Harmless treatment of waste drilling mud

According to the 'National Petroleum and Natural Gas Resource Exploration and Exploitation Situation Report', in 2020, my country completed a total of 2,956 exploration wells and 17,297 development wells; based on the calculation that each well produces about 300 cubic meters of waste mud, the amount of drilling waste generated in my country each year is as high as millions of cubic meters.

There are various chemical reagents (bentonite, fluid loss reducer, anti-collapse agent, flocculant, weighting material, etc.) in different proportions in the waste mud, which may contain a large amount of pollutants. 

In addition, the waste mud also contains petroleum substances in the formation, rock cuttings and heavy metals carried from underground to the ground. Therefore, although coalbed methane is a high calorific value clean energy, the composition of the waste mud generated during its mining process is very complex, and it is one of the main sources of pollution in the coalbed methane production process.

In general, waste drilling mud is a colloidal solution containing clay, chemical treatment agents, inorganic salts, oils, soluble trace elements and drill cuttings. If not properly handled, it will cause serious harm to the ecological environment of the mining area and further affect the production and living safety of local residents.

In recent years, my country has made great progress in the treatment of waste drilling mud, but some oil and gas mining companies still lack a systematic and comprehensive understanding of the treatment process. They only rely on natural drying, which takes up a lot of space, and most of the funds invested are used for transportation, resulting in less investment in the research and development of treatment technology. In addition, the improper selection of treatment equipment and mud treatment agents makes it impossible to fundamentally achieve harmless treatment.

At the same time, the operating environment of oil and gas fields also increases the difficulty of harmless treatment. Therefore, it is urgent to develop a simple, efficient and low-cost treatment method. Relevant studies have shown that the use of chemical solid-liquid separation can better achieve the harmless treatment of waste drilling mud, especially for water-based drilling mud with high water content. 

The most commonly used chemical solid-liquid separation is the gel-breaking flocculation technology. The chemical flocculation technology can effectively accelerate the solid-liquid separation speed of waste drilling mud, and some of the separated water resources can be reused to increase the solid content of the separated mud cake. However, there is little research on the process conditions during the separation process.

Using cement to treat mud cake can greatly shorten the curing cycle of mud cake, and will not cause secondary pollution after curing, so abandoned mud pits can be reclaimed in the same year. However, simply using cement to solidify the solid phase after solid-liquid separation of abandoned drilling mud will increase the total cost of waste mud treatment, so further research is needed on solid phase treatment methods.

The current waste mud solidification technology usually uses cement as the main solidifying agent, and at the same time, according to the pollution characteristics of the drilling mud waste, an appropriate amount of debonding agent, fly ash, gypsum and other treatment agents are added to bury and seal the pollutants and harmful substances in the waste in a high-strength solidified body. After the waste drilling mud is solidified with a solidifying agent mainly composed of cement and fly ash as the auxiliary agent, all indicators of the leaching liquid can reach the first-level emission standard (GB9878-1996).

These methods reduce the amount of cement by adding industrial waste fly ash, and have a certain 'waste treatment' effect, but the solidifying agent is still mainly cement, and there are still certain cost issues.

In recent years, the study of solidifying agents based on industrial waste slag has found that the compressive strength of the solidified block of slag-based solidifying agents increases with the increase of the amount of slag solidifying agents during a certain curing period, indicating that its solidification performance can be explored, so slag-based solidifying agents have a certain feasibility for solidifying waste mud.

The harmless treatment of waste mud is very necessary. 

Degelling, flocculation and solidification of waste drilling mud is convenient for secondary utilization and easy to operate. However, the optimal process conditions for solid-liquid separation are unclear during its treatment process. The cost of treating mud cake with cement-based solidifying agent is high. Therefore, it is urgent to find the best degelling and flocculation material according to the characteristics of waste mud and optimize its treatment process conditions. At the same time, a low-cost solidifying agent is used to solidify the mud cake, and the mechanism of degelling, coagulation and solidification is explored.