Carbonate reservoirs in the Middle East are highly heterogeneous with complex porosity systems and mixed-wet matrix properties. These characteristics strongly affect reservoir performance under waterflooding. This paper concerns a highly layered limestone reservoir with various levels of cyclicity in properties and can be described at a high level as consisting of two main bodies, i.e., an Upper zone and a Lower zone with permeability contrast of up to two orders of magnitude. The main part of the reservoir is currently under waterflooding. Field observations show that injected water tends to channel quickly through the Upper zone along the high permeability layers and bypass the oil in the Lower zone. Past studies have indicated that this water override phenomenon is caused by a combination of high permeability contrast and capillary forces which counteract gravity forces.
In this paper we will investigate different development options for such heterogeneous mixed-wet reservoirs aiming at improving recovery from the Lower zone: 1- Optimized waterflooding with infill wells, 2- Novel EOR options designed to overcome the capillary forces and improve vertical sweep. The EOR options include (a) polymer-assisted solutions consisting of injecting polymer in the Upper zone and water or miscible gas in the Lower zone; and (b) surfactant assisted solutions (foam and enhanced gravity drainage).
The main conclusions of the study are: 1- Waterflooding is an efficient recovery mechanism for the Upper zone and tight well spacing is required to improve recovery from the Lower zone; 2- The EOR processes have the potential of improving recovery from the Lower zone; 3- The most attractive EOR schemes are the polymer-based options which, when compared to the optimized waterflooding/infill scenario, lead to much higher recovery, lower volumes of water injected and significantly less water cycling and the requirement of fewer wells. The polymer-assisted solutions also require injecting much lower polymer volumes compared to conventional polymer flooding. Simulation results show that the process(es) are robust to injection rates, vertical heterogeneity, well completions and a range of polymer viscosities.
Carbonate reservoirs contain more than 50% of world’s remaining conventional hydrocarbon reserves and on average have relatively low recovery factors. With the era of “easy oil” (conventional oil and natural gas that are relatively easy to extract) phasing out, enhanced oil recovery (EOR) becomes increasingly important to maintain and extend the production from existing oil reservoirs.
Nasralla, Ramez A (Shell) | Sergienko, Ekaterina (Shell) | Masalmeh, Shehadeh K (Shell) | van der Linde, Hilbert A. (Shell Global Solution Int.) | Brussee, Niels J (Shell Global Solution Int.) | Mahani, Hassan (Shell) | Suijkerbuijk, Bart (Shell) | Alqarshubi, Ibrahim (Shell)
Low salinity waterflood (LSF) is a promising improved oil recovery (IOR) technology. Although, it has been demonstrated that LSF is an efficient IOR method for many sandstone reservoirs, the potential of LSF in carbonate reservoirs is still not well-established as only a limited number of successful coreflood experiments are available in the literature. Therefore, the aim of this study was to examine the oil recovery improvement by LSF in carbonate reservoirs by performing coreflood experiments.
This paper proposes an experimental approach to qualitatively evaluate the potential of LSF to improve oil recovery and alter the rock wettability during coreflood experiments. The corefloods were conducted on core plugs from two Middle Eastern carbonate reservoirs with a wide variation of rock properties and reservoir conditions. Seawater and several dilutions of formation brine and seawater were flooded in the tertiary mode to evaluate their impacts on oil recovery compared to formation brine injection. In addition, a geochemical study was performed using PHREEQC software to assess the potential of calcite dissolution by LSF.
The experimental results confirmed that lowering the water salinity can alter the rock wettability towards more water-wet, causing improvement of oil recovery in tertiary waterflood in plugs from the two reservoirs. Furthermore, seawater is more favorable for improved oil recovery than formation brine as injection of seawater after formation brine resulted in extra oil production. This demonstrates that the brine composition plays an important role during waterflooding in carbonate reservoirs, and not only the brine salinity. It was also observed that oil recovery can be improved by injection of brines that cannot dissolve calcite based on the geochemical modeling study. This implies that calcite dissolution is not the dominant mechanism of IOR by LSF.
To conclude, this paper demonstrates that low-salinity waterflood has a good potential as an IOR technology in carbonate reservoirs. In addition, the proposed experimental approach ensures the verification of LSF effect, either it is positive or negative. However, more work is required to further explore the most influential parameters affecting LSF response and explain the dominant mechanisms.
Low salinity waterflood (LSF) is a relatively mature improved oil recovery technique for sandstone reservoirs. The concept of LSF, for sandstones, is to lower the ionic strength of the injected brine, which leads to an alteration of the rock wettability towards more water-wet and hence an improvement of oil recovery. Numerous laboratory studies demonstrated the effect of LSF by spontaneous imbibition tests and coreflood experiments (Bernard 1967, Jadhunandan and Morrow 1991, Yildiz and Morrow 1996, Tang and Morrow 1997, Lager et al. 2006, Ligthelm et al. 2009, Masalmeh et. al. 2013). Furthermore, published data confirmed the positive response of LSF at the field scale (Webb et al. 2004, Lager et al. 2008, Vledder et al. 2010). However, the potential of LSF for carbonate reservoirs has not been well investigated.
Sorop, Tiberiu Gabriel (Shell Global Solutions International) | Suijkerbuijk, Bart M.J.M. (Shell Global Solutions International) | Masalmeh, Shehadeh K (Shell Technology Oman) | Looijer, Mark T. (Shell Global Solutions International) | Parker, Andrew R (Shell Global Solutions International) | Dindoruk, Deniz M (Shell Exploration & Production Co) | Goodyear, Stephen Geoffrey (Shell E&P UK) | Al-Qarshubi, Ibrahim S.M. (Shell Global Solutions International)
Low Salinity Waterflooding (LSF) is an emerging IOR/EOR technology that can improve oil recovery efficiency by lowering the injection water salinity. Field scale incremental oil recoveries are estimated to be up to 6% STOIIP. Being a natural extension of conventional waterflooding (WF), LSF is easier to implement than other EOR methods. However, the processes of screening, designing and executing LSF projects require an increased operator competence and management focus compared to conventional waterflooding. This paper discusses the practical aspects of deploying LSF in fields, focusing on the maturation stages, while highlighting the key success factors.
LSF deployment starts with a portfolio screening against specific surface and subsurface screening criteria to prioritize opportunities. Next, the identified opportunities are run through reservoir conditions SCAL tests to quantify the LSF benefits, while de-risking the potential for any injectivity loss due to clay swelling or deflocculation. Standardized LSF SCAL protocols have been incorporated into the general WF guidelines, so that any suitable new WF project conducts LSF SCAL. For mature waterfloods, this SCAL program provides additional reservoir condition relative permeability data, enabling operating units to optimize well and reservoir management (WRM). The next steps in the process are production forecasting, facilities design, and project economics for the LSF opportunity. The multidisciplinary nature of LSF deployment requires integrated (sub)surface technology teams closely collaborating with R&D and asset teams. The standardization of the facilities design, including cost models, can significantly accelerate the deployment effort.
In Shell, LSF is currently at different stages of deployment around the world and across the whole spectrum of WF projects, from the rejuvenation of brown fields to green field developments (offshore and onshore). The LSF deployment effort is combined with the screening of other EOR technologies, to identify where LSF may be able to unlock additional value by creating the appropriate conditions for subsequent chemical flooding.