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As efficient technologies boost oil yields and economic benefits, horizontal wells and hydraulic fracturing are widely used in low- permeability reservoirs. To better evaluate the reserve and improve recovery, it is essential to determine fluid flow patterns and transport mechanisms. Laboratory experiments, field operations, and analytical studies have identified nonlinear flow and microfracture networks during the fluid flow in a reservoir with fractured horizontal wells. However, the interactions between nonlinear flow and microfracture networks are still not fully understood. In this study, nonlinear flow experiments and triaxial compression tests were carried out to analyze nonlinear flow characteristics in the vicinity of microfracture networks. By analyzing the effects of microfracture networks on nonlinear flow, two-phase flow, rock stress sensitivity, and artificial fractures, we found that fluid capacity in capillaries with smaller dimensions decreased along with a drop in the pressure gradient, generating a nonlinear flow pattern. The area of nonlinear flow was diminished by the presence of microfractures, which improved flow efficiency and reservoir quality. Considering the size of fracture apertures, microfractures behave more like matrix pores than natural fractures. Also, microfractures significantly increase rock stress sensitivity and reduce the threshold permeability, which enhances fluid flow capacity. This study contributes to our understanding of flow behavior, predicting production and improving recovery in low-permeability reservoirs.

OBJECTIVES Fifteen years after the roll-out of antiretroviral treatment (ART) in China, there is limited information available on transmitted HIV drug resistance (TDR). This study aimed to characterize the epidemiology of TDR in China. DESIGN We conducted a prospective cross-sectional observational study. METHODS We analyzed the demographic, clinical, and virological data of individuals with newly diagnosed HIV infection using data from the Beijing HIV laboratory network collected between 2001 and 2017. We did population-based sequencing of the pol gene on plasma specimens and identified TDR mutations using the WHO list for surveillance of TDR mutations. RESULTS Data on TDR were available for 91% of the 10 115 individuals with newly diagnosed HIV infection tested, of whom 19.2% were from rural areas. The overall prevalence of TDR was 4.1% [95% confidence interval (CI): 3.7-4.5%], with a declining trend over the period 2001-2017. In the multivariable analysis, the risk of TDR differed significantly according to sex [odds ratio (OR) for women vs. men: 0.41, 95% CI: 0.22-0.69, P = 0.002]; infection type (OR for CRF07_BC vs. CRF01_AE: 0.24, 95% CI: 0.16-0.36, P

HIV subtypes convey important epidemiological information and possibly influence the rate of disease progression. In this study, HIV disease progression in patients infected with CRF01_AE, CRF07_BC, and subtype B was compared in the largest HIV molecular epidemiology study ever done in China. A national data set of HIV pol sequences was assembled by pooling sequences from public databases and the Beijing HIV laboratory network. Logistic regression was used to assess factors associated with the risk of AIDS at diagnosis ([AIDSAD], defined as a CD4 count

