(636f) Nanoscale Static and Dynamic Behavior and Optimized Controlling Technology of Gas Channeling in Shale Oil Reservoirs

Nanoscale Static and Dynamic Behavior and Optimized Controlling Technology of Gas Channeling in Shale Oil Reservoirs

Kaiqiang Zhang, Songyan Li, Na Jia

Keywords: Phase behavior; Flow mechanics; Gas channeling; Optimized controlling technology; Shale oil reservoirs.

OBJECTIVES/SCOPE:

In this paper, static phase behavior and dynamic flow mechanics of gas channelings in shale reservoirs during CO₂ injection processes are experimentally and theoretically studied, on the basis of which two controlling technologies are proposed and evaluated. The developed gas channeling controlling technologies are finally applied in an oilfield located in Northeast China, whose implementation schemes, including the technical parameters and surface/subsurface infrastructures, are optimized.

METHODS, PROCEDURES, PROCESS

In theory, a nanoscale-extended equation of state (EOS), which considers strong confinement effects and intermolecular interactions, is developed to calculate the vapour-liquid equilibrium of CO₂ streams in nanopores. Moreover, complex mathematical models of wellbore pressure distributions and heat transfer are proposed.

In experiment, first, a series pressure-volume-temperature (PVT) tests of the crude oil-CO₂ system are conducted at P = 5-20 MPa and T_res = 97 °C. Second, fifteen laboratory dual-permeability CO₂ coreflood tests at five injection rates and reservoir conditions are completed to investigate the characteristics of gas channelings. Finally, some additional PVT and coreflood tests are performed to develop and evaluate the controlling technology in shale oil reservoirs.

RESULTS, OBSERVATIONS, CONCLUSIONS

The nanoscale-extended EOS is accurate for calculating the phase behavior of CO₂ streams in bulk phase and nanopores. It is found that the phase behavior in nanopores become significantly different, for example, CO₂ pressures in nanopores are always larger than the bulk pressure at the same volume. The shifts of critical properties are concluded to dominate the phase changes from bulk phase to nanopores. Phase transitions from subcritical to supercritical CO₂ are critically important, which is determined to occur at the respective depths of 1052 and 795m for Wells DB33-2-2 and DB33-2-4 in the target reservoir. The gas channeling, which is detrimental to recover oils, easily occurs at high gas injection rates and/or in large permeability (fractured) cores. In this study, the oil recovery factor is found to increase as high as 20-40% before gas breakthrough/channeling. On the basis of results from this study, the HY-2 CO₂ foam, which owns a stable foaming concentration of 1% and resistance factor of 40-80, and polymer gel of AP-P4(0.3%)+YG107(0.3%), whose optimum conditions are at pH range of 3-8 and temperatures of 40-97°C, are determined to effectively control the gas channeling. The comprehensive implementation scheme for controlling gas channelings in shale formations is optimized. Also, a foam surface generator and a subsurface injector with the temperature and pressure tolerances of 200 °C and 50 MPa are designed and applied in the oilfield.

NOVEL/ADDITIVE INFORMATION

Static phase behavior and dynamic flow mechanics of CO₂ gas channelings in nanopores are studied for the first time. Accordingly, the optimized technology and implementation schemes are initially proposed and successfully applied to control the gas channeling and enhance oil recovery in the actual shale oil reservoir.