Multiphase Flow Simulation Of Geothermal Wells

Abstract

Deep geothermal wells producing supercritical water can contribute to greatly reducing the investment costs by increasing the energy output per well by an order of magnitude. The flow of supercritical water in geothermal wells is still an unknown territory, and there is a need for better understanding of the flow behaviour, both during steady state production and transient events. In the Descramble EU project, the Venelle 2 well has been further drilled down to 2900 m, in a supercritical water reservoir (highest measured temperature 443.6°C at 2810m). This well has been used as an example case to set up and run a modified version of the oil and gas multiphase flow simulator LedaFlow®. Temperature and pressure profiles have been obtained using logging tools specifically developed in the Descramble project to tackle such extreme conditions. LedaFlow® is a multiphase flow simulator in pipes originally developed for the oil and gas industry. It can simulate flows of oil, gas and water in networks of pipelines, risers and wells. LedaFlow® solves a 1D model, relying on experiment-based physical correlations to simulate any kind of relevant flow situation. A variety of devices can be attached to the pipes, as valves or reservoir inflow. In addition, a dynamic heat transfer model can be coupled to the pipe to model the effect of the ground around. An accurate equation of state for steam and water, including the supercritical region, has been added to LedaFlow® to simulate supercritical water flow in geothermal wells. The well is modelled by a vertical, 3000 m long pipe (the model was set up before the drilling was completed). This first version of the model uses fixed-pressure boundaries at both ends to model the reservoir pressure and the topside pressure, assumed regulated. A more accurate model can be designed based on the characterisation of the reservoir, and the technological design of the topside equipment. A valve can close the well at wellhead. The ground model extends to 100 m around the well, and is divided in concentric annular cells. We present different simulation results, first for steady state production, then for selected transient events. Since the final reservoir pressure and temperature were not known at the time of running the simulations, four cases have been used, all with a water temperature of 450°C, and pressure in the set [150, 250, 350, 450] bar. Setting the wellhead pressures to different values, we obtain different production flow rates, and for each of them, the pressure-temperature profile along the well is presented. The enthalpy flux out of the well is evaluated, showing that low specific enthalpy reservoirs (which means high pressure) have the highest potential. At the same time, low specific enthalpy reservoirs are more prone to induce multiphase flow in the well. As to transient events, start-up and shut-in events have been run. The conclusions of the start-up simulations are that even if the steady state production is in single phase all along the pipe, it may not be possible to avoid transient multiphase flow during start-up. Shut-in simulations showed that strong temperature spikes can be observed (80°C in a few minutes) close to wellhead if there was multiphase flow during production, due to compression of the steam.

AAPG Datapages/Search and Discovery Article #90346 ©2019 AAPG European Region, 3rd Hydrocarbon Geothermal Cross Over Technology Workshop, Geneva, Switzerland, April 9-10, 2019