Cancer cells are well recognized by immunotolerance characteristics, which include low immunogenicity, weak antigen presentation, low T cell infiltration, and high expression of inhibitory receptors and cytokines. This typical mechanism allows cancer cells to easily avoid the attack by immune cells, resulting in immune escape. Due to unique properties, such as adjustable size, unique surface properties, and ease of modification, nanomaterials have shown great potential in the field of immunotherapy. In this review, we summarize several ways in which nanomaterials can enhance the immune response against tumor cells. Therefore, we presently discuss the potential mechanisms involved in the induction of immune response by a series of nanomaterials. As a result, we also propose the generation of more advanced and tumor-driven nanomaterials, as well as their potential applications in tumor immunotherapy. According to the route of tumor immunity, we have currently classified nanomaterials into three categories: (1) Nanomaterials with inherent immunoregulatory functions, (2) nanomaterials that, upon an exogenous response, trigger immunogenic cell death and/or enhance immune responses, and (3) nanomaterials that are used as delivery vehicles to unload other immune-activating molecules (e.g., chemical drugs, photosensitizers, proteins and DNA plasmids) to tumors or immune cells as well as immune-activating molecules that may enhance an immune response. Some natural or synthetic nanomaterials can inherently stimulate macrophage polarization and enhance immune response. As such, nanomaterials may polarize M2 macrophages into M1 macrophages by regulating the cellular phenotype, reversing the tumor immune microenvironment and, ultimately, improving an immune cell killing activity. Cytokines or compounds with immune stimulating functions can enhance an immune response by promoting the proliferation of immune cells such as T lymphocytes and NK cells. Moreover, some nanomaterials rely on their own electrical properties or responsiveness to the tumor microenvironment, leading to tumor cell immunogenic death and/or enhancing anti-tumor immune responses. Nanomaterials have been widely used in photothermal, magnetocaloric and radiation therapies as well as into other fields, due to their unique optical, electrical, and magnetic properties. Recently, it has been discovered that some of these properties may “switch” tumor cells from non-immunogenic to immunogenic cells (i.e., immunogenic cell death) and, consequently, enhance an anti-tumor immune response. Compared with an endogenous response that mostly relies on characteristics of the tumor microenvironment, rationally designed nanomaterials that are responsive to exogenous responses can more effectively enhance the immune response at the tumor site and then achieve a more precise and tractable treatment. Hence, here we introduce the notion of an enhanced immune response, dependent on the use of nanomaterials co-stimulated by exogenous responses such as near-infrared light response, magnetic field response and others. Lastly, some substances, such as drugs, photosensitizers, proteins, plasmids, are unable to elicit a robust immune response when used without a carrier. In addition, some agents can also cause tissue damage and/or local toxicity when solely used for treatment. Thus, these substances often need to be combined with nanomaterials to achieve high biological loads, targeted delivery, controllable release and other functions to safely and efficiently enhance the anti-tumor immune response. Nevertheless, current research on nanomaterial-based immunotherapy is still under development, and the translation for clinical application is still challenging. Therefore, advances in the development of innovative therapeutic nanomaterials will offer great potential for improving the effectiveness of immunotherapy.

Cancer immunotherapy provides an effective way to deal with cancer. Although immunotherapy strategies have shown encouraging therapeutic effects, the inherent limitations of immunotherapy, such as multiple tumor immune evasion methods, low response rate, and systemic toxicity, still hinder its clinical applications. In recent decades, nanomaterials have been considered promising in cancer immunotherapy since they can realize targeted delivery and interact with the immune system to induce or enhance the antitumor immune responses. Among them, organic/inorganic nanocomposites are ideal candidates for cancer immunotherapy since they could combine the advantages of both organic and inorganic components. Multifunctional organic/inorganic nanocomposites could help overcome the shortcomings of current cancer immunotherapy, and realize the combination of immunotherapy and other therapeutic strategies with synergistic antitumor effects. Herein, we review the recent progress of organic/inorganic nanocomposites designed for cancer immunotherapy. The immunotherapy strategies of nanocomposites are summarized from the perspective of achieving immune enhancement. The challenges of nanocomposites in cancer immunotherapy are also discussed.

HIV-1 CRF01_AE and CRF07_BC have been two mainly circulating HIV-1 strains in the men who have sex with men (MSM) population in Beijing for years. These two subtypes together were accounting for 78.1% of HIV-1 positive cases newly diagnosed in Beijing, 2016. In this study, we report a novel CRF01_AE/CRF07_BC second-generation unique recombinant form (URF) (named DT1427_NFLG) of HIV-1 identified in MSM population. The near full-length genome of DT1427_NFLG is about 8.8 kb with four CRF07_BC fragments inserted into the CRF01_AE backbone. In China, several novel second-generation URFs were reported in recent years and Beijing, as the capital of China, attracting a huge number of people all over the country to work and live, is confronted with the risk of the epidemic of recombinant HIV-1 strains. Therefore, it is necessary to take measures to monitor the emergency of novel recombinant of HIV-1.

Predicting well performance in low-permeability reservoirs has attracted attentions in the last decades. Decline curve analysis is treated as the most convenient and efficient approach to forecast production. However, each single decline model has its own limitations due to the low permeability and porosity. Hence, we cannot directly apply these models in the low-permeability reservoirs, especially the dramatically drop in the early period. In this study, based on the analysis of conventional production decline models and the development characteristics of low-permeability reservoirs, a new production decline model was proposed, which can quickly and accurately predict the future performance of horizontal wells. Three fitting parameters were employed representing the production performance in early, middle and late-life period respectively. A decline exponent parameter of n was also included in the proposed model. Subsequently, the proposed model was validated, and good agreement was obtained with the production data of a low-permeability block in China. The comparison between new method and other models are analyzed in detail, which illustrates that this proposed approach is more flexible and confident. This work can give a useful insight into accurately and efficiently predicting well performance in low-permeability reservoirs.

