RIASSUNTO
Abstract
This paper presents the concept of harnessing geothermal energy from heavy oil fields that have undergone steam-flooding and so accumulated substantial heat from steam injection. Once the steam-flooding process reaches economic cut-off, due to high water cut and/or high steam-to-oil ratio, the reservoir would be abandoned, leaving behind stored energy in the form of heat. From this point, the reservoir could be regarded as an artificial geothermal system, and its intrinsic heat recovered by water circulation.
A compositional, thermal simulator was used to model the overall heat transfer efficiency throughout the reservoir. The Fourth SPE Comparative Solution Project (problem 3) - complemented by parameters obtained from well-known analogue fields - was adopted to build the base case scenario. Sensitivity analyses were then performed to identify critical parameters that may affect the potential for harnessing the geothermal energy.
The reservoir simulations were then coupled with semi-analytical computational of the heat exchange between wellbore and surrounding formation to estimate the total surface heat recovery. The latter could be used for either direct heating or electricity generation, depending on the arrival fluid temperature.
State-of-art technology for converting heat from low-enthalpy resources (""waste energy??) into electricity were evaluated, together with an estimate of the parasitic load associated with ancillary facilities necessary to run operations beyond the oil-recovery phase.
This study shows that it could be possible to advantageously extend the life of heavy oil fields by means of a heat-recovery phase after the oil-recovery phase.
Introduction
Unconventional heavy-oil resources have become increasingly attractive to the industry because of global increase in energy demand and decline in conventional oil production. These resources, which once were deferred from production because of the inherent additional costs for enhancing recovery, are revisited when the oil price is higher. Production from heavy oil reservoirs can be improved by reducing the oil viscosity (thermal processes). Some examples of thermal methods are steam-assisted gravity drainage (SAGD), in-situ combustion, and cyclic steam stimulation (CSS). The most effective methods are steamflooding and hot waterflooding, with recovery factors additionally increased 20 to 30% by these two techniques.
Heavy oil reservoirs where steamflood or hot waterflood processes have been applied for a long period of time reach economic cutoffs resulting from high water cut, high steam-to-oil ratio (SOR), or steam breakthrough. Reservoirs are then either produced until depleted or abandoned. Thanks to the thermal properties of reservoir rock, great amount of heat is accumulated within it during thermal processes. However, concurrent heat dissipation to overburden and underburden takes place during the production period and also after abandoning the fields. The heat trapped during thermal processes could be exploited as geothermal energy potential.