Water flooding is one of the most common enhanced oil recovery (EOR) methods, however its application in naturally fractured reservoirs (NFR), suffers from the flow of water in high permeable channels and fractures which leads to low areal and volumetric sweep efficiency. Preformed Particle Gels (PPG), as a subset of gel treatments, can play a vital role in plugging super-permeable zones either near wellbore or deep in the reservoir. Nevertheless the mechanical strength and thermal stability of the designed PPGs, especially under harsh environmental conditions, are subject to further research and development. In spite of recent studies, the effects of nano-materials on the rheological and morphological properties of the PPGs are not systematically investigated and understood. In this study novel PPGs were designed and synthesized (using free radical polymerization process) for application at harsh environments in terms of temperature, pH and salinity. To investigate the effectiveness of the designed PPG samples static swelling bulk tests as well as dynamic Hele-Shaw tests are performed. The examined parameters in static tests include various pH of the environment, the type and concentration of ions in the brine, temperature, dehydration durability and presence of carbon dioxide. In addition, to enlighten the mechanisms behind the performance of the new designed PPGs, comprehensive set of rheological tests (evaluation of both storage and elastic modules) as well as SEM and ESEM image analysis of dry and swollen particles are performed. Taking advantages of silicate sodium gelation capacity and functionalization of graphene nano-platelets (GNP), novel monolithic gel samples (SNGP) with unique qualities are synthesized. Although the addition of sodium silicate and GNP reduced the swelling capacity of the gels, they have significantly improved the rheological properties of the PPG samples. Rheological tests, confirmed that by adding only 0.1 wt% graphene nano-platelets and 0.5 wt% sodium silicate, the storage module of the swollen gel increased three times. Additionally the main advantage of the samples SNGP sample is its stability at harsh conditions such as high salinity, high as well as low pH, high temperature and its dehydration durability. SEM images of dry particles shows that presence of both silicate sodium and GNP has improved the harmonization of the networked structure, with making it thicker and denser. This has increased the porous interrelated structure which in turn is responsible for strong and improved mechanical properties of the SNGP gels. ESEM images of the swollen samples shows that the uniformly porous network structure of hydrogels is led to the maximum water holding capacity, which contributes to good stability of the 3D network structure of swollen PPGs under dehydration and thermal tests. Additionally, this improved microstructure can explain the compressive strengths increment with adding up nano-graphene and silicate content. The two PPG samples with the most suitable specification in the bulk tests were selected for further performance analysis in dynamic tests through Hele-Shaw cell setup. The experimental results in this transparent fracture model showed that increasing salinity causes the increase of the residual resistance factor (Frr). The results showed that the plugging efficiency and the damage factor are directly related to the PPGs size, water salinity and concentration of both GNP and silicate sodium. It is demonstrated that SNGP sample containing 50% size of 1190–595 μm and 50% size of 297–177 μm (larger size distribution) has a higher plugging capability compared to the samples containing each of these particles sizes alone. Additionally both bulk tests and Hele-Shaw cell tests approved that synthesized PPGs are also applicable for CO 2 injection. • New PPG is synthesized to be applicable in harsh environments such as low/high pH, high salinity and high temperature. • Sodium silicate and graphene nano-platelets improved temperature resistance and dehydration durability properties. • Static swelling bulk tests and dynamic Hele-Shaw cell tests are performed to evaluate the water shut-off performance. • Different rheological tests are performed to evaluate the mechanical and rheological properties of the designed hydrogels. • SEM and ESEM are used to shed light on the morphology of the samples and the reason behind improved properties.
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