MULTI-SCALE QUANTITATIVE ANALYSIS OF MICROPOROSITY IN CARBONATE FABRICS OF THE U.S. SOUTHERN MID-CONTINENT Ibukun Bode-Omoleye 1, Adam Molnar 2 and Michael Grammer 2

1) KICC, Kansas Geological Survey, University of Kansas, Lawrence, KS 2) Boone Pickens School of Geology, Oklahoma State University

------------------------------------------------------------------------------------------------------------Key Findings

This study identified and defined types of microporosity from micro-milled SEM images and captured the geometries of these pores to determine their contribution to the pore system.

In the work presented here, a continuum of pore geometrical data collected from multi-scale image analysis was used to define a quantitative petrographic microporosity cutoff – this is a significant step towards a standardized definition of petrographic cutoffs in future studies.

Cluster analysis indicates that independent of rock type, small and intricate pore systems with higher amounts microporosity are likely to host higher amounts of capillary bound fluids.

Results from multiple linear regression modelling indicate that micropore type, and the shape and complexity of pores, i.e., the and the PoA are theϒ more important parameters for predicting NMR T2 cutoff and bound water saturations.

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Assessing Pore Systems and Microporosity in Carbonates Pore architecture, i.e., type, structure, and overall geometry, is related to fluid

flow and saturation properties in carbonate reservoirs. By combining conventional thin-section microscopy with ion-milled SEM image mosaics, this study takes a multiscale digital image analysis (DIA) approach to capture several orders of pore architectural parameters – including those associated with microporosity – in carbonate samples. The information extracted from the multiscale image analysis is considered to be representative of a continuum of pore properties that contribute to flow and capillary forces in each sample. We integrate these observations with key measurements derived from nuclear magnetic resonance (NMR).

NMR transverse relaxation (T2) measurements provides information on porosity, pore size distribution and the estimation of bound fluid volumes (BFV) and bulk volume movable (BVM) (Chang et al., 1994; Kenyon et al., 1995). BFV and irreducible saturation (Swb) used in this study represents the fluids that are presumed to be trapped by capillary forces in micropores, and is used in industry to estimate formation permeability (Timur, 1968; Coates et al., 1991) and improve saturation estimates from resistivity tools. A standard approach to evaluating BFV is by applying a cutoff T2 value to partition the T2 distribution spectra. The T2cutoff is generally assumed to be the threshold value below which pores are saturated with non-producible fluids – typically micropores.

Recent studies on carbonate pore systems have documented that apart from the amount of microporosity, different micropore types and their associated pore geometries have unique effects on petrophysical properties (Norbistrath, 2015; Kaczmarek, 2015). The work presented here helps improve our understanding of micropore type and geometry, and pore system heterogeneity in carbonate rocks, and extend the application of core-based NMR measurements of transverse relaxation time and T2cutoff.

Microporosity TypesThree main pore types are observed in our dataset, including nano-

intercrystalline pores in cherty matrix/cement, intercrystalline porosity associated with micritic matrix in mudstones to packstones, and intercrystalline micropores in dolomitized rocks.

Figure 1: SEM images showing distinct microstructures of different micropore types from ion-mill polished surfaces. A) Zoomed-in intercrystalline porosity between silica microcrystals in spiculitic packstone.

B) Intercrystalline microporosity between microspar crystals in micritic matrix of skeletal wackestone.C) Large intercrystalline microporosity between dolomite crystals. D) Spiculitic molds that fall in the

micropore size range set in a microporous silica matrix. E) unconnected matrix micropores in a dense matrix of skeletal wackestone to mudstone. F) intraparticle microporosity in crinoid grain (dashed green

outline).

Defining Microporosity CutoffsMost studies utilize approximations of pore geometry and abundance from

petrographic thin sections measurements alone (e.g. Weger et al., 2009). The systematic quantification of microporosity across all scales to include the pore geometry of micropores is still relatively limited. This oversight has mostly been due to the prevalence of milling artifacts that accompany the manual polishing of samples such as plucking and pulling. Utilizing SEM to view ion-milled surfaces allowed us to acquire 2D pore sizes at resolutions down to 20 nm.

Figure 2: Correlation between percentage of porosities at different size cutoff lengths in 20 samples• By comparing values at multiple size cutoff boundaries, we determine that geometrical parameter

below 1 micron and above 100 microns have the most significant variation when compared with other cutoff values.

