Research On The Influence Of Tight Gas Produced Water Quality On Reinjection Reservoir

The high-salinity wastewater produced during the large-scale exploration and development of tight gas mainly includes the initial fracturing flowback fluid and the drainage water produced during the gas production process,collectively referred to as drainage water.The characteristics of Drainagewater includes complex water quality,water yield fluctuates greatly due to the mining stage,difficult treatment,high cost and so on.In order to effectively prevent pollution and protect the environment,the drainage water must be effectively treated and disposed to ensure the green and sustainable development of the industry.The risk of using waste liquid pool for storage drainage water is too high and waste liquid pool cannot meet the needs of large-scale rolling development in the future;Reuse after treatment has the disadvantages of limited reuse water and high requirements for reuse water quality;As for the treatment of high-salt drainage water by evaporative crystallization,its treatment cost is high.Treating the high salinity drainage and reinjection into the stratum has been gradually recognized by the industry in recent years.Moreover,there have been some practical projects in many places in China to treat high-salinity water and reinject the stratum,but our current basic research is lagging behind practice.Whether the same-layer reinjection of water produced by tight gas drainage is feasible and whether it will cause environmental risks is a key issue that the industry is generally concerned about.This paper analyzed the drainage water quality and the characteristics of the reinjection layer in the tight gas block,and the compatibility of water quality at different stages after treatment,the effect of different water quality on clay swelling,the sensitivity of the reinjection layer,the damage of suspended solids and oil content to the reservoir are studied.The feasibility analysis and evaluation of the reinjection of the drainage water after the treatment were carried out,and the recommended water quality index for reinjection was put forward combined with the current relevant standards.In this study,the "Analysis foroil and gas field water"(SY/T 5523—2000)and " Water quality standard and practice for analysis of oil field injecting waters in clastic reservoirs"(SY/T5329—2012)were used to analyze the water quality of the drainage water in the study area.The water quality compatibility was analyzed through the measurement of scale samples and the analysis of scale types.Experimental results showed that the initial fracturing flowback fluid CDJ-07 and CDJ-10 had good compatibility,and the calcium loss rate was stabled below 10%;The fracturing flowback fluid was compatibled with the produced water,and there were a large amount of scaling.Before reinjection,it was descaled on the ground first,and then reinjected at the best mixing ratio.The "Formation damage evaluation by flow test"(SY/T 5358—2010)and "Practices for core analysis "(GB/T 29172—2012)were used to investigate the sensitivity of the rock components of the reinjection layer,the sensitive type and degree of sensitivity of the reinjection layer were obtained.Experimental results showed that the speed sensitivity,acid sensitivity and Alkali Sensitivity damage indexes of the Shihezi Formation 8th Reinjection Layer were 46.59%,21.11% and 82.12%,respectively,which belonged to the medium to weak speed sensitivity,weak acid sensitivity and strong alkali sensitivity.According to the alkali sensitivity experiment,when the p H was less than 8,the damage rate of the reservoir was lower than 20%.Through the core displacement experiment,the influence of suspended solids content and oil content in the drainage water on the damage of the reinjection layer was studied.Experimental results showed that the suspended solids content in water was controlled below10 mg/L and the oil content was controlled within 6 mg/L,reinjection layer damage rate was less than 20%.The clay swelling experiment was used with 3% potassium chloride as a reference to evaluate the stability of the clay after the treatment.Through the analysis of clay expansion experiment,compared with 3% potassium chloride,the maximum clay expansion rate of each water sample was 32.44%,indicated that this type of water will not cause the expansion of the clay after being reinjected into the formation.Based on the above experimental results and referred to the relevant reinjection standards of the oil and gas industry,specific recommended water quality indicators suitable for the reinjection of tight gas and high salinity drainage water in the study area were proposed.