Stress

Formation Sand Control

Physical Properties
of Proppants

Laboratory Testing
of Proppants

Conductivity Cell
Measurements

Crush Testing

Gel Damage

Stress Cycling

Mesh Table

Physical Properties of Proppants

Particle Size

Particle size has a significant impact on proppant pack permeability. Figure 1 shows reference conductivity plots for a variety of different mesh sizes of CarboLite, and Econoprop. Typically, a larger proppant size will result in greater permeability at lower closure stresses. As the closure stress is increased, the effect of particle size on conductivity is reduced due to increased crushing of the larger proppant sizes. Ultimately, as seen in Figure 1 above 10,000 psi, the smaller 20/40 mesh size product will have a larger retained conductivity than 16/20 or 12/18 mesh CarboLite.

Figure 1. Reference Long-Term Fracture Conductivities

Although a large proppant size might give more conductivity it also requires a larger fracture width to enable it to be effectively transported without “bridging out” during the treatment. A “bridge out” condition occurs when proppant is prevented from being transported further down the fracture due to the width limitation of the fracture. This is typically thought to occur when the maximum proppant particle size is greater that one-half the created fracture width.

Proppant Concentration

The proppant concentration refers to the mass of proppant per unit area of fracture placed by the treatment. It is usually expressed in lb/ft2 or kg/m2. Most laboratory testing is conducted between hard sandstone core samples at a concentration of 2 lb/ft2 (10 kg/m2). Laboratory testing has shown that 1/2lb/ft2 of proppant may be lost to embedment in the fracture face of moderately hard formations. In soft frac-pack formations this can be much higher, in particular for the case of chalk formations which may actually extrude into the proppant pack.

Figure 2 shows the fairly linear impact of proppant concentration upon fracture conductivity when tested between hard sandstone platens exhibiting minor embedment. The higher the concentration, the higher the reference fracture conductivity.

Figure 2. Reference Long-Term Fracture Conductivities of Various
Concentrations of CarboLite

Proppant Strength

Fracturing of deeper formations requires the use of stronger proppant materials. Sintered bauxite, the first ceramic proppant, contains corundum, one of the hardest materials known. As a result, it has application in the deepest of wells. Light weight ceramic (LWC) proppants have a large mullite component, another hard alumina-silicate material. Also, when ceramic proppants fail, they do not shatter in the same manner as a sand proppant, but instead cleave into discrete shards still capable of providing significant conductivity. Figure 3 shows a comparison of the main types of 20/40 mesh proppants: sand, resin coated sand, light weight and heavyweight ceramic proppants.

Figure 3. Reference Long-Term Fracture Conductivities of Various
Concentrations of CarboLite

Proppant Density

Table 1 lists the apparent specific gravity (ASG) and bulk density (BD) of the standard proppants. The BD describes the mass of proppant that fills a unit volume, and includes both proppant and porosity void volume. The ASG represents the apparent density of a single pellet and although it does include any internal porosity of the proppant particle, it excludes the porosity between grains. Generally, the ASG is used to estimate proppant settling times, while the BD is used to estimate transportation volumes and actual achieved volume of the fracture. Both properties are measured without closure stress, so the bulk density will increase substantially if the proppants crush or if pack rearrangement results in loss of porosity.

Table 1. Bulk Density and Apparent Specific Gravity of Common Proppants

It is important to note that although proppant is typically purchased by mass, the benefit of a proppant is based on its volume. It is apparent that a fracture containing 100,000 pounds of a light weight ceramic will occupy more volume than a fracture containing 100,000 pounds of bauxite. For tip screenout designs, the fracture may be packed full, to retain the entire hydraulic width generated, regardless of the proppant chosen. However, for typical fractures that are allowed to close on the proppant, the density of the proppant will significantly impact the achieved fracture width. For a given proppant concentration in the fracture there will be a proportionate decrease in propped fracture width for a more dense proppant.

Conductivity data are generally measured in the laboratory at 2 lb/ft² proppant loading and during testing the fractures containing denser proppants have smaller fracture widths. It is incorrect to compare proppants based on measured reference permeability and Beta factor data alone as it is necessary to include the density effects of varying propped width and fluid velocity. This is done by comparing products based on reference fracture conductivity (k_f*w) and non-darcy inertial losses (Brv²) in the fracture. The magnitude of the impact, due solely to density effects is shown in Table 2.

Table 2. Density Effects on Flow Properties (Relative to Sand)

Table 2 shows that at low stresses a bauxite propped fracture has only 78% the width
of a sand propped fracture when both contain the same mass per unit area (lb/ft2) of
proppant. This results in a fluid velocity in the fracture that is 28% higher and for a given permeability will result in 28% greater viscous pressure drop. Similarly, since the inertial pressure drop is related to the square of fluid velocity, density effects will result in the bauxite-filled fracture having 64% greater inertial pressure loss if the Beta factors are equal. This is somewhat offset in deep gas wells where lower strength proppants crush to an extent where fracture width is reduced and/or the Beta may be an order of magnitude higher than bauxite proppant.

This raises an issue regarding proppant substitution in fractures when the fracture is allowed to close on slurry. Often, an operator will attempt to compare proppants by pumping identical fracture designs without changing the slurry concentrations or ramp schedule. However, if creation of the identical fracture geometry is the goal, it is necessary to pump higher concentrations (typically measured in pounds/gallon) of the more dense products. If this correction is not made, the comparison of "similar" fracture designs is often compromised, as fracture width will be narrower with the more dense proppants.

Lighter proppants can provide clear benefits due to greater retained fracture widths. The increased fracture width of a LWC compared to an ISP or bauxite proppant typically results in the highest effective fracture conductivity for a given proppant loading in all but the deepest stress environments where the effective stress on the proppant is above 10,000 psi. For larger proppant sizes (e.g. 16/20 or 12/18 products) crush is increased as the stress is concentrated upon fewer proppant grains, so it may be advantageous to switch to higher strength products at stresses as low as 8000 psi.

Proppant Grain Shape

Roundness and sphericity are important properties because they impact the porosity and packing of the proppant pack. Grain roundness is a measure of the relative sharpness of grain corners, and particle sphericity is a measure of how closely the grain approaches the shape of a sphere. Improved roundness and sphericity will enable greater porosity (and permeability) than a pack made up of an angular proppant after stress is applied and grain rotation occurs. Also, at higher closure stresses, the rounder particles will distribute the load better and have less crush and fines production.

Proppant manufacturers reference the Krumbein shape factor for roundness and sphericity. Figure 4 shows the roundness and sphericity chart. LWC proppants have a roundness and sphericity of 0.9 while sand proppant has typical values of 0.7.

Figure 4. Krumbein Roundness and Sphericity