting state of the sample is minimal. Additionally, laboratory-analysis time is cut from weeks to days. This is particularly valuable because FF and RI play an essential role in estimating hydrocarbon in place using Archie’s equation, and in providing insight into partial oil-wetting conditions. Samples used in the method are from the Avalon and Wolfcamp formations; however, it is also applicable to permeable, conventional rocks.

Introduction

The estimation of volume of hydro-carbon in place (HIP) is a key formation-evaluation objective. In 1942, Gus Archie established the algorithm relating bulk resistivity (Rt), porosity , formation water resistivity (Rw), and water saturation (Sw). The Archie equation, and modified Archie methods, are the most-common petrophysics application for estimating HIP, and remain at the center of log-based formation evaluation.

The Archie equation provides the method to estimate the volume of HIP in terms of Sw and phi. To solve Archie’s equation in terms of Sw, the petrophysicist starts by deriving Rt and phi directly from well logs. The remaining parameters must be determined independently, using cores and fluid samples brought back to the surface. In some situations, some of these parameters can be determined purely from logs, but in low-permeability formations, one must make assumptions that cause an increase in the uncertainty of Sw and HIP.

Conventional electrical properties-based core analysis requires lengthy core cleaning and wettability restoration. However, in low-permeability formations, it is difficult to extract all hydrocarbon from samples without rigorous solvent cleaning, which potentially could alter the original fluid distribution and wetting state of the sample. Furthermore, with ultralow- permeability formations, complete de-saturation is nearly impossible without crushing the rock. Small errors in the cementation exponent (m) or the saturation exponent (n) can cause large errors in the Sw. In organic mudstones, with the presence of kerogen, the matrix becomes partially oil-wet, resulting in increasing n values significantly greater than 2.0. Under estimating the value of n will result in errors in Sw and may mischaracterize an oil-wet formation.