After the gas drilling completed, free water in drilling fluid will influx into the formation rapidly in large volume after the drilling fluid pumped downhole. That will cause clay mineral to swell, which leads to open hole instability. In addition, it may create thick mud cakes on the open hole wall in short period, which results in borehole problems. Tight pull during tripping operation is a typical example. Whether open hole instability or the presence of thick mud cakes, it needs more time to solve problems for unrestricted borehole. Therefore, a new concept - anti-sloughing film-forming drilling fluid technology was raised in order to control open hole instability when the drilling fluid pumped downhole after the gas drilling. Essence of the technology is to film a dense waterproof diaphragm on the open hole wall to effectively prevent free water in the drilling fluid from influxing into the formation, which can avoid hydration swelling and eliminate the formation of thick mud cakes.
This technology is just under laboratory test. So far there is no on-site application.
Experimental tests show that a total fluid composed of macromolecular organics and inorganics (called film forming agent in this article) can quickly form a smooth and strong waterproof diaphragm on the surface of laboratory core within 10 secends to 30 secends. The mentioned diaphragm effectively prevents free water in the drilling fluid from influxing into the core internal. According to the laboratory appearance and results, it can be boldly deduced that if flim forming agent is used in the well after gas drilling, the waterproof diaphragm can effectively prevent free water in the drilling fluid from influxing into the formation once formed, which realizes open hole stabilization.
At present, many wells need 3 days to 4 days or more time to deal with restricted borehole problems cuased by open hole instability or thick mud cakes when drilling medium changed to drilling fuild during gas drilling. It has affected the overall effect of the gas drilling to some extent. Research on the anti-sloughing film-forming technology can help to solve such problem and recover to mud drilling normally in short period, which benefits to improve drilling efficiency and save drilling cost.