Numerical simulation of gas and water flow mechanism in hydraulically fractured shale gas reservoirs

Abstract

a b s t r a c tThe problem of the fracturing water remaining in hydraulically fractured shale gas reservoirs has become one of the major concerns in terms of gas productivity and operating costs. The fracturing water retention is influenced by reservoir properties and production parameters, such as matrix porosity and permeability, fracture porosity and permeability, Langmuir pressure and volume, diffusion coefficient, shut-in time, drawdowns and injection rate. In this study, a horizontal well with six-stage hydraulicfracturing treatment was constructed to understand the water retention and gas production performance in shale gas reservoirs. Gas diffusion, gas adsorption/desorption and Darcy flow as well as non-Darcy flow were considered in this model. The process of water retention and gas production performance was analyzed, and the effects of reservoir and production properties on this problem were performed. The results show that only 34% of the fracturing water can flow back to the surface, most of which remains in shale formations to interfere with gas production. The increasing of matrix porosity, fracture porosity, Langmuir pressure and drawdowns will reduce water retention while water retention in shale matrix will increase with the increasing of matrix permeability and Langmuir volume, and consequently impact gas production. But the trapped water and gas rate increase with the higher fracture permeability. Furthermore, the diffusion coefficient, shut-in time and injection rate do not have a significant effect on water retention and gas productivity. These results can provide insights into a better understanding of gas and water flow in the shale gas reservoirs and the effects of reservoir and production parameters on water retention and gas production.

Methane Diffusion and Adsorption in Shale Rocks: A Numerical Study Using the Dusty Gas Model in TOUGH2/EOS7C-ECBM

“…Shale is a very fine-grained and clastic sedimentary rock, which is characterized by extremely low porosity, ultra-low permeability between 0.001 and 0.00001 mD (Shen et al 2015(Shen et al , 2016. The permeability plays an important role in gas flow transport in shale gas reservoirs (Sun et al 2017;Tokunaga et al 2017).…”

“…Horizontal drilling and hydraulic fracturing are the main technologies to produce gas from the ultra-low-permeability shale reservoirs (Shen et al 2016. In the recent years, due to the advances in horizontal drilling and hydraulic fracturing, gas production from shale reservoirs has drastically increased in North America (Kuuskraa et al 2011;Wei et al 2016;Tokunaga et al 2017).…”

“…Compared with the conventional sandstone and carbonate reservoirs, shale gas reservoirs are characterized by relatively low porosity (≤ 10%), which have ultra-low permeability (≤ 0.001mD) and high clay content (≥ 2%) (Boyer et al 2006;Shen et al 2016). Besides, the pore structures in shale gas reservoirs are complex and varied, which includes organic matter, nonorganic matrix, natural fractures and pore space induced by hydraulic fractures ).…”

“…As there exist complex pore structures and distinctive storage in shale rock, compared with conventional gas reservoirs, the gas transport in shale gas reservoirs is a complex flow process, which is strongly influenced by diffusion, adsorption and desorption (Moridis et al 2010). And diffusion, adsorption and desorption are the primary mechanisms controlling transport in shale matrix, while the Darcy flow is dominant in fractures and the non-Darcy flow is in hydraulic fractures (Shen et al 2016). Thus, understanding methane diffusion and adsorption, and gas flow and equilibrium in the low-permeability matrix of shale is crucial for gas shale formation evaluation and for forecasting gas production in shale gas reservoirs.…”

Acceleration of Gas Reservoir Simulation Using Proper Orthogonal Decomposition

“…They proved that the POD model can achieve good precision in an appropriate range. With fast progress of the exploration and development of gas reservoirs, gas reservoir simulation attracted more and more attention [25][26][27][28]. Additional computation should be made to calculate the compressibility of the gas, which does not exist in incompressible oil flow.…”