Performance and enhanced oil recovery efficiency of an acid-resistant polymer microspheres of anti-CO_(2) channeling in low-permeability reservoirs

CO_(2) flooding is a vital development method for enhanced oil recovery in low-permeability reservoirs,However,micro-fractures are developed in low-permeability reservoirs,which are essential oil flow channels but can also cause severe CO_(2) gas channeling problems.Therefore,anti-gas channeling is a necessary measure to improve the effect of CO_(2) flooding.The kind of anti-gas channeling refers to the plugging of fractures in the deep formation to prevent CO_(2) gas channeling,which is different from the wellbore leakage.Polymer microspheres have the characteristics of controllable deep plugging,which can achieve the profile control of low-permeability fractured reservoirs.In acidic environments with supercritical CO_(2),traditional polymer microspheres have poor expandability and plugging properties.Based on previous work,a systematic evaluation of the expansion performance,dispersion rheological properties,stability,deep migration,anti-CO_(2) channeling and enhanced oil recovery ability of a novel acid-resistant polymer microsphere(DCNPM-A)was carried out under CQ oilifield conditions(salinity of85,000 mg/L,80℃,pH=3).The results show that the DCNPM-A microsphere had a better expansion performance than the traditional microsphere,with a swelling rate of 13.5.The microsphere dispersion with a concentration of 0.1%-0.5%had the advantages of low viscosity,high dispersion and good injectability in the low permeability fractured core.In the acidic environment of supercritical CO_(2),DCNPM-A microspheres showed excellent stability and could maintain strength for over 60 d with less loss.In core experiments,DCNPM-A microspheres exhibited delayed swelling characteristics and could effectively plug deep formations.With a plugging rate of 95%,the subsequent enhanced oil recovery of CO_(2) flooding could reach 21.03%.The experimental results can provide a theoretical basis for anti-CO_(2)channeling and enhanced oil recovery in low-permeability fractured reservoirs.

Surface-functionalized cellulose nanocrystals(CNC)and synergisms with surfactant for enhanced oil recovery in low-permeability reservoirs

Nanocellulose,a natural polymeric nanomaterial,has attracted significant attention in enhanced oil recovery(EOR)applications due to its abundance,nanoscale,high oil-water interfacial adsorption ef-ficiency.In this study,surface-functionalized cellulose nanocrystals(SF-CNCs)were prepared via hy-drochloric acid hydrolysis and chemical modification,with adaptable nanosize and considerable dispersion stability in low-permeability reservoirs.The SF-CNCs were structurally characterized by FT-IR,Cryo-TEM,which have a diameter of 5-10 nm and a length of 100-200 nm.The SF-CNC dispersions possessed higher stability and stronger salt-tolerance than those of corresponding CNC dispersions,due to the strong hydrophilicity of the sulfonic acid group.It was synergistically used with a non-ionic surfactant(APG1214)to formulate a combined flooding system(0.1 wt%SF-CNC+0.2 wt%APG1214).The combined flooding system exhibits strong emulsification stability,low oil-water interfacial tension of o.03 mN/m,and the ability to alter the wettability for oil-wetting rocks.Furthermore,the combined system was_able to provide an optimum EOR efficiency of 20.2%in low-permeability cores with 30.13×10^(-3)μm^(2).Notably.it can enlarge the sweep volume and increase the displacement efficiency simultaneously.Overall,the newly formulated nanocellulose/surfactant combined system exhibits a remarkable EoR performance in low-permeability reservoirs.

Advances in enhanced oil recovery technologies for low permeability reservoirs

Low permeability oil and gas resources are rich and have great potential all over the world, which has gradually become the main goal of oil and gas development. However, after traditional primary and secondary exploitation, there is still a large amount of remaining oil that has not been recovered.Therefore, in recent years, enhanced oil recovery(EOR) technologies for low permeability reservoirs have been greatly developed to further improve crude oil production. This study presents a comprehensive review of EOR technologies in low permeability reservoirs with an emphasis on gas flooding, surfactant flooding, nanofluid flooding and imbibition EOR technologies. In addition, two kinds of gel systems are introduced for conformance control in low permeability reservoirs with channeling problems. Finally,the technical challenges, directions and outlooks of EOR in low permeability reservoirs are addressed.

CO_2-triggered gelation for mobility control and channeling blocking during CO_2 flooding processes

CO2 flooding is regarded as an important method for enhanced oil recovery （EOR） and greenhouse gas control. However, the heterogeneity prevalently dis- tributed in reservoirs inhibits the performance of this technology. The sweep efficiency can be significantly reduced especially in the presence of ＂thief zones＂. Hence, gas channeling blocking and mobility control are important technical issues for the success of CO2 injection. Normally, crosslinked gels have the potential to block gas channels, but the gelation time control poses challenges to this method. In this study, a new method for selectively blocking CO2 channeling is proposed, which is based on a type of CO2-sensitive gel system （modified polyacry- lamide-methenamine-resorcinol gel system） to form gel in situ. A CO2-sensitive gel system is when gelation or solidification will be triggered by CO2 in the reservoir to block gas channels. The CO2-sensitivity of the gel system was demonstrated in parallel bottle tests of gel in N2 and CO2 atmospheres. Sand pack flow experiments were con- ducted to investigate the shutoff capacity of the gel system under different conditions. The injectivity of the gel system was studied via viscosity measurements. The results indi- cate that this gel system was sensitive to CO2 and had good performance of channeling blocking in porous media. Advantageous viscosity-temperature characteristics were achieved in this work. The effectiveness for EOR in heterogeneous formations based on this gel system was demonstrated using displacement tests conducted in double sand packs. The experimental results can provide guideli- nes for the deployment of theCO2-sensitive gel system for field applications.

