Volatilization behavior of polycyclic aromatic hydrocarbons from the oil-based residues of shale drill cuttings

Highlights

• The concentrations of PAHs gradually increased with prolonged volatilization time.

• The C-history method could fine describe PAHs volatilizing in oil-based cuttings.

• Different environmental factors had different impacts on the volatilization of PAHs.

Abstract

Shale gas, an emerging oil-bearing and pillar industry at home and abroad, has a very large impact on economic development and industry, but the resulting emerging pollutants pose a serious threat to the environment. Drilling cuttings, the primary byproduct of the exploration and mining of shale gas, are potentially hazardous types of waste that seriously deplete land resources and pose environmental safety problems. In this paper, a long-term static volatilization experiment was conducted to study the volatilization of polycyclic aromatic hydrocarbons (PAHs) in the oil-based residue of shale gas drill cuttings. Furthermore, the effects of some relevant environmental factors controlling the volatilization behavior were evaluated, including different particle sizes, temperatures and illuminances. The results showed that (1) the volatilization concentrations of PAHs gradually increased with prolonged volatilization time. PAHs with smaller ring numbers were present at the highest concentrations among the detected PAHs, and they were more readily volatilized and could be detected earlier. (2) The C-history method was found to significantly describe the kinetic process of the volatilization of PAHs. (3) Different environmental factors had different effects on the volatilization of PAHs. We found that increasing the temperature and illuminance and decreasing the particle size increased volatility. Through canonical correspondence analysis, PAHs volatilization was found to be a complex process, so volatilization tests under simulated environmental conditions are of scientific and environmental interest.

Introduction

Following the growing demand for energy and the depletion of oil and conventional gas, unconventional natural gas extraction has received increasing attention worldwide. As an important unconventional gas, shale gas, natural gas trapped inside fine-grained sedimentary rocks called mudrocks (mudstone or shale) that are rich in oil and gas, has a high economic value and relatively low greenhouse gas potential when combusted compared with other fossil fuels (Burnham et al., 2012; Yang et al., 2017). The global shale gas reserves are estimated to be larger than 200 trillion m³, and China is the third largest global producer of shale gas, surpassed only by the United States and Canada (Wang and Li, 2019). In 2018, the shale gas production values in the US and China were 624.4 × 10⁹ m³ and 10.88 × 10⁹ m³, accounting for 64.71% and 6.8% of the total natural gas production of the countries, respectively (Castro-Alvarez et al., 2017; Dai et al., 2020).

However, the problems associated with shale gas development cannot be ignored. For instance, large amounts of drilling cuttings emerge during the exploration and mining of shale gas, which result in a serious waste of land resources and environmental safety problems (Annevelink et al., 2016). It is estimated that approximately 1000 m³ of cuttings can be found in one well and that approximately 400,000 m³ of cuttings are produced every year in China (Liu et al., 2018). Drill cuttings can be broadly categorized into three main groups depending on the drilling fluid: water-based cuttings, oil-based cuttings and synthetic-based cuttings. Water-based cuttings are generally defined as industrial solid waste, while oil-based cuttings contain heavy metals, benzene series (BTEX), polycyclic aromatic hydrocarbons (PAHs) and many other toxic and hazardous pollutants and have been included in the China Hazardous Waste List (HW08) (Michalski and Ficek, 2016; Huang et al., 2020). With the rapid development of oil and gas exploitation in China, especially in emerging oil and gas fields such as shale gas, the disposal and utilization of oily sludge from oil and gas exploitation and the remaining solid phase after comprehensive utilization have received increasing attention. When Wu (Wu et al., 2019) studied the pollution characteristics of PAHs in oil-based drilling cuttings in a certain area of Chongqing, it was found that the concentration of PAHs was in the range of 302–595 mg kg⁻¹, and the concentrations of BaP, BbF, BkF and DBA all exceeded the standard limit. Chen (Chen, 2017) analyzed the organic content of a vertical well oil-based drill cuttings and horizontal well oil-based drill cuttings in Chongqing. The results showed that both oil-based drill cuttings contained PAHs, and the vertical well content reached 19.5%. Many methods, such as incineration, thermal desorption and resource utilization, have been employed for the disposal of drill cuttings (Ball et al., 2012), but the residue after utilization is currently placed in temporary storage or reused as road fill or other construction materials.

To date, most studies have focused on the distribution and sources of PAHs(Abdollahi, 2013; JincuiWANG et al., 2019). Studies have been conducted on the impact of PAHs on human/agricultural systems (Peng et al., 2018; Chiang et al., 2020), but the release of PAHs in drill cutting residues is usually excluded. Several PAHs are regarded as priority pollutants by the United States Environmental Protection Agency (USEPA). Although many researchers report the characteristics of PAHs in drill cutting residues, knowledge of organic volatilization behavior from drill cutting residues is scarce, which limits the large-scale utilization of such residues. The volatilization behavior of polycyclic aromatic hydrocarbons from the oil-based residues of shale drill cuttings is mainly controlled by the mass transfer process of PAHs from the inside of the oil-based drilling debris from the porous media source to the surface. For the diffusion and release process of the oil-based drilling debris from the surface of the oil-based drilling debris to the air in a closed environment, this mass transfer process is mainly controlled by the diffusion mechanism. Therefore, the objectives of our work are (1) to describe the main mass transfer processes involved in PAHs volatilization, (2) to simulate the PAHs volatilization process for long-term behavior assessment, and (3) to identify the influencing factors of contamination in the volatilizing system. The purpose of this work is to provide a scientific basis for standardizing and guiding the disposal and utilization of the remaining solid phase after the comprehensive utilization of oily sludge from oil and gas exploration and to provide a theoretical basis for the environmental effects and risk management of oil-based drilling cuttings during storage, disposal and transportation.