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‘Fracturing Sand – Prospective Asian Sources’
- Jajati Nanda, Sr Scientist
- Pallavi Managave, Sr Lab Professional
- Omprakash Pal, Pr Scientist
Analytical Science, Halliburton Technology Center – Pune (India)
Increased hydraulic fracturing activities around the globe have substantially increased the demand for hydraulic fracturing sands. Sands used for hydraulic fracturing must be aligned with the quality standards framed by the American Petroleum Institute (API) and, more recently, the International Organization for Standardization (ISO). White sand from Ottawa & brown sand from the Southeast US are considered best quality sands for hydraulic fracturing. The study was performed on samples from at least ten sources from each Asian country/region. Some samples passed the stringent norms of API and ISO. However this paper represents comparative data of only four of those Asian sands which not only passed the norms set by API and ISO but also represented close characteristics with Ottawa sand.

Hydraulic fracturing is a well stimulation method specially performed on reservoirs with low permeability to increase the flow of hydrocarbons into the wellbore. Specially engineered fracturing fluid is pumped into the pay zone or desired area to be fractured at a rate and pressure high enough to extend and wedge open the fracture hydraulically (Veatch et al 1989). Proppants such as strong grains of sand are added to the fracturing fluid to keep the fracture open after fluid injection has stopped. The amount of proppant used, the manner in which it is carried into the fracture, and properties of the proppant material play a pivotal role in maintaining productivity throughout the life of the well (Martinez et al. 1987). It is estimated that up to 90 per cent of the wells currently operating have been fractured and that, in the future, 60 to 80 per cent of new wells might need to be fractured to remain viable (Kamat et al 2011). All the properties of sand grain proppants, including roundness, size distribution, resistance to crush under the influence of closure stress, grain size distribution, and proppant density, can affect the resultant fracture conductivity. Conductivity of a propped fracture is one of the most important factors that directly affect well productivity, along with the propped fracture area, reservoir permeability, and drainage radius (Montgomery and Steanson 1985).

Selection of a high-quality sand grain type to use as proppant is crucial for a successful hydraulic fracturing treatment. Several global standard proppants are available in the market for hydraulic fracturing stimulation. However, procuring these proppants for remote locations can be challenging in terms of both cost and time. This paper evaluates different proppants from Asian countries to support the local production. Use of these locally available proppants for fracturing treatments can help to save time and reduce production costs. The properties of several 20/40-mesh sands were examined for grain size distribution, proppant strength, quantities of fines and impurities, roundness and sphericity, and proppant density. The purpose of this study was to compare the characteristics of Asian sands with the existing best quality proppants in the market. The following experiments were performed on sand samples in accordance with API RP 56 (1989) and ISO 13503-2 (2006).

Sand Sampling
Sand samples were collected from different global sources, and their properties were compared with Ottawa sand. The samples used are shown in Figure 1.
Experimental Work and Results
Sieve Analysis: Samples were first dried at a temperature of 110 ±5ºC (230 ±9ºF). Suitable sieve sizes (US 16- to 50-mesh) were used to obtain the required information as specified and nested in order of decreasing size of opening, where the pan was placed below the bottom sieve. The sample was placed on the top sieve, and then a lid was placed over the top sieve. The sieves were then agitated using a commercial sieve shaker for 10 min. The weight of material retained was determined on each sieve. The grains passed and retained were calculated by percentage. The sieve nest arranged from top to bottom as 16-, 20-, 30-, 35-, 40-, 50-mesh, and pan. To meet API requirements of a 20/40-mesh proppant, the following criteria must be met:
  • A minimum of 90 per cent of the tested sand sample should fall between the designated sieve sizes, 20 (840 micron) to 40 (419 micron).
  • No more than 0.1 per cent of the total sample should be larger than the first sieve size.
  • No more than 1 per cent of the total sample should be smaller than the last sieve size.
Bulk Density: An empty 100-mL measuring c ylinder was placed on an electronic balance and the weight was recorded. Next, the measuring cylinder was filled with a free-flowing sand sample up to the 100-mL mark. The weight was recorded again and bulk density was calculated using the following equation:

Bulk density = volume of dry sand in grams / volume of dry sand in cc....................Eq 1

High-density proppants are more difficult to suspend in fracturing fluid and transport into the fracture. Fracture width will be narrower using denser proppant. Thus, higher-density proppants require more mass of material to create the same fracture as lower-density proppants (Table 2).

For typical hydraulic fracturing treatments, the density of the proppant will significantly impact the achieved fracture width. Fracture width will be narrower with denser proppant.


Sphericity and Roundness: Par ticle sphericity is a measure of how closely a sand par ticle or grain approaches the shape of a sphere. A per fectly spherical par ticle provides the greatest amount of pore space and minimum resistance for hydrocarbon flow (Gottschling 2005). Grain roundness is a measure of the relative sharpness of grain corners or of grain cur vature. Roundness is measured on the same grain for which sphericity is measured. The most widely used method of determination is visual comparison of images of the samples with the char t developed by Krumbein and Sloss (1963) (Figure 2). Individual grains are obser ved under a high-resolution microscope with 40× magnification. Recommended values of sphericity and roundness should not be less than 0.6 for fracturing sand.
Images of the samples were taken using a microscope, and the individual images were compared with the Krumbein and Sloss char t. Results were obtained from an average study of 20 grains of each sample (Table 3).


