Flow behavior of N2 huff and puff process for enhanced oil recovery in tight oil reservoirs
In the present work, the potential of N2 huff and puff process to enhance the recovery of tight oil reservoir was evaluated. N2 huff and puff experiments were performed in micromodels and cores to investigate the flow behaviors of different cycles. The results showed that, in the first cycle, N2 was dispersed in the oil, forming the foamy oil flow. In the second cycle, the dispersed gas bubbles gradually coalesced into the continuous gas phase. In the third cycle, N2 was produced in the form of continuous gas phase. The results from the coreflood tests showed that, the primary recovery was only 5.32%, while the recoveries for the three N2 huff and puff cycles were 15.1%, 8.53% and 3.22%, respectively.The recovery and the pressure gradient in the first cycle were high. With the increase of huff and puff cycles, and the oil recovery and the pressure gradient rapidly decreased. The oil recovery of N2 huff and puff has been found to increase as the N2 injection pressure and the soaking time increased. These results showed that, the properly designed and controlled N2 huff and puff process can lead to enhanced recovery of tight oil reservoirs.
Enhanced light oil recovery from tight formations through CO2 huff ‘n’ puff processes
Abstract The major objective of this paper was to evaluate the viability of CO2 huff ‘n’ puff processes as primary means to enhance oil recovery in low-pressure tight reservoirs and thereby optimize the corresponding key operating parameters of the process. In this study, CO2 huff ‘n’ puff corefloods were conducted by using a 973 mm-long composite core with an average porosity of 9.6% and an average permeability of 2.3 mD. The effects of primary parameters, such as slug size, injection rate, and the maximum and minimum pressures during production, chasing gas (N2) and soaking time on the performance of the process were investigated and operating strategies were optimized to lead to successful field applications. The experimental results indicate that 0.1 reservoir pore volume (PV) seems to be an optimal slug size for the first cycle, with the cycle recovery factor (RF) up to 14.52% when reservoir pressure is depleted from the maximum pressure to 8 MPa. RF is suggested to be sensitive to the maximum pressure and therefore, a maximum pressure should be built up to as high as the formation can hold. In the subsequent cycles, injecting N2 as a chasing gas flowing CO2 slug has great potential to significantly improve the cycle performance while reducing the CO2 utilization. The optimal operation should have three cycles and the ultimate RF for these three cycles could reach above 30%. The observations of this study suggest that the CO2 huff ‘n’ puff process is a viable technique to enhance light oil recovery in low-pressure tight reservoirs.
Neutron Residual Stress Measurements on Rail Sections for Different Production Conditions
Rail sectioning with subsequent neutron diffraction experiments has been used to assess residual stresses in the rails. In this study we present the results of neutron stress measurements performed at the NIST Center for Neutron Research (NCNR) on rail sections from rails that were produced under various conditions. Specifically, these are air-cooled, air-cooled and roller-straightened, head-hardened and head-hardened and roller-straightened. More significantly, a head-hardened and roller-straightened rail was also studied after service to elucidate evolution of the service-induced residual stresses. In the latter case both a transverse-cut slice and the central region of a 0.53 m long piece were studied. Measurements on this piece are the first in which triaxial stresses have been determined for an intact rail. Neutron strain measurements with 3×3×3 mm3 spatial resolution were successfully employed for transversally cut slices to verify the difference in the stress state depending on the production process. Although examination of slices allows determination of only two-dimensional stresses in the plane of the slice, additional measurements on obliquely-cut slices, which were also carried out, and utilization of FEM gives the possibility of reconstructing the full triaxial stress distribution. Together, these approaches provide a better understanding of rail fabrication and the possibility of improving the durability and safety of rails in the future.Copyright © 2004 by ASME