Effect of high temperature and high pressure on the biodegradability and biotoxicity of typical additives in drilling fluid

Under the high-temperature and high-pressure (HTHP) conditions in the ground, the environmental features of drilling fluid additives may change to some extent. But the relevant information of the influence of HTHP environment on drilling fluid additives is very limited. Four typical additives, namely, sulfonate asphalt (SEB), potassium polyacrylamide (KPAM), potassium humate (KHm) and xanthan gum (XG), were treated for 40 min under two typical bottom hole conditions of 7 MPa, 90 degrees C and 11 MPa, 120 degrees C. The biodegradability and biotoxicity of these additives before and after treatments were determined. The structures of the four additives were also analyzed by Fourier transform infrared spectroscopy (FT-IR), X-ray photoelectron spectroscopy (XPS), nuclear magnetic resonance spectroscopy (NMR), thermogravimetric analysis (TGA) and gas chromatography-mass spectrometry (GC-MS). The results revealed no remarkable difference in biotoxicity and biodegradability of each sample between the two conditions. SEB became nontoxic from microtoxic after being treated with HTHP, with the value of EC50 notably increased from 2463 to 33,398 mg L-1, which might be due to the relative increase of light components observed by GC-MS. Original XG was nontoxic (EC50 = 21,000 mg L-1) and easily biodegraded (BC ratio = 0.469), but HTHP treatments increased the toxicity (EC50 = 5701 mg L-1) and decreased the biodegradability (BC ratio = 0.02), which could be attributed to the reduce of the C-OH/C-O-C bonds observed by FT-IR and XPS spectra. Both KPAM and KHm were midtoxic and had low degradability, and only KPAM became highly toxic (EC50 < 100 mg L-1) after HTHP treatments, which might result from the slight increase in the relative peak area of the amide group. The results could provide preliminary insights into the mechanism of how the HTHP conditions impacted the pollution properties of the additives and pave a new and facile route for the screening of environmentally friendly additives, and consequently mitigate the pollution from the source in the oil and gas exploitation industry.

Automated summary

The environmental features of four typical drilling fluid additives may change under high-temperature and high-pressure conditions in the ground. However, the biotoxicity and biodegradability of the additives did not show significant differences between the two conditions. Overall, sulfonate asphalt became nontoxic, xanthan gum increased in toxicity and decreased in biodegradability, while potassium polyacrylamide became highly toxic under HTHP conditions.