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Exploration and development of coalbed methane (CBM) reservoirs is ongoing in the Asia Pacific region, with low permeability, undersaturated coals being the target in some areas. Horizontal wells are typically required to produce low permeability CBM reservoirs at commercial rates, and may experience a period of single-phase (water) transient flow, followed by a transition to two-phase (water + gas) transient flow. Wells exhibiting this behavior are particularly difficult to model analytically, yet simple analytical and semi-analytical tools are desirable for making quick, but reasonably accurate forecasts. The purpose of this work is to therefore provide a new forecasting methodology for low permeability, undersaturated CBM wells. The new method for forecasting CBM wells introduced in this work is based on the concept of a dynamic drainage area (DDA), recently adapted to forecast tight black oil and gas condensate wells exhibiting two-phase flow. A critical advancement provided in the current work is modeling the transition from single-phase flow of water to two-phase flow of gas and water during the transient linear flow period, which was not attempted for the black oil/gas condensate cases. Above desorption pressure, material balance, performed on the dynamic drainage area to determine average pressure in the distance of investigation, is combined with the linear flow productivity index equation to forecast water production at each time step. For two-phase flow below desorption pressure, the two-phase version of the linear productivity equation and material balance equations for water and gas are solved iteratively for pressure, saturation and fluid production rate within the DDA. The gas material balance equation accounts for desorption effects. In order to calculate the DDA at each time step, the linear flow distance of investigation equation is used. Importantly, using the DDA method, variable operating conditions (flowing pressures) may be accounted for, which is critical for CBM wells during early stages of production. The new semi-analytical DDA model is used to history-match production data from an actual horizontal well completed in a low permeability, undersaturated, western Canadian CBM reservoir. Further, the match is compared to that obtained from rigorous numerical simulation. The history-match of the field data is reasonable with the new model and comparable to numerical simulation. The new model is however easier to setup and less data-intensive than numerical simulation, providing a practical alternative to CBM reservoir engineers who are responsible for generating a large number of forecasts. [ABSTRACT FROM AUTHOR]

• A novel test method coupling NMR + RSM data is proposed. • Pore-throats are mainly in size of 0.1 μm, taking a maximum of 48.39% of the space. • The relations of grouting effect with the permeability and pressure are discussed. • The optimal grouting pressure and the critical value of injectable permeability are found. 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 μm; 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. [ABSTRACT FROM AUTHOR]