Catalytic Pyrolysis And Pollution Migration Of Oil-based Drilling Cuttings In Shale Gas Exploitation

In today’s world,the contradiction between energy supply and demand is becoming more and more prominent.China,as the world’s number one shale gas reserve,tried to find a breakthrough in this unconventional natural gas.Since 2012,Fuling District in Chongqing has been authorized as one of the four national shale gas exploration demonstration zones,which has developed the exploitation of shale gas.However,as shale gas production continued to rise,a large amount of hazardous waste,shale gas oil-based drilling cuttings（OBDC）,was also produced.OBDC are generated during the drilling process and are a mixture of separated formation cuttings,silt and a portion of the drilling fluid.Unlike traditional natural gas exploitation,shale gas exploitation requires 10 times more wellheads than conventional gas reservoirs,and the well spacing is small,which leads to a significant increase in the amount of OBDC.The research report on treatment,disposal and resource utilization is very limited.The basis and innovative research of this paper will provide basic support for the development of future treatment and disposal technologies.This paper firstly explores the content and leaching toxicity of heavy metals,VOC,polycyclic aromatic hydrocarbons（PAH）and other pollutants in OBDC.Based on the value of resource recovery of OBDC,The basic physical and chemical properties of OBDC were determined by means of XRD,FT-IR,SEM,EDS and BET.The optimal pyrolysis experimental parameters were obtained by orthogonal experimental design.Based on this,the composition and distribution of pyrolysis liquid products were determined by GC-MS piecewise integration method,and the pyrolysis mechanism of the catalyst was discussed.The weight loss and heat absorption and release in the pyrolysis process of OBDC were obtained by TG-DSC.The pyrolysis kinetic mechanism function G（α）,activation energy E and the pre-exponential factor A were obtained by both single and multiple scan rate method.The migration and transformation of pollutants in the pyrolysis process were studied by testing the distribution of pollutants in the solid-liquid-gas three-phase products and the leaching toxicity after pyrolysis.Finally,in order to protect the health of the people and the ecological environment in the exploitation area,the population health risk assessment and ecological risk assessment were carried out on the OBDC samples before and after pyrolysis.Through the above aspects of research,the main conclusions are as follows:（1）OBDC exhibited a high-viscosity semi-solid form with graininess.Except for Cr,other heavy metals such as Mn,Cd,Cu,Hg,Ni,Pb and Zn were in a polluted state and were impacted by human activities.Although the total content of pollutants in oil-based drill cuttings was high,the leaching toxicity of each pollutant can meet the corresponding control standards.The average moisture content and oil content of OBDC were 5.05%and 9.35%,respectively,and have resource recovery value.（2）OBDC showed a typical mineral plate/sheet extension under scan electron microscope（SEM）.The main constituent minerals were SiO₂ and BaSO₄,containing chemical groups O-H bonds,direct alkanes（-CH3,-CH2,and-CH）,methoxy（-CH₃O）and C=O bond,Si-O and Al-O bond on the aromatic ring,the main elements were O,C,Si,Ca,Al,K,Mg and Na.（3）The most suitable pyrolysis conditions were:temperature 550°C,heating rate10°C/min,and using fly ash as catalyst.The degree of influence of various factors on the experimental results was:temperature>catalyst type>heating rate.Temperature and catalyst The type were significant factors and the rate of temperature increase was not significant.The proportion of aliphatic hydrocarbons in the pyrolysis liquid phase product was more than 90%,which was related to the type of catalyst.The proportion of aromatic hydrocarbons was less than 10%,which was related to the choice of temperature and catalyst type.（4）Using TG-DSC to study the weight loss and heat absorption and release of pyrolysis process,it was found that pyrolysis can be divided into three parts:evaporation of water and small hydrocarbons,cracking of light hydrocarbons and cracking of heavy hydrocarbons.The corresponding temperature segments were room temperature-100°C,100°C-300°C and 300°C-550°C.The absorption and release heat peaks corresponding to the three stages are endothermic,exothermic and endothermic,respectively.The results of kinetic curve fitting showed that the pyrolysis of drill cuttings under various catalysts in each stage only obey two mechanisms,namely random nucleation and subsequent growth mechanism（Z-L-T equation）and three-dimensional diffusion mode（Jander equation）.The activation energy E gradually rose during the three pyrolysis stages.（5）Cd,Cr and Ni with lower melting point after pyrolysis were likely to cause secondary pollution and enter the gas-liquid two-phase product,Zn was partially volatilized,Cu,Mn and Pb were not volatile and appeared slightly enriched in OBDC.However,the leaching toxicity of the OBDC after pyrolysis was very low.The leaching toxicity results using the sulfuric/nitric acid method and the pure water leaching method met the control standards in the"Hazardous Waste Identification Standards"and the primary emission standard of"Sewage Integrated Emission Standards",respectively.After pyrolysis,the main components of the OBDC were still SiO2 and BaSO4,and the oil content was reduced to less than 0.1%.（6）The non-carcinogenic risk and carcinogenic risk of OBDC before pyrolysis were above the critical value,which had an impact on the health of drilling workers.After pyrolysis,the non-carcinogenic risk of OBDC decreased by 59.6%,the cancer risk decreased by 62.8%,and the non-carcinogenic risk decreased to a safe level,but there was still a risk of cancer;if the optimal experimental conditions are used,all risks can be reduced below the critical value.OBDC had a very high ecological risk before pyrolysis,and this value dropped by 75%after pyrolysis,but there were still significant risks;using the optimal experimental conditions,the ecological risk will be reduced to moderate.If one can reduce the Cd,Hg,Pb content in the drilling fluid,one can reduce all of the above environmental risks to a safe level.