The Group III Shaly Sand Data Set

Abstract

Summary In the development of shaly sand saturation models, core data are of exceptional value. These data are required to check the descriptive and predictive power of existing models and compare them with new concepts. This paper aims to add to the modest set of generally available core data on shaly sands by presenting a variety of special core-analysis measurements on a number of shaly sand core samples. The experiments presented here include standard petrophysical measurements, continuous injection (CI) resistivity measurements (at ambient and elevated temperatures), nuclear magnetic resonance experiments (t1 and t2 acquisitions on fully brine-saturated samples; t2-diffusion measurements with an external magnetic gradient on partially brine-saturated samples), and scanning electron microscopy (SEM) images that have been subjected to image analysis. Introduction Whereas the number of theoretical models to describe the resistivity behavior of shaly sands is rather large, published data sets used to validate these models are scarce. This is hardly surprising if one realizes the amount of work that is involved, and the precision with which this work has to be carried out, to arrive at high-quality data. Examples of such data sets are the measurements on the Group 1 samples of Hill and Millburn, 1 Group II samples of Waxman and Smits,2 the high temperature results of Kern et al.,3 and the extensive clay property measurements of Hill et al.4 Information on the response of the conductivity of the samples at partial hydrocarbon saturations is missing from the aforementioned data sets. The Waxman and Thomas data set5 partially resolved this drawback with the presentation of C o-Cw curves at elevated temperatures in combination with resistivity index measurements at room temperature. However, the range in which the hydrocarbon saturations varied, from 0 to 50%, was rather limited. To arrive at a more complete set, shaly samples with various cation exchange capacities, originating from different wells, were selected for measurement of their electrical behavior at 100% brine saturation and at partial hydrocarbon saturation. The cation exchange capacity (Qv) of these samples ranges from 0.1 to 0.3 meq/mL pore volume (PV). SEM confirmed that the shale is not laminated in these samples; the clay minerals were not analyzed explicitly. Data from these samples form the currently presented Group III data set. This data set has formed the basis of the recently developed effective-medium saturation model SATORI.6* This article contains a host of relevant petrophysical and related properties obtained from the Group III sample set that can be used for testing shaly sand saturation models. The resistivity data presented here comprise resistivity index curves (at ambient temperature and some at elevated temperature), formation resistivity factors, and excess clay conductivities derived from concentration membrane potentials. Moreover, the nuclear magnetic resonance and SEM data provide more detail and information about the pore structure and constitution of the samples. The analyses were conducted with our routine procedures, as briefly outlined later. We have chosen a graphical presentation of the acquired data because of the size; however, if readers are interested in the digital form of the data set, they are invited to contact the first author. Standard Core Analysis Before each measurement, the samples were cleaned by hot solvent extraction at 52°C with an azeotropic mixture of chloroform/methanol/water. Subsequently, the samples were dried in a vacuum oven at 95°C. The porosity of the samples at ambient conditions was determined by the buoyancy method, both before and after the various special analyses. Before these, the air permeability of the samples was measured at a radial confining stress of 15 bar. Formation Resistivity Factor (F R). The samples were saturated with brine (100 g NaCl/L) and mounted individually in a Hassler-type core holder at a confining stress of 70 bar (the same stress as used for the resistivity index measurement). The sample was flushed with brine until equilibrium was reached in the resistance measurement. The resistivity of the brine was determined separately. From the resistivity of the 100% saturated sample, Ro, and the brine resistivity, Rw, the formation resistivity factor, FR=Ro/Rw, was calculated. Concentration Membrane Potential (Mc). The samples for measurement of the membrane potential were saturated with NaCl-brine of 8.4 g NaCl/L and mounted individually in a core holder.7 A salinity contrast was applied across the sample by alternating flushing one end-face of the sample with brine of 11.1 g NaCl/L and the other end-face with brine of 5.6 g NaCl/L. The equipment was maintained at 25°C during the experiment. The potential difference across the sample was recorded, and the maximum difference between this potential and the separately measured diffusion (or liquid junction) potential of the two brine solutions is a measure of the clay conductivity Ce (=B Qv) as defined in the Waxman-smits model.2 The factor, B, is the equivalent counter ion mobility, and the Qv is the cation exchange capacity of the clay per unit PV (Qv in meq/mL Vpore). The clay conductivity, Ce, is calculated from the difference between the membrane potential of a shaly sample and the liquid junction potential of the brine, and the difference between the potential of a perfect membrane (Nernst potential) and the liquid junction potential of the brine. The brine salinity and temperature determine the B-value, and the Qv of the sample can, hence, be derived. Wet Chemistry Cation Exchange Capacity (Qv). Cation exchange capacity (Qv) experiments were carried out according to a titration method, with barium chloride and magnesium sulfate as the reagents. The measurements are performed on trim-ends from the samples. The clean sample is crushed, and de-mineralized water and barium chloride are added. The clay present in the sample is converted into a monoionic barium clay. The excess barium is washed from the sample. Subsequently, the amount of barium present in the clay is titrated conductometrically with a standard magnesium sulfate solution. The calculated Q v value is expressed in meq/mL PV. Resistivity Measurements Standard CI Equipment. The sample is mounted in a Hassler-type core holder in a rubber sleeve between two plungers with platinum electrodes. A semipermeable membrane is placed between the sample and the bottom electrode.

Electrical conductivity of partially saturated porous media containing clay: An improved formulation

“… (E + Y1 and E + Y2 from Emerson and Yang [], H1 to H15 from Hofman et al . [], and W + T1 to W + T11 from Waxman and Thomas []).…”

“…The 28 data sets consisted of 15 data sets from Hofman et al . [], 11 data sets from Waxman and Thomas [], and two data sets from Emerson and Yang []. From Hofman et al .…”

“…From Hofman et al . [], all data sets in their Table that were measured at 21.5°C were used, except for the high salinity data for core 45, 53A, and 416, as these could not accurately be extracted from the corresponding graphs. From Waxman and Thomas [], all data sets in their Figures and the high salinity data in Figure were used.…”