Grain size vs. permeability and porosity: 24-Pet-B4 Well Testing, December 2016, Q21

Question 21 of 22: Grain size vs. permeability and porosity

Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)

Notes on this paper

EGBC National Exam — Petroleum Engineering, 2016-Dec. 3 hours duration; closed book. This sitting's own cover page reads “98-Pet-B4, Petroleum Geology” and every question is descriptive/interpretive petroleum geology (source rocks, hydrocarbon chemistry, oil sands and tight-oil development, shale gas and coalbed methane, carbonate traps, siliciclastic/deltaic traps) – no well-test pressure-transient content anywhere. Five (5) of the paper's six 20-mark sections are marked (NOTES item 5); all six are solved in full below so this set also serves as a complete study reference. This sitting's Section 6 is a Siliciclastic Traps section (wave-dominated delta, grain-size–permeability–porosity relations, sandstone diagenesis). The paper is entirely qualitative (draw/describe/define/list), with no numeric given data anywhere.

Reference texts: Selley, R.C. & Sonnenberg, S., Elements of Petroleum Geology, 3rd ed., Academic Press (source rocks, migration, traps, carbonate systems, oil sands); Tissot, B.P. & Welte, D.H., Petroleum Formation and Occurrence, 2nd ed., Springer (kerogen typing, maceral groups, catagenesis); Boggs, S. Jr., Petrology of Sedimentary Rocks, 2nd ed., Cambridge (source-rock, deltaic and carbonate lithofacies; sandstone diagenesis); James, N.P. & Jones, B., Origin of Carbonate Sedimentary Rocks, Wiley-Blackwell (carbonate platform/ramp/sabkha facies models); Butler, R.M., Thermal Recovery of Oil and Gas, Prentice Hall, 1991 (SAGD, CSS, oil sands thermal recovery); Green, D.W. & Willhite, G.P., Enhanced Oil Recovery, SPE Textbook Series Vol. 6 (thermal EOR mechanisms); Law, B.E. & Curtis, J.B., “Introduction to Unconventional Petroleum Systems,” AAPG Bulletin 86, 2002 (shale gas, tight gas, coalbed methane); Lee, W.J. & Wattenbarger, R.A., Gas Reservoir Engineering, SPE Textbook Vol. 5 (unconventional gas reservoir characterization).

Section 6, Q6-2: Grain size vs. permeability and porosity (2 marks)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

For a given, roughly constant sorting, both permeability and porosity increase with increasing grain size, but permeability increases far more steeply (over several orders of magnitude from clay/silt to coarse sand/gravel) than porosity, which increases only modestly (typically from ≈35% to ≈45–50% across the same range).

Both curves rise with increasing grain size, but at very different rates: porosity depends mainly on grain packing and sorting, so it increases only modestly with grain size for a well-sorted sediment (roughly 35–50% across the full clay-to-gravel range), whereas permeability depends on pore-throat size (which scales with the square of grain/pore-throat radius, per the Kozeny–Carman relation), so it increases by several orders of magnitude – from effectively impermeable clay/shale (<0.001 mD) to highly permeable clean gravel/coarse sand (>1,000 mD) – over the same grain-size range.

Topic. Grain size as the dominant, but not sole, control on porosity vs. permeability.

Key relations. Kozeny–Carman: k ∝ φ³d²/(1−φ)², where d is a characteristic grain/pore-throat diameter – permeability's quadratic dependence on pore-throat size (itself set by grain size) is why k responds so much more steeply than φ does.

Why this works. Porosity is a volumetric (geometric packing) property, essentially scale-independent for a given sorting and packing arrangement, while permeability is a flow-conductance property that depends on the actual size of the connecting throats between pores – doubling grain size roughly quadruples typical pore-throat size, and Poiseuille-type flow through a wider throat scales with (throat radius)⁴, compounding into the dramatic permeability increase seen here.

Common pitfall. Assuming high porosity always implies high permeability – a well-sorted lagoonal or prodelta mud (Q1-3, Q6-1) can have high total porosity yet extremely low permeability because its pore throats, though numerous, are individually far too small to transmit fluid readily.

Source. Boggs, Petrology of Sedimentary Rocks, ch.3 (porosity/permeability fundamentals); Selley & Sonnenberg, ch.3.