Bottomhole Stress Analysis And Breaking Mechanism Research In Underbalanced Drilling

It is very effective that underbalanced drilling technologies are adopted to enhance the drilling rate which is several times than conventional drilling rate, reduce reservoir damage and timely evaluation of low-voltage low-permeability reservoir. the using of underbalanced drilling technologies has saved more time of drilling and reduced the drilling costs. At the same time , hole enlargement is a disadvantage of underbalanced drilling, it has a greater adverse impact on Cementing and completion of the well . However , presently the people studying on the mechanisms of enhancing drilling rate and creating hole enlargement is few. therefore ,the paper will do some study on the mechanisms of high drilling rate and hole enlargement.At underbalanced drilling process, the drilling fluid pressure is lower than the strata pressure, so the fluid is influxing from the stratum to the well and all of these lead to the change of the strata pressure finally. The change of the strata pressure brings on the change of rock skeleton stress. At the same time, because of the temperature difference which is between the drilling fluid and the stratum ,heat conduction exists between the drilling fluid and the stratum ,the changes of formation temperature affect the changes of rock skeleton stress. This process is the typical question of THM. Basing on the basis of the classics THM and combining the factual state of the bottomhole rock, this paper builds the coupled equations of the analyzing the stress state of bottomhole rock in the underbanlanced drilling and expound the mechanisms of raising the drilling rate in the underbanlanced drilling. at the same time, the paper reveals the mechanism of hole enlargement from the change of the stress state of bottomhole rock ,the stress state of the borehole rock, the erosion by the drill fluid and the debris in the underbanlanced drilling. According the analyzes of this paper, the rock breaking mechanisms of underbanlanced drilling will be revealed more depth, the essential theoretical foundation will be provided for setting up suitable Rop for underbanlanced drilling, bit optimization, prediction of formation drill ability and the new bit design as well as to prevent hole enlargement.