Preparation and performance of fluorescent polyacrylamide microspheres as a profile control and tracer agent

Polyacrylamide microspheres have been suc- cessfully used to reduce water production in reservoirs, but it is impossible to distinguish polyacrylamide microspheres from polyacrylamide that is used to enhance oil recovery and is already present in production fluids. In order to detect polyacrylamide microspheres in the reservoir pro- duced fluid, fluorescent polyacrylamide microspheres P（AM-BA-AMCO）, which fluoresce under ultraviolet irradiation, were synthesized via an inverse suspension polymerization. In order to keep the particle size distribu- tion in a narrow range, the synthesis conditions of the polymerization were studied, including the stirring speed and the concentrations of initiator, NaaCO3, and dispersant. The bonding characteristics of microspheres were deter- mined by Fourier transform infrared spectroscopy. The surface morphology of these microspheres was observed under ultraviolet irradiation with an inverse fluorescence microscope. A laboratory evaluation test showed that the fluorescent polymer microspheres had good water swelling capability, thus they had the ability to plug and migrate in a sand pack. The plugging rate was 99.8 % and the residual resistance coefficient was 800 after microsphere treatment in the sand pack. Furthermore, the fluorescent microspheres and their fragments were accurately detected under ultra- violet irradiation in the produced fluid, even though theyhad experienced extrusion and deformation in the sand pack.

Study of the reinforced mechanism of fly ash on amphiphilic polymer gel

Amphiphilic polymer gels are widely used in heterogeneous reservoirs for conformance control technology.However,in high temperature and high salinity of calcium and magnesium reservoirs,amphiphilic polymer gels cannot maintain effective performance.In this work,a novel reinforced amphiphilic polymer gel(F-PADC gel)was prepared by physically mixing polymer solution and fly ash(FA),which is an extremely low cost material.The viscoelasticity and stability of the F-PADC gel were studied by rheometry and micro-rheometry.The reinforced mechanism of FA on amphiphilic polymer gels was revealed.The results show that the addition of FA can make the gel more robust with a denser network structure.On the fifth day,the elastic modulus(G’)increases from 5.2 to 7.0 Pa and the viscosity modulus(G")increases from 0.4 to 0.6 Pa at the frequency of 1 Hz,which improves the viscoelasticity of the gel system.More importantly,the F-PADC gel does not degrade after aging at 85℃for 180 d.And its viscoelasticity increases obviously,G′and G"increase to 110.0 Pa and 3.5 Pa,respectively,showing excellent anti-aging stability.Moreover,FA amphiphilic polymer gels have a good injectivity and a perfect plugging rate of 98.86%,which is better than that of sole amphiphilic polymer gels.This novel mixed FA amphiphilic polymer gels can prove to be a better alternative to conventional polymer gels to enhance oil recovery in high temperature and high salinity reservoirs.

Interferon-Gamma Release Assay is Not Appropriate for the Diagnosis of Active Tuberculosis in High-Burden Tuberculosis Settings： A Retrospective Multicenter Investigation

Background：Interferon-gamma release assay （IGRA） has been used in latent tuberculosis （TB） infection and TB diagnosis,but the results from different high TB-endemic countries are different.The aim of this study was to investigate the value of IGRA in the diagnosis of active pulmonary TB （PTB） in China.Methods：We conducted a large-scale retrospective multicenter investigation to further evaluate the role of IGRA in the diagnosis of active PTB in high TB-epidemic populations and the factors affecting the performance of the assay.All patients who underwent valid T-SPOT.TB assays from December 2012 to November 2015 in six large-scale specialized TB hospitals in China and met the study criteria were retrospectively evaluated.Patients were divided into three groups：Group 1,sputum culture-positive PTB patients,confirmed by positive Mycobacterium tuberculosis sputum culture;Group 2,sputum culture-negative PTB patients;and Group 3,non-TB respiratory diseases.The medical records of all patients were collected.Chi-square tests and Fisher＇s exact test were used to compare categorical data.Multivariable logistic analyses were performed to evaluate the relationship between the results of T-SPOT in TB patients and other factors.Results：A total of 3082 patients for whom complete information was available were included in the investigation,including 905 sputum culture-positive PTB cases,914 sputum cultmre-negative PTB cases,and 1263 non-TB respiratory disease cases.The positive rate of T-SPOT.TB was 93.3% in the culture-positive PTB group and 86.1% in the culture-negative PTB group.In the non-PTB group,the positive rate of T-SPOT.TB was 43.6%.The positive rate of T-SPOT.TB in the culture-positive PTB group was significantly higher than that in the culture-negative PTB group （x2 =25.118,P 〈 0.01）,which in turn was significantly higher than that in the non-TB group （x2 =566.l 16,P 〈 0.01）.The overall results were as follows：sensitivity,89.7%;specificity,56.37%;positive predictive value,74.75%;negative predictive value,79.11%;and accuracy,76.02%.Conclusions：High false-positive rates of T-SPOT.TB assays in the non-TB group limit the usefulness as a single test to diagnose active TB in China.We highly recommend that IGRAs not be used for the diagnosis of active TB in high-burden TB settings.