Enhanced oil recovery by low-salinity water spontaneous imbibition (LSW-SI) in a typical tight sandstone formation of mahu sag from core scale to field scale

Abstract

Accelerating mass exchange between matrix and fractures is the essence of enhanced oil recovery (EOR) in tight formations after natural depletion. Low salinity water (LSW) injection has been commercially-proven in conventional reservoirs EOR, with scale projects in progress worldwide. There is, however, a lack of understanding of the EOR effect in tight formations. Therefore, in this work, we introduced LSW-EOR to a target tight formation using huff-puff mode. Spontaneous imbibition (SI) tests were firstly performed on homogenous Berea sandstone cores with decreasing salinity brine to observe the production response. The results indicated that the oil recovery of the tight rock was boosted by tuning brine salinity. Of all the used brines with salinity ranging from 0.021% to 2.1% TDS (total dissolved salinity), the 0.21% TDS brine showed a rapid increase in oil production over imbibing time, which finally led to an incremental oil recovery of 4.5% OOIP (original oil in place). Core-scale modeling was conducted by history-matching the oil recovery dynamics of the SI results through modifying capillary pressure and relative permeability. A full-scale reservoir model was constructed using micro-seismic data to model fracture geometry combing fracturing results and scaling parameters obtained from core scale history-matching. It is proven that LSW huff-n-puff stimulated the oil production after natural depletion and improved MEE (mass exchange efficiency) of the target formation, but the EOR benefit was not comparable to CO₂ and surfactant-assisted water huff-puff methods.

Keywords

Fractured tight formation / Enhanced oil recovery / Low-salinity effect / Spontaneous imbibition / Huff-n-puff

Declaration of competing interest

The authors declare that they have no conflict of interests.

Acknowledgements

The authors gratefully acknowledge the financial support of National Natural Science Foundation of China (51974265 and 51804264), and Youth Science and Technology Innovation Team of SWPU (2017CXTD04). The authors also thank the Computer Modeling Group Ltd. For providing the CMG software for this study and the anonymous reviewers for their valuable comments.

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