CO2 Injectivity in a Multi-lateral Horizontal Well in a Low Permeability Coal Seam: Results from a Field Trial

Abstract

Enhanced coalbed methane (ECBM) production by CO2 injection offers the potential of increasing recovery of the gas in place over primary recovery methods and at the same time storing CO2. A field ECBM trial via CO2 storage has been carried out in the east margin of the Ordos Basin, Shanxi province, China. A unique aspect of the trial was using a multi-lateral horizontal well, which was clearly beneficial for production or injection for low permeability formations. This paper described a reservoir model constructed to simulate the CO2 injection behaviour using the coal seam reservoir simulator SIMED II. The simulation work included history matching the bottomhole pressure while using injection rate as control. A complexity in the modelling was representation of the multi-lateral horizontal well as its branches were not aligned with x or y coordinates. Thus approximation was taken to represent the well in the model. The simulation result showed good match for some data but could not match well the whole range of data, suggesting possible well opening or closing to flow during injection and shut-in periods. It has been suggested that the permeability decrease due to gas adsorption induced coal swelling may play an important role in the gas flow behaviour, especially near the wellbore where the amount of CO2 adsorbed was highest. However, due to the coarse grid size applied in this finite difference model and the difficulty to refine them near the well branches while maintaining overall size of the model, the permeability loss due to adsorption induced coal swelling was diluted by the relatively large grid size. Thus this may not accurately reflect the permeability change and the quality of the history matching results are affected. A suggestion is to use refined grid blocks near wellbore to examine the impact of swelling on the overall CO2 flow behaviour for this project.