Experimental investigation on migration and retention mechanisms of elastic gel particles(EGPs)in pore-throats using multidimensional visualized models

Knowledge of migration and retention mechanisms of elastic gel particles(EGPs)in pore-throats is essential for the effective application of EGPs as a smart sweep improvement and profile control agent for enhanced oil recovery(EOR).The matching coefficient(defined as the ratio of particle size to pore-throat size)is used to investigate its influence on migration,retention and profile control performance of EGPs.A 1-D continuous pore-throat visualization model(PTVM),a 2-D heterogeneous PTVM and a 3-D heterogeneous core model were constructed and used to investigate pore-scale migration,retention and controlling mechanism of migration and retention characteristics on EGPs profile control.The results of the 1-D continuous PTVM indicated that while the matching coefficient was in the optimal range(i.e.,0.20-0.32),the EGPs could not only smoothly migrate to the deeper pore-throats,but also form stable retention in the pores to resist the erosion of injected water,which was conducive to the effective indepth profile control.The results of the 2-D heterogeneous PTVM verified that the sweep efficiency in low-permeability regions could be significantly improved by in-depth migration and stable retention of EGPs in the pore-throats with an optimal matching coefficient(0.29),which was much better than that in cases with a smaller matching coefficient(0.17)or an excessive matching coefficient(0.39).Moreover,the NMR displacement experiments of 3-D heterogeneous cores were carried out to simulate the EGPs profile control in actual reservoir porous media.Saturation images and T2 spectrum curves of crude oil showed that EOR in the low-permeability layer was highest(56.1%)using EGPs profile control with an optimal matching coefficient,attributing to the in-depth migration and stable retention of EGPs.

Impact of flow rate on dynamic imbibition in fractured tight sandstone cores

Dynamic imbibition,which is significantly affected by flow rate,plays an important role in the development of tight oil.This study investigated the impact of flow rate on dynamic imbibition in fractured tight sandstone cores via online nuclear magnetic resonance core-flooding experiments.The oil expulsion efficiency and capillary number of multiscale pores were quantitatively analyzed to elucidate the influence of flow rate on the oil recovery during dynamic imbibition.The pores of the cores used were divided into micropores(0.01-1.00 μm in diameter),mesopores(1.00-30.00 μm in diameter),and macropores(30.00-400.00 μm in diameter) by matching the T_(2)spectrum and the mercury intrusion data.The volume proportion of micropores was 52.0%,and that of macropores was 19.0%.The total oil recovery of the core was found to reach 29.8% at the optimal flow rate of 0.1 mL/min.At the optimal flow rate,the oil recovery of micropores reached 50.4%,followed by that of macropores(28.6%),and that of mesopores was the lowest(15.8%).The oil expulsion efficiency,the capillary number,and the contribution to total oil recovery of micropores significantly increased with the decrease in flow rate,while those of macropores decreased.This was caused by the synergy of capillary force and displacement pressure.During dynamic imbibition at a low flow rate,the oil in micropores was effectively expelled driven by capillary force,and the effect of displacement pressure was weak,leading to large amounts of remaining oil trapped in macropores.On the contrary,when the flow rate was too high,large amounts of remaining oil would be trapped in micropores.Only at a moderate flow rate did the capillary force and displacement pressure both have significant effects on oil expulsion,and the oil in different sized pores was effectively expelled,thus generating a relatively high total oil recovery.

A novel triple responsive smart fluid for tight oil fracturing-oil expulsion integration

The traditional multi-process to enhance tight oil recovery based on fracturing and huff-n-puff has obvious deficiencies,such as low recovery efficiency,rapid production decline,high cost,and complexity,etc.Therefore,a new technology,the so-called fracturing-oil expulsion integration,which does not need flowback after fracturing while making full use of the fracturing energy and gel breaking fluids,are needed to enable efficient exploitation of tight oil.A novel triple-responsive smart fluid based on“pseudo-Gemini”zwitterionic viscoelastic surfactant(VES)consisting of N-erucylamidopropyl-N,N-dimethyl-3-ammonio-2-hydroxy-1-propane-sulfonate(EHSB),N,N,N′,N′-tetramethyl-1,3-propanediamine(TMEDA)and sodium p-toluenesulfonate(NaPts),is developed.Then,the rheology of smart fluid is systematically studied at varying conditions(CO_(2),temperature and pressure).Moreover,the mechanism of triple-response is discussed in detail.Finally,a series of fracturing and spontaneous imbibition performances are systematically investigated.The smart fluid shows excellent CO_(2)-,thermal-,and pressure-triple responsive behavior.It can meet the technical requirement of tight oil fracturing construction at 140°C in the presence of 3.5 MPa CO_(2).The gel breaking fluid shows excellent spontaneous imbibition oil expulsion(∼40%),salt resistance(1.2×104 mg/L Na+),temperature resistance(140°C)and aging stability(30 days).

Flow characteristic of polymer solutions in porous media:Influence of the molecular weight and concentration

The polymer solution flow in porous media is a central research topic related to hydraulic fracturing measures,formation damage and fracture propagation.Influenced by molecular weights and concentrations,various flow patterns of polymer in pores are presented,resulting in different filtration loss.In this work,the effectiveness of various polymer solutions for filtration loss was assessed by utilizing the core flooding experiment firstly.The result shows that lesser filtration loss normally is inextricably linked to solutions with high molecular weight and concentration.Subsequently,the flow behaviors of polymer solutions investigated by designed micro pore-throat structure and micro-particle image velocimetry(m-PIV)further confirmed the above result.It was found that the central convergent flow pattern benefiting from higher viscous force loss and less filtration loss was observed at high flow rates(0.5 mL/h),and higher molecular weight and concentration were more prone to convergent flow patterns.The viscosity force loss increases by about 4 times varying the molecular weight of polymer from 5×10^(6)to 18×10^(6)g/mol or the concentration from 0.05 to 0.3%.It interprets higher molecular weight and concentration in core studies and field observations with decreased filtration loss of HPAM.This work provides a theoretical foundation for the application of fracturing fluids as well as fresh perspectives on how to access the filtration loss of fracturing fluids.