Interaction of silica nanoparticles with chalk and sandstone minerals: Adsorption, fluid/rock interactions in the absence and presence of hydrocarbons

Abstract

Conventional oil production from petroleum reservoirs generally leaves more that 50% of the original oil in place unrecovered. This residual oil is the target of various enhanced oil recovery (EOR) techniques that involve fluid injection into the reservoir which supplements oil recovery by interacting with the rockoil-brine system. Silica nanofluids have emerged as a promising fluid for EOR. Nanofluids are colloidal suspensions of nanoparticles (NP) dispersed in a suitable fluid. Over the past decade, a lot of research has focused on investigating silica nanofluids for EOR applications. This thesis addresses the mechanisms for silica NP adsorption and fluid/rock interactions during nanofluid injection. Understanding these processes would aid efficient design of nanofluid floods. In chapter 1 of the thesis, a brief background of the research conducted into silica nanofluids for EOR is discussed. Wettability alteration, interfacial tensionreduction and structural disjoining pressure due to NP wedge formation are the major mechanisms attributed to incremental oil recovery by silica NPs. However, the adsorption mechanisms of silica NPs and their effect on fluid/rock interactions are not well understood. This thesis focusses on the adsorption of silica NPs for sandstone and chalks mineral surfaces and their effect on fluid/rock interactions. The materials and methods used in this study are presented in chapter 3. Chapter 4 addresses the surface modification of berea sandstone by the in-house silica nanofluids. Fines migration during water injection, especially in the case of low salinity, is a potential problem in sandstone reservoirs. It is shown that adsorption of silica NPs in berea sandstone reduces production and migration of fines. This is due to reduction of direct contact between the flooding fluid and rock minerals. The reduction of the fines was indicated by the reduced pressure drop, i.e. reduce the flow resistance of the fluid during the post flush of the NPs’ slug. In addition, it was shown that the adsorption of silica NPs modify sandstone surface and make the interaction between the modified surface and the fine particles more attractive. So, modified surface acts as a “collector” for the fines. The in-house silica nanofluids show limited stability of the dispersed NPs. To proceed with the objectives of this work, it was decided, then, to acquire a more stable commercial silica nanofluid (DP9711 from Nyacol Nano technologies). The nanofluids’ stability was confirmed at our laboratory. Two types ofadsorption experiments were performed: (1) static adsorption of silica NPs on minerals and (2) dynamic adsorption of silica NPs injected into sandstone and chalk cores. The kinetic aspects of silica NP adsorption were also addressed. The static adsorption was done to address the silica NPs adsorption affinity to the different minerals (calcite, quartz and kaolinite) and the kinetics of the adsorption process (chapter 5). The dynamic adsorption of the injected silica NPs was performed to address the extent of the fluid/rock (sandstone and chalk) interactions in chapter 6. Fluid/rock interactions during oil recovery by continuous injection of silica nanofluids are addressed in chapter 7. Silica NPs shows high adsorption affinity towards calcite mineral followed by quartz, and the lowest adsorption affinity towards kaolinite. The scanning electron microscopy (SEM) images did not show pore throat blockage. This was also confirmed by the improved injectivity during nanofluids injection. Silica NPs’ adsorption process on quartz and calcite was best fitted to pseudo second order kinetic model. Both the rate of adsorption and the level of equilibrium adsorption increases with the salinity. The adsorption of NPs is largely influenced by the fluid pH for chalk and sandstones. Increased alkalinity during low salinity flooding favours NP desorption. However, dynamic adsorption of NPs injected into chalk and sandstone core showed high irreversible adsorption at elevated salinity (synthetic seawater: SSW). It is interesting to see that in the limited oil recovery experiments; mineral dissolution, suppression of the ion exchange process and loss of cementing minerals caused by low salinity injection, were reduced by silica nanofluids. It is also shown that the silica NPs modifies the mineral surface and made the interaction energy between the fines and the mineral surface more attractive for both LSW and SSW. In other words, the silica nanofluids reduce the probability for formation damage associated with low salinity water injection in sandstone reservoirs. Some incremental oil recovery was observed with silica NPs. However, limited experiments were performed on oil recovery, hence the recovery by nanofluids has not been optimized in this work. NP adsorption on chalk significantly reduced calcite dissolution by about 30%. That is the silica nanofluid reduced the detrimental effect of low salinity flooding on chalk matrix integrity which is one of the major concerns in chalk reservoirs. As mentioned earlier oil recovery optimization was not performed. The results from this work identified that silica nanofluids can potentially increase oil recovery from chalks as compared to low salinity injection alone. The main outcome of this work suggests a synergy between silica NPs and low salinity flooding technique for EOR wherein, addition of silica NPs to low salinity water can reduce formation damage in sandstone reservoirs and reduce the risk of reservoir subsidence due to calcite dissolution in chalk reservoirs. The results from this work identified that silica nanofluids can potentially increase oil recovery from chalks as compared to low salinity injection alone. The main outcome of this work suggests a synergy between silica NPs and low salinity flooding technique for EOR wherein, addition of silica NPs to low salinity water can reduce formation damage in sandstone reservoirs and reduce the risk of reservoir subsidence due to calcite dissolution in chalk reservoirs.