Section 5 Proppants

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Hydraulic FracturingSection 5 Proppants

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5. Proppants

The word proppant comes from the abbreviation of two words - propping agent. Proppantsare granular materials, which are placed inside the fracture in order to prop the fracture

open as the pressure falls below closure. The conductivity of the fracture is directly related tothe quantity of proppant within the fracture, the type of proppant, and the size of the proppantgrains.

The purpose of hydraulic fracturing is to place the right amount of proppant in the right place.When this is done correctly, the well is effectively stimulated.

5.1 Proppant Pack Permeabi l i ty and Fracture Conduc t iv i ty

As discussed in section 1, the fracture conductivity is the product of the proppant packpermeability and the width of the fracture. In other words, it is a function of the type of materialholding the fracture open, and the amount of this material in the fracture.

The permeability of the proppant pack is controlled by several factors:-

i) Proppant Substrate. The material that the proppant is made from obviously has abig effect on the permeability of the proppant pack. Some materials are stronger thanothers, and are better able to withstand the enormous forces trying to crush theproppant as the fracture closes. The softer the material, the more the proppant grainwill deform. This reduces the porosity of the pack and reduces the overall fracturewidth. the more brittle the proppant is, the more likely it is that the proppant willproduce fines as the grains are pushed together in a series of point to point contacts.Basically, the stronger the material, the better the proppant.

ii) Proppant Grain Size Distribution. A normal rock has a wide variety of grain sizes,

depending upon how well sorted it is. However, in general, a sandstone will be amixture of small, medium and large grains. This acts to reduce the permeability andporosity, as the smaller grains will occupy the pore spaces between the larger grains,and will also tend to plig up the pore throats. If, however, a set of grains are of almostidentical size, then there are no fines to block up pore spaces and throats, so thatporosity (and hence permeability) are maximised.

This is why proppants are generally sold within a specific grain size distribution. Thisuniformity of grain size is one of the main reasons why proppant is usually severalorders of magnitude more permeable than the formation, and also one of the mainreasons why so much effort is spent in ensuring this uniformity of size.

Proppants are supplied within a specific grain size range. This grain size refers to the

size of sieve used to sort the proppant. For instance, 20/40 size means that the vastmajority of the proppant will fit through a size 20 sieve (20 holes per square inch), butwill not fit through a size 40 sieve (40 holes per square inch). Common proppant sizesare 8/12, 12/20, 16/30, 20/40 and 40/60, although theoretically any combination ofsizes can be produced.

iii) Average Proppant Grain Size. Generally, the bigger the average proppant grainsize, the higher the permeability of the proppant (provided the grain size distribution isreasonably uniform). This is because larger grains produce larger pore spaces andpore throats, allowing an increased flow rate for a similar porosity. However, thelarger grains are more susceptible to producing permeability reducing fines, than thesmaller grain sizes. This is because larger grains distribute the closure pressureacross fewer grain-to-grain points of contact, and so the point contact loads tend tobe greater.


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iv) Sphericity and Roundness. These quantities define how spherical the proppantgrains are and how many sudden, sharp edges the grains have. Obviously, thesmoother and more spherical the proppant grain is, the higher the pack permeability.There are standard API procedures for checking these quantities, but unfortunatelythey rely on some subjective analysis. Consequently, it is often difficult to see a cleartrend between one proppant type and another. However, in general, artificial

proppants will have better sphericity and roundness than naturally occurring types.

v) Frac Fluid Quality. The amount of residue left by the fracturing fluid can also have abig influence on the permeability of the proppant pack. In order to assess the effect ofthese fluids, a quantity called Regained Permeability is measured. Put simply, asample of the proppant is put into a load sell, and subjected to a closure pressure, atan elevated temperature. A standard, non-damaging fluid is then flowed through thetest cell. By analysing the pressure drop and flow rate, the permeability of the packcan be calculated. Next, the frac fluid is flowed through the test cell, and allowed toremain there for a specific time, during which it is designed to break. Once the fluidhas broken, the permeability of the pack is measured again, by the same method asbefore. The two permeabilities are compared and the result (the regainedpermeability) is given as the percentage of the original permeability that remains after

the test.

Proppant packs can lose significant proportions of their permeability to fluid damage.Cheap, poorly designed fluids can cause regained permeabilities to be as low as only30% or even less, whereas the state-of-the-art fluids can produce values in excess of90%.

vi) Closure Stress. As the proppant is crushed by the closure of the formation, it willstart to produce fines. As discussed above, these fines will reduce the permeability ofthe pack. The stronger the proppant, the fewer fines are produced, but all proppanttypes experience a decrease in permeability as closure stress increases, thedifference between types being the extent to which this occurs. In addition, mostproppants also have a maximum stress, above which whole-scale disintegration of

the proppant substrate starts to occur, rather than simple fines production. At thispoint, pack permeability falls dramatically.

