61st Annual Report on Research 2016

SUMMARY

This project is a first step to characterize spatial connectivity of kerogen in organic-rich mudrocks using electromagnetic measurements, which significantly affects production from these formations. Achieving this goal requires understanding of kerogen electrical properties and developing models that incorporate spatial connectivity of conductive rock components. During the first year, the PI’s research group demonstrated how sensitive can electrical conductivity measurements be to directional connectivity and volumetric concentration of kerogen using numerical modeling.

During the second year, the PI and her research team (a) performed geochemical characterization on pure kerogen samples at different thermal maturity levels, (b) quantified aromaticity of the samples and investigated the possibility of graphitization as thermal maturity increases, (c) measured electrical conductivity of pure kerogen and mudrock samples as a function of thermal maturity, and (d) incorporated directional connectivity of rock components into evaluation of electrical resistivity measurements.

OBJECTIVES

The objectives pursued during the second year included:

Objective No. 1: Quantify the impact of kerogen maturity and chemical structure on its electrical resistivity

Objective No. 2: Develop new electrical rock physics models, which quantitatively take into account directional connectivity of conductive rock components

ACCOMPLISHED AND ONGOING TASKS: METHODS AND RESULTS

Task 1: Quantify Electrical Resistivity and Chemical Properties of Kerogen as a function of Thermal Maturity

To enable direct measurements of the chemical and electrical properties of kerogen, we first isolated kerogen from organic-rich mudrocks. We synthetically matured mudrock and isolated kerogen samples to select heat treatment temperatures. We quantified thermal maturity of non-heated and heat-treated mudrock and isolated kerogen samples using geochemical parameters evaluated with Rock-Eval pyrolysis. To measure variation of kerogen aromaticity in samples with different thermal maturity levels, we estimated relative amounts of aliphatic and aromatic components using solid-state ¹³C nuclear magnetic resonance (NMR) spectroscopy. Finally, we measured electrical resistivity of the mudrock and isolated kerogen samples, prepared in the form of cylindrical molds.

Subtask 1.1: Isolation and Thermal Maturation of Pure Kerogen Samples

To prepare isolated kerogen samples, we first crushed and sieved mudrock samples to obtain particles that are less than 88 microns in size. Next, we performed chemical treatments to remove bitumen and minerals using chloroform, HF, and HCl treatments. Lastly, we performed a density-based separation method to remove pyrite using zinc bromide. The kerogen and mudrocks samples were synthetically matured by increasing temperature to certain limits.

Subtask 1.2: Measure Electrical Conductivity of Kerogen as a function of Thermal Maturity

Figure 1a shows electrical resistivity of non-heated and heat-treated kerogen isolated from three organic-rich mudrocks. Measurements are made at room temperature (76±2°F). Electrical resistivity increases then decreases as a function of heat treatment temperature. The initial increase in electrical resistivity can be due to reduction in conductive pathways (e.g., saline water) and the decrease in resistivity can be due to aromatization and graphitization in kerogen. Figure 1b shows the correlation between the heat-treatment temperature and HI (Hydrogen Index) in all the samples.

Figure 1: (a) Electrical resistivity and (b) HI measured for kerogen samples as a function of heat-treatment temperature.

Subtask 1.3: Quantify Aromaticity and Graphitization

Figure 2 illustrates solid-state ¹³C NMR spectra of (a) non-heated and (b) heat-treated to 450°C isolated kerogen samples from Formation D. Aromaticity of isolated kerogen from Formation D increases 145% (from 0.4 to 0.98) with a decrease of 600 mg hydrocarbon/g organic carbon in HI.

Figure 2: Formation D: Solid-state ¹³C NMR spectra of (a) non-heated and (b) heat-treated isolated kerogen samples.

Figure 3 shows aromaticity of kerogen samples isolated from naturally matured organic-rich mudrocks with different origins as well as aromaticity of synthetically matured kerogen samples from the same formations as a function of HI. Aromaticity increases from 0.40 to 0.95 as HI decreases from 603 to 36 mg hydrocarbon/g organic carbon.

Figure 3: Aromaticity of isolated kerogen samples from four organic-rich mudrocks.

Figure 4 shows TEM (Transmission Energy Microscope) images of (a) non-heated and (b) heat-treated isolated kerogen samples from Formation D. The TEM image of the non-heated sample indicates the presence of amorphous carbon structures. It appears that graphite-like fringes have formed in adjacent layers, after heat treatment. This evolved carbon structure is an evidence of graphitization in the heat-treated sample.

Figure 4: Formation D: TEM images of (a) non-heated and (b) heat-treated isolated kerogen samples.

Task 2: Assimilate the Impact of Spatial Connectivity of Rock Components in Evaluation of Electrical Measurements

We proposed a model which relates the directional conductivity of the rock in the j-direction with the directional connectivity of each conductive network Ψ_i,j (i.e., introduced in the previous report) according to    

where σ_i,j is the electrical conductivity of the i^th conductive component (e.g., saline water, kerogen, pyrite) in the j-direction, C_i,j is the corresponding constriction factor, c_i is the volumetric concentration of the i^th element, and τ_i,j is the electrical tortuosity of the i^th component in the j-direction. We successfully tested this model in cases where saline water was the only conductive component of the rock. The next step includes testing the model in the presence of kerogen and to use it to estimate spatial connectivity of kerogen where enough sensitivity to that parameter exists.

CONCLUSIONS

We quantified the impact of thermal maturity on aromaticity and electrical resistivity of organic-rich mudrocks and isolated pure kerogen. We observed up to nine orders of magnitude decrease in kerogen resistivity as thermal maturity increases. The increase in conductivity of kerogen with respect to both synthetic and natural maturation processes could be due to increase in aromaticity, increase in aromatic cluster size, and/or graphitization in kerogen. To the best of the PI’s knowledge, this is the first time that aromaticity and electrical resistivity are quantified simultaneously in both mudrock and isolated kerogen samples as a function of thermal maturity, which can advance interpretation of geophysical measurements for assessment of kerogen content and connectivity, as well as hydrocarbon reserves. We also introduced a model which incorporates spatial connectivity of different rock components in evaluating electrical measurements and will be used for assessment of spatial connectivity of rock components (e.g., organic matter).