Page 160 - A Practical Companion to Reservoir Stimulation

PRACTICAL COMPANION TO RESERVOIR STIMULATION

expected stresses and can be readily converted to conductiv- P-2.3: Proppant Size and Conductivity
ity. The latter is simply the product of the proppant-pack At lower closure stresses, higher fracture conductivities can
permeability and propped width. be obtained by simply increasing the mesh size of the proppant.
Permeability reduction at higher stresses is attributed to This larger flow capacity is a result of the corresponding
the dislodging of fragments from particles (thus reducing larger pore sizes between the grains. However, as the closure
their sphericity), the crushing of other particles and the partial stress increases, the larger mesh sizes begin to lose their
plugging of the flow path by the created fines. advantage. The rate of permeability reduction is always higher
Short-time measurements fail to account for long-term in larger mesh sizes because larger sizes have lower strength
effects resulting from exposure to given stresses. Fatigue of and their resistance to higher stresses is impaired. This causes
the particles can be quantified through an extended time test a more pronounced loss in sphericity and increased fines
where proppant is kept at the expected stress value for up to generation. As a result, the permeability of the larger proppants
200 hr. Figure P- 1 is a representative test showing on the left is reduced to a much greater degree. It is conceivable that
the closure stress impact on proppant-pack permeability and above a certain stress level a larger mesh size may, in fact,
fracture conductivity (2 lb/ft2). For example, at 2000 psi the exhibit a lower permeability than a similar proppant of smaller
permeability is 600,000 md and the conductivity is 2500 md- size. The permeability curves in Fig. P-2 show the stress-
ft; at 5000 psi these values are 180,000 md and 1000 md-ft, sensitive values of three different sizes of proppants and the
respectively. However, as can be seen on the right, from the crossover of their permeability at a high stress.
continuation of the figure at an extended time exposure (at
5000 psi), the values level off after 50 hr and are substantially P-2.4: Proppant Slurry
lower than the short-time measurements. The permeability is The easiest way to improve fracture conductivity is to in-
73,000 md, and the conductivity is 430 md-ft. This major crease the slurry concentration. Higher concentrations result
reduction would have a significant effect on the forecast of in wider propped fractures and therefore in improved con-
fractured well performance. ducib&j. Slumy cmcfimiiws skou\d be designed to pre-
In addition, proppant embedment results in width reduc- vent the proppant concentration in the fracture from falling
tion and thus fracture conductivity reduction. The previously below 0.5 lb/ft’ and should be above 1 lb/ft2 whenever pos-
described apparatus is used, but instead of steel plates, the sible. To obtain a 1 lb/ft2 concentration throughout the frac-
proppant is enclosed by reservoir rock material of medium ture, slurry concentrations need to approach or even exceed
hardness. A common rock used for this purpose is Ohio 10 ppga. Slurry concentrations of this magnitude often lead to
Sandstone with a Young’s modulus equal to 6 x lo6 psi. For fears of screenouts. However, as Fig. 9- 16 shows, there is no
example, while long-term fracture conductivity of 20/40 ISP more risk of bridging with a slurry concentration of 20 ppga
at 5000 psi between steel plates is 6200 md-ft, it becomes than there is with one of 5 ppga.
5700 md-ft between the rock plates, an 8% reduction. Need- There are two disadvantages in the placement of large-
less to say, this effect will be far more severe in soft rock such mesh proppants. First, there is an increased risk of premature
as a chalk or at higher closure stresses. Table P-3 contains screenout because of the larger grain size. Hydraulic fracture
fracture conductivity values at 1 Ib/ft2 concentration for widths must be three to four times wider than the diameter of
various proppants and closure stresses, including embed- the proppant to prevent bridging. Based on the average di-
ment. Table P-4 contains the same information for 2 lb/ft2 ameter of sizes listed in Table P-1, a 12-20 mesh proppant
concentrations. requires twice the hydraulic fracture width of a 20-40 mesh
proppant. The other problem with larger mesh sizes is that
their settling rate is greater. Therefore, deep-penetrating
fractures will be more difficult to obtain when using these
proppants.

P-4