It should be noted that the reservoir pressure has an influence on the closure stressexperienced by the proppant. This phenomenon is discussed in greater detail in thenext section of this manual. However, as the reservoir pressure increases, the closurepressure decreases and visa versa. This means that as the reservoir pressuredeclines with time (and in some reservoirs especially gas this can be verydramatic), the closure stress experienced by the proppant actually increases.Additionally, there can be localised areas of low reservoir pressure (such as near thewellbore during drawdown) where once again the proppant experiences higherclosure pressure. This potential increase in stress with the life of the well must beallowed for when selecting a proppant.

vii) Non-Darcy Flow. This effect will be discussed on more detail in section 7. However,as the flow rate through the proppant pack increases, the pressure drop will increaseat a rate faster than predicted by Dacrys law. This is due to the effects of inertialenergy loses, as the fluid rapidly changes direction as it moves through the porespaces. As the fluid velocity increases, the pressure drop due to inertial flow effectsincreases with the square of the velocity. So at low flow rates, (such as in a reservoirrock), non-Darcy effects can safely be ignored, whilst at high rates (such as in aproppant pack), the proppant permeability has to be reduced to reflect this effect.

viii) Multi-Phase Flow. Multi-phase flow has a similar effect upon proppant packpermeability as it does on formation permeability. It reduces it, by an amount that isdependent upon the absolute permeability, and the relative saturation of each phase.

As it is very rare for a reservoir to produce a single phase (which the exception ofsome gas reservoirs), it is also very rare for proppant to conduct only a single phase.


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Therefore, the actual effective permeability of the proppant pack may be significantlyless than the published data, which is generally produced for single-phase flow only(although this situation is improving).

5.2 Propp ant Selectio n

As illustrated in section 5.1, there are a substantial number of variable which must be takeninto account when selecting proppant. However, in many cases the selection process hasbeen simplified.

All proppant suppliers and manufactures publish data for pack permeability against closurestress, for all their proppant types and grain size distributions. Provided the closure stress isknown (taking into account any subsequent loss in reservoir pressure), the absolutepermeability of the proppant pack can be easily found. This eliminates the need for the FracEngineer to hold data on sphericity, roundness, crush resistance, grain size distribution,substrate material etc. Simply look up the proppant you are interested in, and see what thepermeability is for a given closure stress.

Most fracture simulators already have this data for most major proppant types. This allows thesimulator to predict the fracture conductivity for most given proppant/closure stresscombinations. Usually, there is also a proppant damage factor, which allows the user tosimulate the regained permeability effects of the fracturing fluids.

Some - but not all - fracture simulators will also model the effects of non-Darcy flow, showinga decrease in effective permeability as production rate rises.

However, no current fracture simulators allow for the effects of multi-phase flow. Data on thishas been published by a few sources, the most notable of these being the Stim-LabConsortiums PredKsoftware.

Table 5.2a gives guidelines as the maximum closure stress each of the major proppant typescan withstand, before substrate failure begins to occur. Obviously, these limits are verygeneralised, and are highly dependent upon factors such as grain size and the quality of themanufacturing process and/or source of sand. More detailed information is available from themanufacturers or in the references;

Type MaximumClosure

Stress, psi

ProductExample

Frac Sand 5,000 Brady, Ottawa Colorado

Low Density Ceramics 9,000 CarboEconoprop, CarboLite,ValueProp

Intermediate Density Ceramics 12,000 CarboProp, InterProp

Sintered Bauxite 14,000 CarboHSP, Bauxite

Table 5.2a Generalised maximum closure stresses for the main proppant types.

Important Note

The quality of the proppant, and the subsequent conductivity of the fracture, has a biggereffect on post treatment production than virtually anything else under the Frac Engineerscontrol. In most cases, an economy made on proppant selection is a false economy. Forinstance, although low-density ceramics cost twice as much as frac sand, they have three tofour times the effective pack permeability, even at low closure stresses, due to their highsphericity and roundness.


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References

Technical Data Interactive CD ROM, Carbo Ceramics Inc, 2000 onwards.

PredKSoftware, Stim-Lab Consortium, 1999 onwards

Vincent, M.C., Pearson, C.M., and Kullman, J.: Non-Darcy and Multiphase Flow in ProppedHydraulic Fractures: Case Studies Illustrate the Dramatic Effect on Well Productivity, paperSPE 54630, presented at the SPE Annual Technical Conference and Exhibition, Houston, Oct1999.

API Recommended Practice 56 Testing Sand Used in Hydraulic Fracturing Operations, 2nd

Edition, American Petroleum Institute, December 1995

API Recommended Practice 58 Recommended Practices for Testing High StrengthProppants Used in Hydraulic Fracturing Operations, 2

ndEdition, American Petroleum Institute,

December 1995.

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