Brittleness and the Sweet Spot

It Has to Frac

A shale holds its gas in pores far too tight to flow on their own, so it is produced by hydraulic fracturing: the rock has to break, and the break has to stay open. Whether a rock breaks or merely deforms is its brittleness, and clay is what takes it away. Rickman et al. (2008), writing on shale petrophysics for stimulation design, put it this way: "Ductile shale is not a good reservoir because the formation will want to heal any natural or hydraulic fractures." The mineral index below is the simplest way to put a number on that.

Counting the Brittle Minerals

The simplest measure counts minerals. Take the rock's weight as quartz $Q$, calcite $Cal$, dolomite $Dol$, clay $Cl$ and organic carbon $TOC$. The form the figure opens with calls quartz and carbonate brittle and puts everything weighed in the total,

a share of the weight between 0 and 1, which the figure prints as a per cent.

Which minerals count as brittle, and what stands in the total, is a choice, and published forms make it differently. Mews, Alhubail and Barati (2019), in a review of brittleness correlations, print three that the figure holds. The one above they give for Glorioso and Rattia (2012); that paper and Wang and Gale's (2009) were not read here, and their forms are as Mews et al. print them. They credit to Jarvie et al. (2007) quartz alone over quartz, carbonate and clay, with no organic carbon in the total (Jarvie et al.'s paper itself prints no formula; it reports that the best Barnett production comes from zones of 45 % quartz and only 27 % clay), and they give the form of Wang and Gale (2009), which counts dolomite as brittle and leaves calcite in the total only. For the figure's default rock, the core bed of the Ogbon Shale's upper rich zone (Section 16.6), a mineral part that is 30 % clay with 4.0 wt% of organic carbon, and a carbonate that is 30 % of the non-clay minerals and is calcite, the three give 67 %, 49 % and 47 %. Call the carbonate dolomite and they give 67 %, 49 % and 67 %. The rock has not changed, and its position against a 50 % line has: a published index is a definition, so a figure that quotes one has to say whose.

The Same Idea from the Moduli

Rickman et al. (2008) asked the same question of the elastic moduli. A brittle rock is stiff, a high Young's modulus $E$, and does not bulge sideways under load, a low Poisson's ratio $\nu$. Their index stretches each over a range and averages the two,

In the copy of the paper read for this section the equations run the modulus term from 1 to 8 and the Poisson's ratio term from 0.4 to 0.15, and no unit is named for the modulus; the book reads it as millions of psi (Mpsi). Mews et al. describe the bounds as the smallest and largest values in the formation of interest, so they belong to the rock Rickman et al. worked in, and their moduli are static ones. The figure's moduli are a model's, the kind a sonic log gives, so a stiff rock can lie beyond the top of the range: the index is then held at 100 %, and the figure says so.

The figure builds the moduli from the Rock Physics tools: a Hill average of the minerals, a frame softened by the porosity, and the anchor's gas put in the pores by Gassmann's equation. For the default rock Young's modulus is 7.1 Mpsi and Poisson's ratio 0.235, an elastic index of 77 % beside the mineral index of 67 %. The two are not one measure. Raise the porosity from 6 % to 12 % and the mineral index stays at 67 % while the pores soften the frame and the elastic index falls to 68 %. Make the carbonate 90 % of the non-clay minerals and the mineral index stays at 67 %, since carbonate counts as brittle, while the elastic index falls from 77 % to 56 %: calcite's Poisson's ratio in the Rock Physics table is 0.32 against quartz's 0.06, and the moduli do not rank carbonate with quartz. Kerogen is left out of the moduli, because the book holds no sourced kerogen modulus.

Two Things at Once

Brittleness alone is not a target, and neither is richness. A rich rock holds gas, but if it is clay-heavy it will not stay fractured; a brittle, lean rock fracs well but has little to give. The sweet spot is the rock over both lines, the upper right of a plot of organic carbon against brittleness. The figure draws its lines at a mineral index of 50 % and 2.0 wt% of organic carbon. They are the book's own, the same in Figure 16.6, set to show the idea; they are not published criteria, and a real play sets its own.

Try both ways out. At 30 % clay the default rock is a sweet spot. At 55 % clay its mineral index is 43 %, under the line, though its organic carbon has not moved: for this mix the index stays over the line only while the clay is under 48 %. Take the organic carbon from 4.0 wt% to 1.0 wt% instead and the index rises to 69 %, but the rock falls under the richness line. Either way it is half a play.

What the Index Does Not Say

Brittleness is not fracability. Bai (2016) argues that the two are different properties: brittleness describes how much a rock deforms plastically before it fails, while fracability describes how easily it fails, which also depends on the confinement and the rock's strength, so a brittle formation can be stronger and no easier to fracture. A mineral index is also a statement about composition only; it knows nothing of the stresses, the natural fractures or the pores. The mineral weights themselves come from the logs by the multimineral solver of Section 12.5, and the organic carbon from the Passey method of Section 16.2. Section 16.6 lays richness and brittleness along one shale on a log and flags the depths where both pass.

References

• Bai, M. (2016). Why are brittleness and fracability not equivalent in designing hydraulic fracturing in tight shale gas reservoirs. Petroleum, 2(1), 1-19. doi:10.1016/j.petlm.2016.01.001.
• Jarvie, D. M., Hill, R. J., Ruble, T. E. and Pollastro, R. M. (2007). Unconventional shale-gas systems: the Mississippian Barnett Shale of north-central Texas as one model for thermogenic shale-gas assessment. AAPG Bulletin, 91(4), 475-499. doi:10.1306/12190606068.
• Mews, K. S., Alhubail, M. M. and Barati, R. G. (2019). A review of brittleness index correlations for unconventional tight and ultra-tight reservoirs. Geosciences, 9(7), 319. doi:10.3390/geosciences9070319.
• Rickman, R., Mullen, M., Petre, E., Grieser, B. and Kundert, D. (2008). A practical use of shale petrophysics for stimulation design optimization: all shale plays are not clones of the Barnett shale. SPE Annual Technical Conference and Exhibition. SPE 115258.