Pores, Spores, Pollen and Pellets: Small, but Significant Constituents of Resource Shales

Slatt, Roger M.; O’Brien, Neal R.; Molinares-Blanco, Carlos E.; Serna-Bernal, Andrea; Torres, Emilio J.; Philp, Paul

doi:10.1190/urtec2013-065

Cited by 8 publications

(6 citation statements)

References 15 publications

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“…These geometrical properties of Barnett samples—small pore size and low porosity—are expected to yield extremely slow transport. However, larger pores cannot be discounted in their roles of mass transport and connection of nanopore regions; the commonly used argon milling approach is biased against these pores [ Slatt et al , ]. We observed appreciable micron‐sized pores, and note that pores >1 µm account for 7.9–18% for Barnett samples.…”

Section: Discussionsupporting

confidence: 90%

Low nanopore connectivity limits gas production in Barnett formation

2015

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Gas‐producing wells in the Barnett Formation show a steep decline from initial production rates, even within the first year, and only 12–30% of the estimated gas in place is recovered. The underlying causes of these production constraints are not well understood. The rate‐limiting step in gas production is likely diffusive transport from matrix storage to the stimulated fracture network. Transport through a porous material such as shale is controlled by both geometry (e.g., pore size distribution) and topology (e.g., pore connectivity). Through an integrated experimental and theoretical approach, this work finds that the Barnett Formation has sparsely connected pores. Evidence of low pore connectivity includes the sparse and heterogeneous presence of trace levels of diffusing solutes beyond a few millimeters from a sample edge, the anomalous behavior of spontaneous water imbibition, the steep decline in edge‐accessible porosity observed in tracer concentrations following vacuum saturation, the low (about 0.2–0.4% by volume) level presence of Wood's metal alloy when injected at 600 MPa pressure, and high tortuosity from mercury injection capillary pressure. Results are consistent with an interpretation of pore connectivity based on percolation theory. Low pore connectivity of shale matrix limits its mass transfer interaction with the stimulated fracture network from hydraulic fracturing and serves as an important underlying cause for steep declines in gas production rates and a low overall recovery rate.

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Section: Discussionsupporting

confidence: 90%

Low nanopore connectivity limits gas production in Barnett formation

2015

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“…Using SEM/field-emission SEM methods to determine porosities of a range of shale, Slatt and O'Brian (2011) and Slatt et al (2013) reported that micropores ([1 lm in pore length and nanopores (\1 lm) are subequal. The micropores are commonly porous floccules (clumps of electrostatically charged clay flakes arranged in edge-face or edge-edge card house structure) of up to 10 lm in diameter, which are not often seen or identified in ion-milled shale surfaces, perhaps due to the collapse of floccules during milling (Slatt et al 2013).…”

Section: Mercury Intrusionmentioning

confidence: 99%

“…The micropores are commonly porous floccules (clumps of electrostatically charged clay flakes arranged in edge-face or edge-edge card house structure) of up to 10 lm in diameter, which are not often seen or identified in ion-milled shale surfaces, perhaps due to the collapse of floccules during milling (Slatt et al 2013). For the Eagle Ford shale with a high carbonate content, Slatt et al (2012) reported the presence of lm-sized pore types from coccospheres (internal chambers and hollow spines are up to 1 lm in diameter and several lm in length) and foraminifera (their internal chambers can be 10 lm in diameter).…”

Section: Mercury Intrusionmentioning

confidence: 99%

Pore structure and tracer migration behavior of typical American and Chinese shales

Liu

Gao

et al. 2015

Pet. Sci.

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With estimated shale gas resources greater than those of US and Canada combined, China has been embarking on an ambitious shale development program. However, nearly 30 years of American experience in shale hydrocarbon exploration and production indicates a low total recovery of shale gas at 12 %-30 % and tight oil at 5 %-10 %. One of the main barriers to sustainable development of shale resources, namely the pore structure (geometry and connectivity) of the nanopores for storing and transporting hydrocarbons, is rarely investigated. In this study, we collected samples from a variety of leading hydrocarbon-producing shale formations in US and China. These formations have different ages and geologic characteristics (e.g., porosity, permeability, mineralogy, total organic content, and thermal maturation). We studied their pore structure characteristics, imbibition and saturated diffusion, edge-accessible porosity, and wettability with four complementary tests: mercury intrusion porosimetry, fluid and tracer imbibition into initially dry shale, tracer diffusion into fluid-saturated shale, and high-pressure Wood's metal intrusion followed with imaging and elemental mapping. The imbibition and diffusion tests use tracer-bearing wettability fluids (API brine or n-decane) to examine the association of tracers with mineral or organic matter phases, using a sensitive and micro-scale elemental laser ablation ICP-MS mapping technique. For two molecular tracers in n-decane fluid with the estimated sizes of 1.39 nm 9 0.29 nm 9 0.18 nm for 1-iododecane and 1.27 nm 9 0.92 nm 9 0.78 nm for trichlorooxobis (triphenylphosphine) rhenium, much less penetration was observed for larger molecules of organic rhenium in shales with median pore-throat sizes of several nanometers. This indicates the probable entanglement of sub-nano-sized molecules in shales with nano-sized pore-throats. Overall findings from the above innovative approaches indicate the limited accessibility (several millimeters from sample edge) and connectivity of tortuous nanopore spaces in shales with spatial wettability, which could lead to the low overall hydrocarbon recovery because of the limited fracture-matrix connection and migration of hydrocarbon molecules from the shale matrix to the stimulated fracture network.

