Fracturing is an important means to increase oil and gas production, especially for the development of low permeability and ultra-deep oil and gas wells. Fracturing proppants are the core products of oil and gas fracturing production, the key materials for supporting artificial fractures in oil and gas reservoir transformation, and the key to improving the success rate of fracturing and the transformation effect.
Proppants are natural sand or artificial high-strength ceramic solid particles with a certain particle size, strength, sphericity, and grading. There are three main types of proppants used in industry: natural quartz sand, artificial ceramic particles, and coated proppants.
Quartz sand proppants
The main component of natural quartz sand is SiO2, and there are also small amounts of Al2O3, Fe2O3, K2O, Na2O, CaO, MgO and other components. The main mineral component of quartz sand is quartz, which is usually above 80%, and high-quality quartz sand can reach above 98.5%.
- Quartz sand is hard, wear-resistant, and chemically stable. It is only soluble in hydrofluoric acid and insoluble in other acids.
- Quartz sand is rich in resources, cheap, low in density (about 2.65g•cm-3), easy to pump, and still has good conductivity after crushing, which has a certain effect on increasing the production of oil and gas wells with low closure pressure.
- Quartz sand proppant has low cost, easy operation, and a relatively smooth surface after being washed by river water.
These characteristics make quartz sand widely used as a hydraulic fracturing proppant. The particle size of quartz sand used as a petroleum proppant is generally between 0.1mm and 2mm, also known as fracturing proppant or fracturing sand.
- Quartz sand is brittle and has low strength. When the crack closure pressure is high, it is easy to break, greatly reducing the effective support area of the crack, and cannot achieve the purpose of increasing oil and gas.
- The surface roughness of quartz sand is not good, which is not conducive to increasing the permeability of the crack.
- Quartz sand has a large thermal expansion coefficient. When the well is deep and the temperature is high, there is a risk of sudden expansion.
These disadvantages make natural quartz sand unable to meet the needs of hydraulic fracturing in deep low-permeability oil and gas development.
Since the strength of quartz sand proppant is not enough to meet the requirements of high-sealing pressure oil and gas wells, researchers have invented an artificial ceramsite proppant.
Ceramic proppant is often made of mineral materials such as bauxite as the main raw material, supplemented by various mineral additives, and sintered in a pressureless atmosphere. According to the difference in bulk density and apparent density after sintering, ceramsite proppant can be divided into three types: low density, medium density, and high density.
Compared with quartz sand proppant, ceramic proppant has high sphericity, smooth surface, good fluidity, high compressive strength, and low crushing rate. Practice has shown that the use of ceramic proppant for fracturing oil wells can increase production by 30-50% and extend the service life of oil and gas wells.
However, ceramic proppant has the following disadvantages:
(1) The particle density is large and the sedimentation speed is fast, which puts higher requirements on pumping conditions and fracturing fluid performance, and increases costs.
(2) Bauxite resources are scattered, processing and mining are limited, and the requirements for raw materials are strict, and resource utilization is limited.
Coated proppant
Coated proppant improves the strength and acid and alkali resistance of the proppant, also known as resin coated proppant. It usually consists of two parts, the inner part is composed of aggregate with a certain strength, and the outer part is coated with resin as a coating.
- Resin coated proppant has the characteristics of low density and high compressive strength.
- Coated proppant has a smooth surface, good water conductivity, and is easy to carry and transport.
- In addition to being used as a fracturing proppant, resin coated proppant can also effectively prevent the backflow of proppant, surface peeling and formation sanding during the reflux of fracturing fluid.
- Each particle of resin coated proppant has a uniform and tough resin shell, and the particles are also bonded together due to a certain polymerization effect, so it has good high temperature stability and acid corrosion resistance.
The disadvantages of resin coated proppant are high cost, poor chemical stability, easy bonding under high closure stress, and significantly reduced conductivity.