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

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 pro-duction 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 com-parable to CO2 and surfactant-assisted water huff-puff methods.(c) 2020 Southwest Petroleum University. Publishing services by Elsevier B.V. on behalf of KeAi Communications Co. Ltd. This is an open access article under the CC BY-NC-ND license (http:// creativecommons.org/licenses/by-nc-nd/4.0/).