Potential of thermal methods to enhance recovery in unconventional oil reservoirs

Abstract

Produced hydrocarbon mixtures from nanoporous unconventional reservoirs consist of predominantly lighter components. This observation may be attributed to the membrane properties of nanoporous media. The average pore size in unconventional, tight-oil reservoirs is usually less than 100 nm and hindered transport in nanopores may impact production from unconventional reservoirs. A consequence of molecular sieving during hindered transport is leaving behind heavier hydrocarbons as they are trapped in the reservoir. In this thesis, hindered transport in nanoporous unconventional reservoirs is considered and the efficiency of molecular filtration or sieving is defined based on the thermodynamics of hydrocarbon fluids in nanopores. Because the flowing hydrocarbons in unconventional reservoirs mostly consist of uncharged, lighter components, steric hindrance is considered to be the main mechanism of molecular sieving. The effect of temperature and pressure on filtration efficiency is documented and implications on improved hydrocarbon recovery from unconventional reservoirs are discussed. A one-dimensional, compositional simulation formulation, which uses an implicitpressure, explicit-saturations and explicit-compositions (IMPESC) direct sequential method, is also presented as a first step toward studying hindered transport in nanoporous media. Filtration efficiency is defined based on the molecular partitioning of the filtrate between micro (bulk) and nanopore phases. Analogy to reverse osmosis is used to find the equilibrium concentrations of the filtrate under a given pressure gradient. Equilibrium concentrations are obtained from the condition of equality of fugacities at equilibrium and fugacities are obtained from the Peng-Robinson equation of state (P-R EoS). The goal in using the P-R EoS was to create a proper model of thermodynamics and compositional effects on phase behavior. Definition of molecular filtration efficiency based on the thermodynamic equilibrium of flowing hydrocarbon components is a major difference from the conventional definition of filtration efficiency due to steric hindrance of solid particles, which is independent of pressure and temperature. Understanding and predicting the impact of pressure and temperature on hindered transport enables us to assess the potential of thermal methods to improve production from tight unconventional reservoirs.