In China, CRF01_AE and CRF07_BC are the most circulating strains of HIV-1, more and more second-generation recombinant forms have been isolated around the two strains. The same situation happened in Beijing in recent years. In our study, we have isolated a new second-generation recombinant form of HIV-1 from a male patient who was infected by homosexual behavior. Polygenetic analysis revealed that the sequence includes five CRF01_AE fragments and four CRF07_BC fragments. CRF01_AE is the skeleton of the recombinant which was inserted with four CRF07_BC fragments. The emergency of such second-generation recombinant forms manifests the diversity of the HIV-1 epidemic. Consequently, further investigation of molecular epidemiology is needed to track the genetic evolution of HIV-1.

Oil production prediction model and water flooding characteristic curves are generally applied to production performance analysis. However, these two methods have some limitations especially in predicting water cut. Growth model can predict oil production and cumulative oil production, but it cannot predict water cut. Water flooding characteristic curves can predict water cut and cumulative oil production, but it cannot establish the relationship between production indexes and time. The Morgan–Mercer–Flodin (MMF) growth model is a kind of production prediction model. This study analyzed features of MMF growth model, then Hu–Chen (HC) model was deduced on the base of MMF growth model. Theoretical basis of HC model was improved and solution of parameters was simplified. Then combining it with water flooding characteristic curve, a new combined solution model was built. This new combined solution model can predict water cut versus time and it overcame deficiencies of production prediction model and water flooding characteristic curves. The applications of field case selected from the Block-A in Daqing Oilfield indicated that the prediction accuracy of this new combining model can meet the demand of production performance analysis and it can guide the determination and adjustment of the development programs in water flooding hydrocarbon reservoirs. It has features of higher prediction accuracy, easier to use and widely practicable compared with other combined models.

A series of experiments was conducted to quantify foamy oil recovery under different conditions, including the pressure depletion rate, solution gas-oil ratio (GOR), and oil viscosity. As indicated by the experimental results, at higher-pressure depletion rates, solution GOR, and oil viscosity, the foamy oil ultimate recovery tends to be higher. The foamy oil phenomenon can occur when the pressure depletion rate, solution GOR, and oil viscosity exceed certain values. Based upon experimental results, multiple regression analysis was used to establish a mathematical expression of the relationship between dimensionless foamy oil recovery ( R eD ) and the main factors affecting it, including the dimensionless pressure depletion rate (Δ p rD ), the dimensionless solution GOR ( R sD ), and the dimensionless oil viscosity ( μ D ). The computed results showed the weighted values of the effects of lnΔ p rD , ln R sD , and ln μ D on ln R eD to be 0.653, 0.211, and 0.136, respectively, which indicates that improving the pressure depletion rate may be the most effective approach to increasing the amount of foamy oil recovered. The uses and limitations of the correlation were validated using verification experiments and are discussed. Suggestions for rendering foamy oil recovery more efficient can provide useful guidance for the development of heavy hydrocarbon reservoirs.

Beijing has seen a rising epidemic of HIV among students. However, little information was known about the molecular epidemiologic data among HIV-infected students. In this study, the diversity and the prevalence of transmitted drug resistance (TDR) in pol sequences derived from 237 HIV-infected students were analyzed. TDR mutations were found in five men who have sex with men (MSM) population among students. The overall prevalence of TDR in students was 2.1%, comprising 1.3% of protease inhibitors and 0.8% of non-nucleoside reverse transcriptase inhibitors. Our finding indicates a low-level prevalence of TDR mutations among students in Beijing.

