An Experimental Method for Obtaining Optimal Proppant Packing Ratios for Micro-Sized Proppant Placement in Unconventional Reservoirs
Authors: Liu Huifeng , Zhang Jiayuan , Luan Guilin , Feng Shengjie , Chen Zhangxin , Wang Duo 
Published: November 2024

Abstract
Coal bed methane and shale gas reservoirs are extensively developed in China, not only because of their large reserves, but also because they are clean energy resources and contribute to the net zero emissions. However, these reservoirs are usually typical unconventional reservoirs with tight matrix and natural fractures. Hydraulic fracturing with micro-sized proppants is used to develop them cost-effectively. However, a quantitative design for micro-sized proppant injection is not mature and is still raising attention in industry.
Different from traditional proppants, micro-sized proppants (&lt;100 mesh) are required to be placed in fractures in a partial-monolayer manner in order to obtain maximum conductivity. Many previous theoretical studies have shown that propped fracture conductivity changes parabolically with proppant concentration and there is an optimal proppant packing ratio. However, no one has observed this phenomenon in the laboratory. In this paper, we will introduce a laboratory method for obtaining an optimal packing ratio and the corresponding test results using 140/200 mesh and 200/300 mesh silica sands. The results are then compared with our numerical modeling results based on the LBM (lattice Boltzmann) algorithm.
Micro-sized sands are selected and sorted from a silica sand mine in Chifeng, Inner Mongolia. The sorting coefficient of the sand particles is strictly controlled within 1.20. The proppant breakage rate under 4000 psi is controlled within 10%. Naturally cracked cores with natural rough surfaces, which are obtained from a shale formation in China, are used to make the test units. A GCTS Rock Triaxial System instead of a traditional fracture conductivity tester is used to test the conductivity of fractures propped by micro-sized proppants. A Transient Pulse Method is used to test the propped fracture permeability. The permeability and conductivity of 140/200 mesh and 200/300 mesh silica sands are measured at six different proppant packing ratios, 0, 0.25, 0.375, 0.50, 0.75 and 1.0, under two different closure pressures, 3000psi (21MPa) and 4000psi (28MPa). The regression analysis results show that the optimal packing ratios of 140/200 and 200/300 mesh silica sands are 40.30% and 52.09% respectively, regardless of the closure pressure. However, the conductivity curves of 200/300 mesh silica sands show that their optimal packing ratio under 4000psi is 78.14%, much higher than the result under 3000psi, which is 52.09%.
This is the first time in industry to obtain the optimal proppant packing ratios for micro-sized proppant placement by direct conductivity tests on real shale cores. Our numerical modeling results based on the LBM algorithm under the same conditions show a lower optimal packing ratio (30%-40%) than the measured results. This makes our laboratory tests more beneficial in targeting the causes of the discrepancies and rectifying mathematical models.