Study on the Pore-throat structure characterization and nano grouting law of the Low-permeability mudstone based on NMR-RSM methods

Mudstone, a common sedimentary rock in coal measure strata, sees the softening and disintegration of its clay minerals such as kaolinite owing to groundwater, water accumulation in goaf, and high in-situ stress, resulting in serious deformation and shrinkage of roadway space. However, it can hardly be reinforced by grouting because of other factors such as dense occurrence and poor permeability. In view of this, the silica sol, a nano-sized grouting material featured with controllable solidification and colloidal compactness, may be able to grout the low-permeability mudstone. In this paper, using the response surface methodology (RSM) and low-field nuclear magnetic resonance (NMR), we designed a two-factor orthogonal experiment and proposed a new data coupling test method integrating the processes of design, monitoring, and analysis, thus illustrating the pore throat distribution characteristics and grouting feasibility of low-permeability mudstone and revealing the diffusion law of nano silica sol in this kind of rock under the dual influences of pressure and permeability. On this basis, the optimal grouting parameters as well as the critical value of injectability were eventually determined. Experimental results proved that low-permeability rocks primarily contained pore throats in the size of 0.1 mu m; and as these rocks became less permeable, a larger proportion of throats narrower than this size were developed, accounting for 48.39% in 1 mD samples. The low-permeability mudstone, which was highly matched with the silica sol solution in the particle size of 6.23 nm, could be modified and reinforced. Based on a calculation model for response values, the experimental data were successively classified by permeability and grouting pressure, from which the following pore-sealing rates were obtained: A (60 -100 mD): 33.74%, B (30 -60 mD): 24.61%, C (0 -30 mD): 0.41%, D (2 -3 MPa): 0.75%, E (1 -2 MPa): 38.92%, and F (0 -1 MPa): 28.02%. This indicated that the grouting effect was proportional to permeability; and it first increased and then decreased under rising pressure. According to the response to these rates, the low-permeability mudstone had better be grouted with nano silica sol under the pressure of 0.804 MPa, with the critical value of injectable permeability being about 40.42 mD.