With the rapid development of shale gas exploitation, large quantities of drilling cuttings (DCs) and flowback waters (FWs) laden with organics are produced. These organics, whose chemical compositions are largely unknown, could be toxic to organisms in the receiving environment. This study collected two time series of DCs and FWs from different shale gas exploitation platforms in the Sichuan Basin, China. We developed a nontarget analytical method to identify organic additives as well as their degradation products in these DCs and FWs and observed variable compositions and levels of organic additives in the DCs and FWs. The variation could be ascribed to the varied geological shale formations and unique drilling techniques. Alcohol ethoxylates (AEOs) and quaternary ammonium compounds (QACs) were identified as the main additives in the DCs from water-based drilling cutting with AEOs and alkylbenzenesulfonate in the related FWs. Meanwhile, nonylphenol ethoxylates (NPEOs) and QACs were the predominant surfactants in oil-based drilling cutting, with polyethylene glycols (PEGs) and AEOs dominating in related FWs. Carboxylated products of PEGs, polypropylene glycols, NPEOs, and octylphenol ethoxylates were widely identified in DCs and FWs, evincing biotic/abiotic transformation and chain-shortening of these surfactants downhole.
Economical and environmental concerns have forced the oil and gas industry to consider reusing flowback and produced water for fracturing operations. The major challenge is that the high-salinity of flowback water usually prevents its compatibility with several fracturing fluid additives. In this paper, the authors explored an economic and effective method to prepare guar fracturing fluids with different salinity waters. The main research idea was to use chelating agents to mask metal ions, such as calcium and magnesium, that are harmful to crosslinking. Firstly, a complexometric titration test was conducted to measure the chelating ability of three chelating agents. Secondly, through viscosity, crosslinking, and hanging tests, it was verified that the complex masking method could cope with the problem of high-valence metal ions affecting crosslinking. Thirdly, the preferred chelating agent was mixed with several other additives, including thickeners, crosslinkers, and pH regulators, to prepare the novel guar fracturing fluid. The comprehensive performances of the novel fluid system were tested such as temperature and shear resistance, friction reduction, gel-breaking performance, and core damage rate. The results show that the organophosphate chelating agent (i.e., CA-5) had the greatest ability to chelate calcium and magnesium ions. There was a good linear relationship between the dosage of CA-5 and the total molar concentration of calcium and magnesium ions in brine water. The main mechanism was that the chelating agent formed a complex with calcium and magnesium ions at a chelation ratio of 1:5. The test results of the comprehensive performance evaluation indicate that the prepared guar fracturing fluid met the requirements for field application, and the lower the salinity of the flowback water, the more it is economical and effective.Keywords:flowback and produced water;chelating agent;complex masking method;crosslinker;chelation ratio
The rising water-use intensity, and lack of cost-effective treatment strategy and reuse of hydraulic fracturing flowback (HFF) has become an increasing cause of concern. The present work evaluates the integration of parallel sets of tandem anaerobic-oxic membrane bioreactor (AMBR) with and without nano-Fe for treatment and reuse of real HFF obtained from Ordos Basin, China. Treatment efficiencies in terms of organic conversions, micro-pollutants degradation, resource recovery, and effects of nano-Fe release on membrane fouling were evaluated. Nano-Fe mediated AMBR (FAMBR) system effectively reduce target micro-pollutants (such as Acenaphthylene) at 94.4 % compared to the parallel AMBR system (17.1 % without nano-Fe). Moreover, recovery of potential economic chemicals like Al and P (1.0 and 0.6 mg/g spent nano-Fe) availed using FAMBR system. However, colonization of FAMBR membrane surface by Fe–protein/peptide hydroxocomplexes initiated by Fe-catalyzed microbial extrusions present a huge fouling challenge relative to the AMBR system. Additional evidences from microscopic/spectroscopic analysis of the FAMBR membrane system revealed that despite having a promising outlook, mediation of nano-Fe with AMBR system might result in a major fouling event during HFF treatment. Engineered design of nano-Fe to reduced leached nano-Fe ions in pre-treatment step prior to AMBR treatment system may be of potential research consideration.
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