Quartz sand is a kind of natural stable mineral with wide distribution and high hardness. Its main chemical composition is silicon dioxide (SiO2). Quartz sand is mainly formed by hydraulic erosion, weathering and denudation of quartz sandstone, so it is mainly produced in river beach, desert and coastal areas.
The content of SiO2, the main mineral component in quartz sand, is closely related to its particle size. Generally, the larger the particle size is, the higher the SiO2 content is. The quartz content of high quality quartz sand used in the world is over 98%. However, due to different uses, fracturing quartz sand does not pay much attention to the content of silica. With the rapid development of the world’s oil and gas exploitation industry, the exploitation difficulty is gradually increasing, the depth of oil and gas wells is getting deeper and deeper, there are more and more low-permeability deposits, and the demand for high-strength fracturing proppant products is gradually increasing. As the resin coated proppant (pre curing resin coated quartz sand) has the characteristics of high strength, strong corrosion resistance and good conductivity, its market demand will continue to expand, and it will have a broad development space in the future.
1. Sphericity and roundness
Sphericity, well understood, refers to the degree to which fracturing proppant (i.e. quartz sand) is close to spherical. In the eyes of unfamiliar people, the concept of roundness is a little abstract. In fact, roundness can be simply understood as “roundness”. The more sharp the edges of particles, the worse the roundness; otherwise, the more smooth the edges, the better the roundness. The sphericity and roundness of natural quartz sand shall not be less than 0.6.
2. Acid solubility
Acid solubility refers to the percentage of the mass of proppant dissolved by acid and the mass of total proppant under the specified acid solution and reaction conditions. Acid resistance is an important index of fracturing proppant. Proppant with good acid resistance can work in acid rock for longer time and maintain good conductivity. The allowable acid solubility values of proppant with various particle sizes are shown in Table 1. It should be noted that the acid solubility indexes of quartz sand and ceramsite proppant are the same.
The turbidity of proppant is the turbidity degree of liquid after adding a certain quantity of proppant into the specified volume of distilled water, shaking and placing for a certain time, and the unit is FTU.
Turbidity mainly represents the surface finish of proppant, reflecting the number and size of particles on its surface. According to the regulations, the turbidity of quartz sand for fracturing shall not be higher than 100ftu.
4. Crushing resistance
For a certain amount of quartz sand proppant, the crushing rate determined by pressure test under rated pressure characterizes the crushing resistance of proppant. The main factors affecting the crushing rate are the roundness, sphericity, surface finish, microstructure and so on. The fracture conductivity of quartz sand proppant is affected by its breaking rate.
5. Bulk density and apparent density
The bulk density of quartz sand proppant is the proppant mass per unit volume (g / cm3), which reflects the overall density of proppant stack. The main factors affecting bulk density are apparent density, roundness, sphericity, surface finish and so on.
The apparent density of proppant is the proppant mass per particle volume (g / cm3). The difference between it and bulk density is that the volume of apparent density does not include the volume of void between particles. The main factors affecting the apparent density of proppant are phase composition, structure and porosity. For different density fracturing proppant, the test indexes are different. In summary, the quartz sand for fracturing has the following requirements: uniform particle size, small density, large strength, small crushing rate, high roundness and sphericity, less impurity content, wide source and low price.