In the late 1990s, large amounts of low permeability resource plays in the south of Songliao basin have been discovered and made a great contribution to regional development, so far most of them need to be made efforts to improve recovery factors by tertiary method, CO2 flooding has an effective way for this kind of reservoir. CO2 injection pilot was implemented in 2008, it provided invaluable information for extensive areas to make gas injection and production scheme including produced gas recycling. Most low permeability reservoirs in Song Liao basin have no nature flow, the economic productivity could not be obtained except fracturing. Therefore, it leads to CO2 breakthrough early and produced gas profile variation greatly during the CO2-EOR process. CO2 is from the deeper natural gas field with CO2 concentration over 20%, after separation, the CO2 purity reaches 99% for injection. So how to treat the EOR produced gas is an environmental and economic issue to achieve zero emissions for the whole operations. According to the actual field test data, the vast majority of composition of produced gas are lighter hydrocarbon and CO2. This paper will analyze the composition of gas samples from Jilin oil field, understanding the components of the output gas variation with time. The phase behavior and mass transfer will be characterized by the minimum miscibility pressure (MMP) test and core flood experiment, and the quantitative description method will be obtained through the relationship between the individual component or segment components and sweep efficiency. Consideration the effect of CO2 impurities on MMP and displacement efficiency, the critical value of CO2 concentration of produced gas is 85%~95%, according to this study, all the produced gas can be reinjection without separation, the value for field application is approximately 90%. That means it would require mixing with pure CO2 for reinjection when CO2 concentration of produced gas is below 90%, otherwise, it could re-inject to oil field directly. All these results have been applied to achieve reasonable CO2 recycle.

To analyze the proportion and associated factors of taking subsequent confirmation test among men who have sex with men (MSM) after being tested positive in oral fluid HIV antibody test.By using successive sampling, 1 003 MSM, who were tested positive in oral fluid HIV antibody test in China-BillMelinda Gates Foundation AIDS prevention Program (Extension program) in Beijing during May 1 to December 31, 2013, were recruited. The inclusion criteria included: the objects were men who reported having sex with men; the objects aged more than 18 years old; the objects were tested positive in oral fluid HIV antibody test; the objects had not been reported as HIV positives in China Information System for Disease Control and Prevention previously. According to the program strategy, MSM grassroots organizations transferred the respondents to seek subsequent confirmation tests in specific Center for Disease Control and Prevention (CDCs) or hospitals. The subsequent confirmation tests included: fingertip blood HIV antibody rapid test, venous blood Enzyme Linked Immunosorbent Assay (ELISA) HIV antibody test and venous blood Western Blot (WB) HIV antibody test. Chi-square test was adopted to compare the proportion of taking subsequent confirmation tests in different groups. Nonconditional multivaritae binarylogistic regression analysis was taken to identify the associated factors with whether taking subsequent confirmation tests and to calculate the OR (95% CI) values.The 1 003 respondents were (30.9 ± 9.1) years old. Among all objects, 87.8% (881/1 003) of them took fingertip blood HIV antibody rapid tests and the positive rate was 85.4% (752/881). 98.0% (737/752) of those who were identified as positive in fingertip blood HIV rapid tests took ELISA and WB tests, and the positive rate was 94.4% (696/737). Comparing with those who were expected to seek subsequent confirmation tests in CDCs, the OR (95% CI) value of those who were expected to seek tests in hospitals was 5.10 (1.69-15.36). The OR (95% CI) values of those who used condom sometimes and those who never used condom in anal sex were 5.81 (2.14-15.77) and 3.45 (2.00-5.97) respectively, in comparison with those who reported not having anal sex or using condom consistently in anal sex during the past 6 months. Comparing with the respondents recruited from the internet, the OR (95% CI) values of those recruited in bathrooms, parks/toilets and bars were 0.17 (0.05-0.53), 0.10 (0.04-0.29) and 0.22 (0.06-0.79) respectively. The likelihood of taking subsequent confirmation test decreased with the increase of number of male sexual partners in the past 3 months, and the OR (95% CI) value was 0.92 (0.86-0.99).The potential HIV positive MSM in the bathroom, park/toilet and bars are less likely to take subsequent confirmation test. Those who do not use condom consistently during anal sex are more likely to seek subsequent confirmation test. Medical organization conducting subsequent confirmation tests is more likely to increase the confirmation test rate of potential HIV positive MSM. The number of male sexual partners has negative correlation with whether to accept the subsequent confirmation test.

