In this paper, a series of fracture conductivity experiments were designed and conducted by an American Petroleum Institute (API) standard fracture conductivity evaluation system. The mixing proportion of quartz sand and ceramic was optimized. By the evaluation of the proppant breakage rate and sphericity analysis of mixed proppant with different sand volume proportions (P S ), the proppant mixture conductivity evolution behavior was analyzed. Results of this study showed that the conductivity of mixed proppant was between that of pure ceramic proppant and pure quartz sand proppant under the same conditions. For 20/40 mesh mixed proppant, a small amount of ceramic (25%) in mixed proppant could obtain 1.27–3 times higher conductivity than pure sand, while 40/70 mesh mixed proppant required the addition of 50% or more ceramic. The crushing resistance of mixed proppant determined the decrease of conductivity with the increase of effective closure stresses. A logarithmic empirical model was further derived from the results, which could be used to forecast the performance of fracture conductivity at different effective closure stresses and sand volume proportions.
For tight oil and gas reservoirs, the porosity and permeability are ultralow compared to conventional reservoirs; thus, large-scale hydraulic fracturing is usually conducted to enhance hydrocarbon production.1,2 Due to the poor petrophysical properties of tight reservoirs, the flow of oil and gas from matrix to fracture is very slow,3−5 which results in excessive high fracture conductivity having a little contribution to improving the productivity of oil or gas wells.6,7 It is necessary to build a just-good-enough fracture conductivity in tight reservoirs rather than to construct super high fracture conductivity.8,9 Quartz sand and ceramic are widely used proppant during hydraulic fracturing. Because of superior crush and chemical resistance,10 ceramic has superior conductivity to quartz sand at the same stress and proppant pack thickness conditions. However, its price is much higher than that of quartz sand. Specifically, the price of ceramic is 3–5 times higher than that of quartz sand proppant.11 To obtain an economic production of the tight reservoirs, only the pursuit of high fracture conductivity is not the best option. It is necessary to figure out a reasonable fracture prop method. By adopting the proposed method, the economic production rate and low conductivity construction investment can be achieved at the same time.
Many scholars have suggested replacing the traditional single proppant design with the combined proppants, which may be segregated by tail-in, evenly blended, or blended with dominant concentrations of one particular size.12−15 In the study of segmental by tail-in, the conductivity variation characteristics of different proppant types, sizes, and mass ratios have been investigated.12,16 And a relevant empirical model based on the experimental results has also been proposed.17 In the study of the mixed proppant, some research on the conductivity of a mixed proppant pack with different sizes and types has produced some interesting findings. It has been proved that small proppants introduced into proppant packs can severely hinder conductivity.18 The adverse effect induced by proppant size difference has been revealed that, as illustrated in , when some different size proppants are mixed, small proppants will fill into the pores formed by large proppants, thus reducing the fracture conductivity, which is very similar to the mechanism of the decrease of conductivity caused by the crushing of proppant particles. At the same time, it is also found that in the actual production, the particle size difference in the mixed proppant is too large, which is easier to aggravate the pore plugging effect caused by the migration of small particles.7,19,20 The research on mixing ceramic and sand shows that even a 1:1 mixture is dominated by the lower conductivity of sand and small amounts of 100 mesh (5%) by weight would result in above 25% reductions in permeability.21 Furthermore, by testing the fracture conductivity at various ratios of the mixed proppant with different sizes, the potential effect of the mixed proppant sizes relative to fracture conductivity has been understood in hybrid completion designs.16 Rui et al. have proposed a coupled flow-stress-damage (FSD) model of hydraulic fracture propagation with gravels based on the characteristics of the glutenite reservoirs.2 In addition, there are some numerical simulation researchers on the mixed proppant.22−24 Guo et al.25,26 have conducted a physical simulation of hydraulic fracturing of large-sized tight sandstone outcrops and have evaluated the unconventional performance and the applicability of the coated proppant. It is not difficult to find that predecessors have done a lot of work on the mixed conductivity of proppants with different particle sizes. However, there are relatively few studies on the conductivity behavior of the mixed proppant in the case of equal particle size proppant mixtures. Meanwhile, there is also a lack of relevant models that can be used to predict the conductivity in the case of equal particle size mixing.
In this paper, a series of experiments have been carried out using ceramic and quartz sand proppant obtained from Changqing oilfield, China. The fracture conductivity behaviors at different proppant mixing proportions are evaluated experimentally. A propped fracture conductivity prediction model of proppant mixtures is established, which can provide a guide for proppant schemes during hydraulic fracturing operations.