Microbial dewaxing of crude oil is an efficient and simple method. In order to further improve the paraffin degradation rate of microorganisms, two strains L and K were used to form paraffin degradation mixed bacteria, and their degradation conditions were optimized. The factors affecting the degradation of mixed bacteria (culture temperature, initial pH value of culture medium, salt concentration, and V（L strain ）/V（K strain）) were determined by orthogonal experiment and single factor experiment. Box-Behnken method was used to design four?factor three?level experiment and response surface optimization, and a mathematical model was established to explore the best conditions for the degradation of paraffin by mixed bacteria. The experimental results showed that the influence of four single factors is as follows: V（L strain）/V（K strain）>culture temperature>salt concentration>initial pH value of culture medium. Their primary term, secondary term and individual interaction term had significant influence on the paraffin degradation rate. The optimal experimental operating conditions for the mixed bacteria to degrade paraffin were determined as follows: culture temperature 37.2 ℃, initial pH value of culture medium 7.3, salt concentration of culture medium 1.2%, ratio of mixed bacteria V（L strain）/V（K strain）=1.0∶1.6, and paraffin degradation rate 58.67%. The mixed bacteria were used to act on crude oil under the optimized conditions, and the biodegradation rate was 41.38%,have good application prospects.

Aiming at the periodic characteristics of severe slug flow an experimental study was carried out through the downward?riser system. Based on the experimental data and the variation of pressure in the pipe, the variation rule of the severe slug flow period was analyzed and verified reciprocally with the calculation model results. It turned out that the flow state in the tube would change under different experimental parameters such as the inclination angle of the downdip tube, the gas phase conversion velocity or the liquid phase conversion velocity. The time of the slug eruption and slug reflux was basically changeless on account of the height of the riser was invariant, so the period of severe slugging depends on the time of slug formation and slug outflow stages. And it was mainly affected by the inclination angle of downward riser, gas superficial velocity or liquid superficial velocity.

The influence of sulfate reducing bacteria (SRB) and CaCO₃ scale layer on the corrosion behavior of 20^# steel was studied by laboratory simulation experiment. The uniform corrosion rate was studied by corrosion weight loss method.The corrosion electrochemical behavior of 20^# steel in the injected aqueous solution was studied by electrochemical impedance spectroscopy (EIS) and open circuit potential (OCP). The morphology and composition of corrosion products on the surface of 20^# steel were analyzed by scanning electron microscopy (SEM), EDS and Raman spectroscopy. The pitting pit depth of 20^# steel was analyzed by laser confocal microscopy (CLSM). The results show that there is a synergistic effect between SRB and CaCO₃ scale layer. The porosity of CaCO₃ scale layer and the anaerobic environment under the scale layer provide convenient conditions for the adhesion and growth of SRB. The inoculation of SRB can promote the uniform corrosion and local corrosion of 20^# steel under the CaCO₃ scale layer.

The geometric model of a certain oil and gas mixed transportation pipeline was established with OLGA software, and the transient flow law during the pipeline shutdown and restart process was studied. First, the distribution characteristics of temperature, pressure and liquid holdup along the pipeline during steady state operation were analyzed, and the location of the minimum temperature and the location of the maximum pressure along the pipeline were determined. The influence of environmental temperature and shutdown time on operating parameters were analyzed. The safe shutdown time when the minimum temperature temperature is higher than its freezing point was determined. In the actual operation process, the shutdown time should not exceed the safe shutdown time, otherwise problems such as pipeline condensation and excessive starting pressure will easily occur, which threaten the safety of the pipeline.