Background Sexual transmission of HIV among men who have sex with men (MSM) increased markedly in China during the past decade. HIV incidence is a critical indicator in HIV surveillance and we use a HIV-1 BED-capture-enzyme immunoassay (BED-CEIA) to examine the incidence among MSM in Beijing from 2008 to 2016. Risk factors related to recent HIV infection were also assessed. Methods Consecutive cross-sectional surveys on MSM were conducted yearly from 2008 through 2016. Demographic and behaviors data were collected. HIV status was determined and HIV positive specimens were tested for recent infection using BED-CEIA. Specimens with ODn values≤0.8 were considered recently infected, HIV incidence rates and prevalence were then calculated. Risk factors associated with recent HIV infection were assessed by univariate and multivariable logistic regression. Results From 2008 to 2016, the numbers of eligible participants in the nine consecutive years ranged from 472 to 616. All the 261 eligible HIV-positive specimens were subjected to recent HIV infection testing. HIV prevalence ranged from 5.0% (3.3%-6.8%) to 10.2% (7.8%-12.7%), and incidence ranged from 1.57% (0.19%-2.95%) to 6.63% (3.65%-9.61%). MSM who never or sometimes used condoms during anal sex with men in the past 6 months (aOR = 1.515, 95%CI: 1.016-2.257, p = 0.041), or having syphilis infection (aOR = 1.561, 95%CI: 0.946-2.575, p = 0.081) were more likely to be recently infected with HIV. Being a Beijing resident (aOR = 0.409, 95%CI: 0.212-0.790, p = 0.008), or having only one male anal sex partner in the past 6 months (aOR = 0.467, 95%CI: 0.220-0.994, p = 0.048) were associated with a lower risk for recent HIV infection. Conclusions The HIV incidence fluctuated among MSM in Beijing. Unprotected anal sex, having multiple sex partners, being a non-registered Beijing resident and having a syphilis infection play important roles in the recent HIV infection. Effective intervention measures for HIV and syphilis control and prevention should be continuously strengthened.

To understand the epidemiologic characteristics on newly reported HIV cases and those recently infected HIV-1 cases in the past few years in Beijing so as to find out their correlates.All the qualified newly reported HIV serum samples from both the first quarters of 2009, 2010 and the second quarter of 2011 in Beijing, were tested using the BED HIV-1 incidence capture enzyme immunoassay (BED-CEIA) to identify those recent HIV-1 infection (BED positive) samples and related socio-demographic characteristics. The proportions of BED positives were determined and the correlates analyzed by SPSS software.The Annual qualified newly reported HIV serum samples of each year from Beijing were 274, 236 and 356, with BED positive rate among them as 35.0%, 31.8% and 33.7%, respectively. The majority of newly reported HIV cases of each year were males (89.1%, 88.6%, 91.9%), range of age as 20-39 years (77.4%, 75.8%, 82.0%), Han nationality (77.7%, 88.6%, 85.7%), and most of them were not permanent residents of Beijing (74.1%, 80.5%, 2011 data missed). The proportions of patients through sexual transmission showed upward trend (67.5%, 76.2%, 86.8%). Especially in the 'man having sex with man' (MSM)group, it showed a significantly increase (44.9% , 45.3% , 62.6%). The proportions of injection drug users (15.3%, 8.1%, 9.0%)declined to some extent during this period. Data from statistics indicated that the proportions of BED positives among newly reported HIV cases of each year were significantly correlated to factors as patients' sex, transmission route and source of samples, but not significantly correlated to patents' age or marital status. Male cases accounted for 96.9% and MSM accounted for 73.5% of all the BED-positives during 2009-2011.MSM accounted for both large portion of newly reported HIV cases and recent HIV-1 infections which suggested an increasing trend in 2009-2011. It seemed that the characteristic and changing trends of HIV epidemic in Beijing was oriented by MSM population in the last years.

