Assessment of Non-Equilibrium Phase Behavior Model Parameters for Oil and Gas-Condensate Systems by Laboratory and Field Studies

Abstract

Abstract Non-equilibrium phase behavior of hydrocarbons has a great influence on the development of gas condensate and oil and gas condensate deposits, as well as oil deposits after evolution of dissolved gas. The key parameter of mathematical models that take into account non-equilibrium phase behavior is the characteristic relaxation time. The paper presents examples of assessing its value according to the results of laboratory experiments and well tests, which also allow verifying the mathematical model and obtaining input data for the production forecast taking into account non-equilibrium processes. A model of non-equilibrium phase behavior in a compositional formulation, based on relaxation of the chemical potentials difference, was verified on the data of a laboratory experiment in a calorimeter cell. The numerical algorithm for calculating the dynamics of phase fractions and compositions in a non-equilibrium process was modified to take into account the isochoric condition, which made it possible to directly compare the simulation results to the real experiment. A good match of the experimental pressure-temperature hysteresis curves was obtained without additional adjustment of the characteristic relaxation time as compared with the estimate obtained from processing the measured pressure relaxation curve. Thus, the adequacy of the mathematical model and the possibility of evaluating its parameters directly from the experimental measurements were confirmed. An example of assessing the characteristic relaxation time from well test data is presented for the pressure buildup (PBU) curve recorded under the conditions of gas dissolution in oil. A sector blackoil-type model was used for the well drainage area. For matching the PBU data, various modeling methods for non-instant gas dissolution were considered, including a model based on relaxation of solution gas-oil ratio (sGOR). The results of simulations with the sector model showed that the traditional options for limiting the sGOR increasing rate do not allow adequate matching of the PBU curve under conditions of gas dissolution. For the model that takes into account the sGOR relaxation, an in-situ estimate of the characteristic relaxation time was obtained which provided a fine match of the PBU data. As expected from theoretical concepts, the characteristic relaxation time for the in-situ process has a much larger order of magnitude than for the laboratory experiments on centimeter-sized cells. The results of the study show the adequacy of relaxation models for non-equilibrium phase behavior in compositional and black-oil formulations for describing real processes and demonstrate the possibility to estimate the characteristic relaxation time from laboratory experiments and well tests, taking into account its dependence on the process scale.