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“…In addition, they vary significantly from shale to shale (Slatt and O'Brien, 2011). Recently, several papers focused on the identification, characterization, and imaging of a wide range of pore types in gas-producing and/or potential shale resources in North America (Ross and Bustin, 2008;Loucks et al, 2009;Nelson 2009;Ambrose et al, 2010;Passey et al, 2010;Sondergeld et al, 2010;Curtis et al, 2011;Slatt and O'Brien, 2011;Chalmers et al, 2012;Loucks et al, 2012;Milliken et al, 2012;Slatt et al, 2013), China (Wei & Qin, 2013;Wang et al, 2013), and Europe (Bernard et al, 2012;Klaver et al, 2012). Clearly, a consensus is emerging that the matrix porosity, organic porosity, and micro-fracture porosity are the prevalent types of porosity in shale gas resources (Wang and Reed, 2009;Slatt and O'Brian, 2011;Loucks et al, 2012).…”

Section: Introductionmentioning

confidence: 99%

Quantitative and Qualitative Evaluation of Micro-Porosity in Qusaiba Hot Shale, Saudi Arabia

Abouelresh

2015

Unconventional Resources Technology Conference

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The Lower Silurian Qusaiba Hot Shale (QHS) has long been known as the major source rock in the Paleozoic petroleum system of Saudi Arabia. However, during the last few years, special focus has been directed toward this formation as a candidate for unconventional shale gas reservoir due to its high organic content, laterally extensive occurrence and reasonable vertical thickness. Yet, one of the main challenges is the lack of understanding of the size, distribution, and types of the micro-porosity systems in this organic-rich shale. This work provides an integrated analysis of the micro-porosity in QHS.Comprehensive sequential analysis includes thin section petrography, X-ray diffraction, scanning electron microscope (SEM) imaging, and nuclear magnetic resonance (NMR) analysis. In particular, these techniques were used to characterize the micro-porosity systems of six subsurface core samples of QHS from the Rub'al-Khali basin, Saudi Arabia.The investigated samples consist mainly of dark to light grey, silty, flaky-to-subflaky, micaceous, non-calcareous, highly-cemented shales. Three lithofacies have been identified from the petrographic analysis of QHS: silica-rich, clay-rich, and mica-rich mudstone facies. Micro-porosity in the studied samples reflects high diversity in both pore size and type. The micro-porosity fraction of the investigated shale comes in three major groups according to origin: (1) inorganic micro-porosity, which is commonly associated with the inorganic components of QHS (Silica-Mica -clay and pyrite mineral grains), particularly due to the random edge-face clay and mica flake orientation as well as the dissolution and degradation of pyrite framboids; (2) organic-related micro-porosity which mainly occurs inside the organic particles as a result of thermal maturity; and (3) microfracture-related micro-porosity.Low field NMR has been used to examine the T1 and T2 relaxation times. The total porosity measured by T1 for the three identified lithofacies ranges between 6.6 and 0.9 p.u., with the highest values corresponding to the mica-rich mudstone facies (6.6 and 6.5) and the lowest values corresponding to the silica-rich mudstone facies (0.9 and 1.4). As for the clay-rich mudstone facies, it had an intermediate porosity (3.2-3.3). The presence of bimodal porosity size in QHS is clearly reflected by the T1 results. NMR results also showed that organic-related porosity which represented by the primary relaxation peak is greater than the inorganic-related porosity, which is represented by the secondary relaxation peak.

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Pores, Spores, Pollen and Pellets: Small, but Significant Constituents of Resource Shales

Cited by 8 publications

References 15 publications

Low nanopore connectivity limits gas production in Barnett formation

Low nanopore connectivity limits gas production in Barnett formation

Pore structure and tracer migration behavior of typical American and Chinese shales

Quantitative and Qualitative Evaluation of Micro-Porosity in Qusaiba Hot Shale, Saudi Arabia

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