Abstract Multiple transverse fractures are hydraulically created to enhance the productivity of horizontal wells drilled along with the minimum principal stress. The fracture properties, such as fracture number, length, conductivity, etc, are very important factors affecting the horizontal well productivity. Therefore, a reliable and practicable method for predicting and optimizing the productivity of horizontal wells is essential to reservoir engineering designation. However, existing mathematical models and evaluation methods did not include the impact of the threshold pressure gradient and the pressure-sensitive effect which cannot be ignored in ultra-low permeability reservoirs. The objective of this paper is to present a method considering the threshold pressure gradient and the pressure-sensitive effect, thus predicting and optimizing the productivity of multiple transverse fractured horizontal wells (MFHW) in ultra-low permeability reservoirs becomes available. The new method couples the non-Darcy elliptical flow, which considers the threshold pressure gradient and the pressure-sensitive effect in the reservoir region, the Darcy linear flow in the fractured region, and the Darcy radial flow to the horizontal wellbore in the fractured region respectively. Based on the equality radius mode and the pressure superposition theory, the evaluation methods for the productivity of multiple transverse fractured horizontal wells are presented. In addition, the new method is validated by comparing with field data, results confirm that those formulas are precise and practical, and the evaluation method is reliable. Based on an example of productivity analysis for a multi-fractured horizontal well, the effects of some factors, such as threshold pressure gradient, coefficient of deformation and fracture parameters (fracture number, fracture length, conductivity, uniformity of fractures length and intervals), on the productivity were studied. The results show that the greater the threshold pressure gradient, the greater the effect of it on multi-fractured horizontal well productivity, so the threshold pressure gradient must be accounted to evaluate the MFHW productivity in ultra-low permeability reservoirs. Also, the greater the coefficient of deformation, the greater the effect of it on MFHW productivity, and the affection of coefficient of deformation on productivity is related to producing pressure drop, and the greater the pressure drop, the larger the influence of deformation coefficient on producing rate, thus for strongly pressure-dependent reservoir, we'd better conduct an optimization design for feasible producing pressure drop. In the given condition, the optimal fracture number of the horizontal well is 4~5, the half length of fracture is about 120 m, and the conductivity is 4.8 D•cm. The fractures with longer length, lower conductivity, variable length and placed in unequal interval are favorable for homogeneous oil reservoir. This paper provides reservoir engineers with a reliable and practical method to predict and optimize the productivity of multiple transverse fractured horizontal wells in ultra-low permeability reservoirs.

Abstract After the development of the oil field, it is an important task to predict the production and the recoverable reserve opportunely by the production data. At present, the forecasting methods include reservoir numerical simulation, forecasting model, material balance and production decline, and so on. Of these methods, the forecasting model is a useful and effective one for entire forecasting. The famous Generalized Weng model, Weibull model, Rayleigh model, HCZ model, Hubbert model, Lognormal distribution model are major forecasting models. But these models are all single peak development model, they can't get the reliable results for the bi-peak and multi-peak development model. In this paper a generalized single peak forecasting model and its derived models were established. From the generalized single peak forecasting model, we can derive some famous forecasting models, such as Generalized Weng model, Weibull model, Rayleigh model, Arps Exponential Decline model, Modern model ?, Modern model?, and so on. Subsequently, a multiple peak forecasting model was proposed based on the generalized single peak forecasting model, by which the multi-peak model production, cumulative production and recoverable reserve can be obtained. Furthermore, the resolution method of multi-peak forecasting model parameters was also introduced by nonlinear automatic matching solving techniques. According to the practical application for some oil fields, both generalized single peak forecasting model and multiple peak forecasting model are all confirmed to be practical and effective. Introduction It is important to forecast the production and recoverable reserve during oil field development and management, and the forecasting model is one of the important methods to predict the future production. Forecasting models can be divided into Periodic model and Growth model according to their characteristics. The Periodic model includes generalized Weng model, Weibull model, Rayleigh model, Gamma model, generalized model, and so on. The Growth model includes HCZ model, Hubbert model (which is also called Logistic model)??logarithmic normal distribution model, normal distribution model, Hu-Chen model and other models1–12. The above mentioned models are all based on the single peak development model. On the other hand, bi-peak and multi-peak development models are more constantly displayed during the development of oil field because of putting into production of new block, large scale development adjustment, implement of tertiary oil recovery, and so on. When the oil field development models are bi-peak model, tri-peak model or multi-peak model, it is hard to get the reliable forecasted result only based on single peak model. Based on generalized single peak forecasting model, this paper established the multi-peak forecasting model and its nonlinear multi-parameter automatic matching method. In addition, real data from three oil fields have been adopted to forecast the production, and the results are satisfactory. So the multi-peak forecasting model was approved to be practical and effective.

The parameters influencing gas alternative water (WAG) for CO2 flooding in the low permeability block of the Jilin oil fields in China were investigated using the numerical simulation software, Eclipse. The minimum miscibility pressure was first determined based on slim tube tests. Comparisons were made between continuous water flooding, continuous CO2 flooding, and WAG flooding methods. The oil recovery ratio of “gas injection first method” was higher than that of “water injection first method” and the mechanism of CO2 displacement was analyzed. The optimum parameters for WAG flooding were 7 for the number of slugs, 0.3845 PV for the total injection volume, approximately 0.5742 for the gas/water slug ratio, and 120 days for the stewing time. The optimum injection timing of the switching depletion development to the WAG injection was 0.25 years and the earlier switching to the WAG injection after water flooding was more suitable for enhanced oil recovery. The maximal cumulative water injection by water fl...