Work Flow Determines Petrophysical Properties From As-Received Core Samples
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.
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.
The estimation of volume of hydrocarbon in place (HIP) is a key formation-evaluation objective. In 1942, Gus Archie established the algorithm relating bulk resistivity (Rt), porosity (ø), 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 ø. To solve Archie’s equation in terms of Sw, the petrophysicist starts by deriving Rt and f 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 desaturation 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. Underestimating the value of n will result in errors in Sw and may mischaracterize an oil-wet formation.
The work flow described in the complete paper measures the Archie parameters—Rw, m, and n—directly from core samples and independent of the log-based method. Initial porosity and Sw are measured using nuclear magnetic resonance. Rw is measured from a patented method based on resistivity dispersion. A two-electrode system provides the Rt measurement. Sample desaturation is by centrifuge at two drainage pressures. Sw and Rt measurement occurs at each desaturation state. Sw measurement at each subsequent desaturation step is by the gravimetric method. Regression analysis of the Rt and Sw data series yields Ro, the resistivity that would be expected for a 100% water-saturated sample, the FF, the RI, and the Archie parameters of m and n.
Because testing on core plugs occurs in the AR state, original fluids are retained, and the wettability of the matrix is not altered.
Classical and Modified Electric- Properties Testing and Results
The complete paper includes several pages describing classical methods for determining FF and RI and the description of the new test method. The discussions include plots and equations, as well as a diagram of the new modified electric-property work flow (Fig. 1). The work flow initially measures porosity, resistivity, connate water resistivity, and water saturation in the AR state. The next step is to desaturate the core using a centrifuge to approximately 600 psi of drainage pressure, and to measure the new saturation and bulk resistivity. A simple mathematical computation or graphical solution yields m and n. Centrifuge desaturation is repeated through a second spin to approximately 1,200 psi of drainage pressure. The AR and first centrifuge spin data are used as the primary source for determining m and n. The full suite of AR, and first and second centrifuge spin results, provide data to validate or simply quality check the results obtained from the AR and first spin set.
Following the testing discussions, several additional pages are devoted to the results, including plots, of application of the new work flow to five cores from the Wolfcamp formation and five from the Avalon.
The results of applying the AR core-analysis work flow to the more-permeable Wolfcamp show consistency between AR, first-spin, and first- and second-spin data. In the Avalon, some of the core permeability was too low to allow desaturation through spinning. The results from these samples were unreliable. Where sample desaturation is possible, the authors believe the method is readily applicable to other formations, including Bone Spring sands and conventional systems.
Sw results from core extractions and log-based computation were compared using the measured values of m and n for two formations. In each case, the log-based computation matched reasonably well with the core. Furthermore, using measured electric properties yielded a better match to core than did assuming values of m and n equal to 2.0. Using n as a measure of a sample’s partial wettability, the authors concluded that the Wolfcamp samples appeared more water-wet than the Avalon samples. The Wolfcamp and Avalon n values range from 1.73 to 2.46, and 3.09 to 4.0, respectively.
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.