Brittleness, shale

Select a tungsten carbide insert bit with a minimum offset and projectile or conical inserts when drilling limestone, brittle shale, nonporous dolomite and broken formations. Use bit type 6-3 to 7-3. 

The terms brittle and ductile are used in the literature in different ways. Here, the following definitions are adopted a brittle shale will increase its permeability by developing dilatant fractures (Fig. 2), whereas a ductile shale is able to undergo plastic deformation without increasing its permeability (it will... 

The third essential requirement for a commercial accumulation of petroleum is a cap rock or seal. This is a sedimentary stratum that immediately overlies the reservoir and inhibits further upward movement. A cap rock need have only one property It must be impermeable. It can have porosity, and may indeed even contain petroleum, but it must not permit fluid to move through it. Theoretically atty impermeable rock may serve as a seal. In practice it is the shales and evaporites that provide most examples. Shales are probably the commonest, but evaporites are the more effective. We saw earlier how mud is compacted during burial into mudstone, or shale. These rocks are commonly porous, but because of the narrow diameter of the pore tluoats, they have negligible permeability. Thus shales generally make excellent seals to stop petroleum migration. When strata are folded or faulted, however, brittle shales may fracture. As described earlier, iiactures enhance permeability most dramatically. In such instances, petroleum may leak from an underlying reservoir and ultimately escape to the surface of the earth. 

In shales, mud weight is usually sufficient to balance formation stress, as these wells are usually stable. With water based mud, chemical differences can cause interactions between mud and shale that lead to softening of the native rock. Highly fractured, dry, brittle shales can be extremely unstable (leading to mechanical problems). 

The shale s ability to fracture is of special interest (King, 2010). Mechanical behaviour of shale is important in hydraulic fracture stimulation where brittleness is an indication of the ability to not only initiate and place a hydraulic fracture, but also maintain its long-term hydraulic conductivity (Harris et al., 2011). Brittle shale may be more easily fractured, opening flow paths. Ductile shales may require more proppants (King, 2010). Two types of criterion are used ... 

The parameters for the model were originally evaluated for oil shale, a material for which substantial fracture stress and fragment size data depending on strain rate were available (see Fig. 8.11). In the case of a less well-characterized brittle material, the parameters may be inferred from the shear-wave velocity and a dynamic fracture or spall stress at a known strain rate. In particular, is approximately one-third the shear-wave velocity, m has been shown to be about 6 for various brittle materials (Grady and Lipkin, 1980), and k can then be determined from a known dynamic fracture stress using an analytic solution of (8.65), (8.66) and (8.68) in one dimension for constant strain rate. 

IRATI FORMATION, by far the largest, which extends from the state of Sao Paulo to the frontier with Uruguai. The very conservative numbers shown in Table II are meant to represent the measured reserves processable by the Petrosix process developed by Petrobras to retort oil shales. These restrictions are mainly related to min ing variables, carbon content, brittleness etc. Indeed, they repre sent only a small fraction (the border) of the Parana Basin. 

The CDM has two additional features that allow it to represent fracture in rocks. First, there is a brittle/ductile transition pressure. Above this pressure, the rock behaves as an elastic/plastic ductile solid, the failure surface is independent of the level of damage, and the damage is not allowed to increase, even if the failure surface is exceeded. Second, the CDM allows for non-vanishing plastic volume strain to approximate the dilatancy observed in certain laboratory experiments on oil shale. 

Damage mechanics principles have been applied extensively for well stability problems, especially for hard, brittle and fractured shale rocks. Constitutive models using damage concept, coupled with hydro-chemical swelling models, have been developed based on experimental results, and applied for design of drilling fluid density, combined with FEM methods (Liu, 1995). 

Brittle fractured Q/l shales, high salinity pore fluids, low porosities,... 

Little information is available about the long term behavior of the filler substances when immersed in sea water and under irradiation. Bitumen is known to become brittle and crack below room temperature. As Furfurol(F) is a patented material, information about its composition and behavior was not available to the Group, and long term behavior is, in any case, difficult to predict. It is known to be a mixture of the following constituents epoxy resin, amine type solidifier, mineral filler, shale distillate and furfuryl alcohol [26]. An effective lifetime of SOO years is quoted in the White Book [1] for this material. 

When the unconsolidated, plastic clays are deposited in a sedimentary basin as pelitic sediments, they are transformed to rigid, brittle pelitic rocks by diagenesis. Claystones and mudstones are non-laminated pelitic rocks, and shales are laminated pelitic rocks. During diagenesis, some new clay minerals may appear through transformation of pre-existing clay minerals and neoformation of some non-clay silicate minerals. 

Fireclay brick is the second most frequently used brick. It is usually described as meeting type H in ASTM C-279. They contain a higher proportion of alumina and lower percentages of silica and iron than do shale brick. Fireclay bricks have a higher absorption rate than shale bricks, although some manufacturers will provide a denser brick that will meet type L for absorption. These brick are usually selected for outdoor exposures where rapid thermal changes occur since they are less brittle than the shale brick. Since they have a low iron content, they are used in process equipment where this characteristic is important in maintaining product purity. 

In general, red shale bricks provide a lower absorption than fireclay bricks but they are more brittle. These bricks are best applied in areas where lowest absorption masonry is desired but where thermal shock is not a major factor. 

New approaches to old problems are emphasized. For example, how do mathematical aerodynamicists turned petroleum engineers view the physical world Stare up the back end of a rocket lifting off Is that a fuselage with stabilizer fins, or is it a circular wellbore with radial fractures Pry open the maintenance box of your typical jet engine Are those cascades of airfoil blades, or are they distributions of stochastic shales Can the solutions that describe brittle failure be repackaged to model arrays of fractures, say, the natural fracture systems that spur horizontal drilling Very often, the problems (inaccurately) crunched by our fastest computers can be solved (accurately) using closed-form analytical solutions found in other scientific disciplines. 

Failure/Strength Properties 278 7.6 Some Remarks About Shale Brittleness 298... 

Criterion based on shale mineralogy Sondergeld et al. (2010) investigated Barnett shale and noted that most brittle section of Barnett shales have abundant quartz, the least brittle have abundant clays,. Studies from Jarvie et al. (2007), Rickman et al. (2008, 2009), and Sondergeld et al. (2010) result in definitirm of a brittleness index in terms of volume fractions ... 

Criterion based on Young s modulus and Poisson s ratio (derived from elastic wave velocities and density) Grieser and Bray (2007), Rickman et al. (2008), and Sondergeld et al. (2010) characterize shale with E > 34.5 MPa and a < 0.25 as brittle and derived a plot for separation of the two types (Fig. 7.29). 

---