Simulation of gas production in the northern regions: the role of thermodynamics

Abstract

Purpose: Natural gas extraction can involve conditions favoring the formation and deposition of hydrates in wells and bottom holes. At the moment, these processes are considered separately, i.e., the dynamics of hydrate formation in various production modes is modeled within the framework of non-isothermal multiphase filtration, while the dynamics of hydrate formation in wells is studied within the framework of tube hydraulics. In the latter case, the temperature and pressure near the bottom hole are specified and are normally assumed to be equal to their reservoir values. In the present paper the first attempt of partial combining these two approaches is done. Methods: The influence of mathematical model parameters on the dynamics of pressure and temperature fields at non-isothermal gas filtration is investigated in a numerical experiment. A nonlinear system of partial differential equations obtained from the energy and mass conservation laws and the Darcy law are used to describe the process, and physical and caloric equations of state are used as closing relations. Results: The pressure and temperature fields in the reservoir are determined by solving the problem of nonisothermal filtration of an imperfect gas and are compared with equilibrium conditions of hydrate formation. This solution is be further used to determine the temperature and pressure in the bottom hole region. The boundary conditions correspond to a given pressure drop at the bottom hole. It is shown that the influence of the temperature field on such integral characteristics as cumulative gas production is most pronounced at moderate pressure drops. The size of the zone of possible hydrate formation in a gas reservoir is determined in two particular examples. Conclusions: Two important conclusions can be drawn: (1) a zone of hydrate formation during gas production can be identified with the use of some geophysical method, for instance, acoustic logging; (2) such a narrow zone can be easily affected by one of hydrate formation inhibitors (methanol or calcium chloride solution). These results again demonstrate the importance of taking into account thermodynamic processes in mathematical modeling of natural gas production.