Turbidity: Turbidity in water results from the presence of suspended clay, silt, or finely divided inorganic matter. Turbidity is a measure of the optical property of a suspension that results from the scattering and absorbing of light by the particulate matter present. The samples were dispersed in deionised (DI) water and manually shaken to suspend the clay, silt, or finely divided matter. The suspension properties of the samples were measured using a Hach® spectrophotometer at 450 nm as formazin turbidity units (FTU). As recommended by API, the turbidity value for fracturing sand should be less than 250 FTU.


Acid Solubility: Acid solubility signifies the presence of acid-soluble materials in the sample. The materials can be carbonates, feldspars, iron oxide, clays, etc., which can be present in fracturing sand as contaminants. For this test, 5 g of a dry sample was dispersed in 100 mL of an acid mixture (12 per cent hydrochloric (HCl) acid and 3 per cent hydrofluoric (HF) acid). Because of the hazards associated with using HF acid, an equivalent quantity of ammonium bi-fluoride was used instead. The test was performed at 150°F in a water bath for 30 min. Then, the samples were cleaned with DI water, dried, and weighed. The weight loss was reported as acid solubility. As per API recommendat ions, the acid solubility of 20- to 40-mesh fracturing sand should not be more than 2 per cent.

Mineralogy: The mineralogical composition of the samples was tested to determine the presence of undesired compounds, such as carbonates, feldspars, and clay. The samples were ground to pass through a 200-micron sieve, mounted on XRD sample holders, and run to produce diffraction using PANalytical X’PERT PRO defraction system in the 2? range of 4 to 65°. The XRD patterns were analysed with the Rietveld technique using data available from the International Centre for Diffraction Data (ICDD) library. The powdered samples were also analysed by XRF using PANalytical Epsilon 3 software for elemental analysis as oxides.

Loss on Ignition: Known quantities of samples were exposed to a temperature of 1000°C in a muffle furnace for 1 hr to determine the temperature stability of the fracturing sands. Any weight loss was reported as a loss on ignition caused by the presence of carbonates, organic compounds, etc.



Crush Resistance: Proppant breakage and fines generation causes decreased pack conductivity because generated fines can flow with hydrocarbons and plug the flow channels. A crush resistance test is conducted to determine the amount of fines that will be generated when a certain pressure is exerted on the sand sample. This test provides an indication of the stress level at which crushing is excessive and the maximum stress to which the material can be subjected. The test pressures are chosen based on closure stresses encountered downhole in an oil or gas well.

The crush resistance test is performed by applying force on a specific weight of sand using an API-recommended-size piston in a cylinder at a particular stress level. The percentage of fines passing through the bottom screen is the crush strength. For 20- to 40-mesh sand, the percentage of crushed sand passing through the 40-mesh screen is the crush strength. API recommends that fines generated should not exceed 14% for US 20/40-mesh size. According to ISO standards, the fines generated should not exceed 10%, irrespective of grain size. In this paper, the crush resistance test was performed according to the procedure given in API RP 56 (1989).

The crush resistance of grains indicates their compressive strength capacity. This is one of the more important criteria for proppant selection. The more fines generated at the prescribed stress level, the greater the probability of fines migration plugging the flow channels. The samples were subjected to a stress level of 4,000 psi (Table 9). The samples passed the test, generating less than 14 per cent fines.



Conclusion
All the properties of the four samples of Asian origin found quite close to Ottawa sand and hence considered as best quality proppants for hydraulic fracturing as per norms of API and ISO. These samples were selected after detailed study of at least 10 samples from each region/country. This satisfies representation of every Asian region as natural habitat of quality fracturing sand, which can serve to minimize production costs.

References
API RP 56, Recommended Practices for Testing Sand used in Hydraulic Fracturing Operations, first edition. 1989. Washington, DC: API.

ISO 13503-2, Measurement of properties of proppants used in hydraulic fracturing and gravel-packing operations, first edition. 2006. Geneva, Switzerland: ISO.

Gottschling, J.C. 2005, Analysis of Non-API Industrial Sands for use in Hydraulic Fracturing, Paper SPE 98019, presented in SPE Eastern Regional Meeting, Morgantown, West Virginia, USA, 14–16 September. http://dx.doi.org/10.2118/98019-MS.

Kamat, D., Saaid, I.M., and Dzulkarnain, I. 2011. Comparative Characterization Study of Malaysian Sand as Proppant. World Academy of Science, Engineering and Technology 57.

Martinez, S.J., Steanson, R.E., and Coulter, A.W. 1987. Formation Fracturing. Petroleum Engineering Handbook, ed. Howard B. Bradley. Richardson, Texas: Society of Petroleum Engineers.

Montgomery, C.T. and Steanson, R.E. 1985. Proppant Selection: The Key to Successful Fracture Stimulation, J. Pet Tech 37 (12): 2163–2172. http://dx.doi.org/10.2118/12616-PA.

Veatch Jr., R.W., Moschovidis, Z.A., and Fast, C.R. 1989. An Overview of Hydraulic Fracturing in Recent Advances in Hydraulic Fracturing, Vol. 12. Richardson, Texas: Monograph Series, SPE.