An experimental study on factors controlling the proppant transport in hydraulic fractures of coal reservoirs

Effectively propping open hydraulic fractures in coal reservoirs plays an important role in ensuring the efficient production of coalbed methane (CBM). This study investigated the effects of fracture morphology and the volume, injection rate, and properties of fracturing fluids on proppant transport in fractures through visualized experiments. The results are as follows: (1) Proppants showed a relatively homogeneous distribution in simple fractures with a constant aperture. In contrast, they form room-and-pillar props composed of proppant pillars and proppant-free rooms in local simple fractures with varying apertures and reticular fractures due to the effects of the varying flow rates of fracturing fluids and the blocking of fracture walls on the proppant transport. These room-and-pillar props are effective for deep severely damaged stress-sensitive reservoirs; (2) Significant bridging effects of particles occurred in simple fractures with varying apertures and reticular fractures, resulting in great differences in the particle size ranges of proppants allowed to pass through fractures with different apertures. Meanwhile, further injecting fracturing fluids into fractures in the case of sand plug diverted fractures, forming new fractures. Therefore, tip screen-out (TSO) is favorable for the stimulation of fracture networks; (3) As the volume and injection rate of fracturing fluids increased, proppants migrated farther and those in reticular fractures entered more branch fractures, thus significantly increasing the propped fracture area. The viscosity of fracturing fluids increased after adding drag reducers to them. As a result, the sand-carrying capacity of fracturing fluids was improved effectively, and the transport distance and propped fracture area of proppants in fractures were further increased. Therefore, the authors of this study recommend using hydraulic fracturing with large injection rate and large-volume fracturing fluids and adopting low-damage, low-friction fracturing fluids and proppants with multiple grain sizes composed of silts and fine-, medium-, and coarse-grained sands. The purpose is to improve the propped effects for fractures of various types and different apertures in reservoirs and thus promote the efficient production of fluids. The results of this study can provide experimental support for the optimization of fracturing techniques for CBM reservoirs and coal-measure gas reservoirs.