This study quantifies microporosity (Xpor) petrographically by defining the percentage of microporosity using different cutoff boundaries. The pairs scatterplots in Figure 1 shows the relationships between these estimated percentages of microporosity using some of the common cutoffs that are published in literature. For example, the first column and last row indicate the relatively lower correlative relationship between the estimated percentage of microporosity using a cutoff of pore sizes less than 1 µm (Xnano) and the microporosity cutoff at pore sizes less than 100 µm (X100). Xnano is only up to 60% in some samples whereas some samples consist of pores entirely smaller than 100 µm (X100). These two groups also show the most scatter and low correlation with other cutoff boundaries.

In essence, we systematically define the pore length cutoff values applied by using different cutoffs to ascertain the pore lengths at which geometrical parameters vary the most and would, therefore, be more likely to affect pore size-sensitive petrophysical parameters. This quantitative approach is better suited for the purpose of our study which is to directly correlate image-derived pore geometric parameters to petrophysical measurements.

Statistical Analysis of Relevant Associated Pore Architecture ParametersDue to the observed inherent heterogeneities among the samples in this dataset,

the scatter plots/cluster analysis did not indicate clear trends and relationships between pore geometrical parameters and the petrophysical properties considered. We see some correlations that show that small and intricate pore systems with higher amounts micro- to nano-sized pores likely to host higher amounts of capillary bound fluids independent of rock type. This relationship holds especially among samples with the same type of microporosity. These results make it apparent that capillary bound fluids in micropores are dependent not only on micropore sizes but also on their architecture as well.

However, multiple regression analysis helps to define the relationships more clearly. Our results indicate that the shape and complexity of the pores, i.e., the ϒ and the PoA (which includes a size component) are much more important for both irreducible water saturation estimates and including aspect ratios for the T2cutoff values. The results from the models is a step toward understanding how micropore types and which pore architecture parameters can assist in predicting irreducible water saturation which are very important in reservoir considerations of permeability and producibility.

Table 1: Results of regression analysis. Coefficients of Determination from the correlation between measured and estimated saturation (Swb) and T2cutoff using the following Digital Image Analysis

parameters and categories as input variables: Gamma, Perimeter over Area, Aspect Ratio, and percentage of microporosity. The geometric parameters PoA and Gamma ( ) in addition to porosity significantlyϒ

improve the correlations, whereas the combination of several geometrical parameters does not produce a significant improvement.

Implications and ConclusionsThis study identified micropore types and then utilizes statistical methods to

investigate the relationships between multiscale pore architecture data and estimations of NMR T2cutoff and bound fluid saturation. We systematically define the percentage of microporosity (Xpor) by applying different cutoffs to determine the pore lengths at which geometrical parameters vary the most and would be more likely to affect petrophysical parameters. Furthermore, multiple regression analysis helps to define the relationships more clearly. Results indicate that micropore type, and the shape and complexity of pores, i.e., the ϒ and the PoA are the more important predictive parameters in the models. Coefficients of correlation improve from 0.1 to 0.78.These results are a step toward understanding how micropore types and which pore architecture parameters can assist in predicting irreducible water saturation which is very important in reservoir considerations of permeability and producibility.

References

Chang, D., Vinegar, H.J., Morriss, C. and Straley, C., 1994, Effective porosity, producible fluid and permeability in carbonates from NMR logging. In Society of Petrophysicists and Well-Log Analysts. 35th Annual Logging Symposium.

Coates, G.R., Miller, M. and Henderson, G., 1991, An investigation of a new magnetic resonance imaging log. Paper DD. In 32nd Society of Professional Well Log Analysts. Annual Logging Symposium transactions.

Kaczmarek, S.E., Fullmer, S.M. and Hasiuk, F.J., 2015, A universal classification scheme for the microcrystals that host limestone microporosity. Journal of Sedimentary Research, 85(10), p. 1197-1212.

Kenyon, W.E., Takezaki, H., Straley, C., Sen, P.N., Herron, M., Matteson, A. and Petricola, M.J., 1995, A laboratory study of nuclear magnetic resonance relaxation and its relation to depositional texture and petrophysical properties-Carbonate Thamama Group, Mubarraz Field, Abu Dhabi. In Society of Petroleum Engineers Middle East Oil Show. 11-14 March.

Timur, A., 1968. An investigation of permeability, porosity, & residual water saturation relationships for sandstone reservoirs. The Log Analyst, 9(04).

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