Abstract
Instability of a well means for whatever reason, the well shape deformed from its circular condition, which may be accompanied by collapse or fracture initiation. The problem of the instability of the well in the production and exploration wells imposed extra costs in the drilling projects; so the proper predictions to prevent instability are imperative. Several analytical and numerical methods have been proposed to analyze the stability of wellbores so far, but in this paper, an analytical study was presented to determine the appropriate rock failure criterion through wellbore stability analysis. Two important factors affecting the analyses are failure criteria and rock behaviour. In this paper, with regard to attain the most suitable failure criterion, 15 rock failure criteria, including Hoek-Brown and Mohr-Coulomb, were compared with the achieved data from 14 different rock samples and finally, the modified Lade criterion with standard deviation of 2% was considered as the most accurate equation. By integrating this failure criterion through the stresses analysis around the well that is in accordance with the Kirsch analytical equations, a new equation was developed to predict the minimum weight of the mud to prevent the wellbore collapse.
In order to verify the model, the equation is compared with the actual data of two wells: one in Norouz field in Iran and another one in the field in Indonesia. In both cases, only the modified Lade equation predicted the proper mud weight to keep stability of the well, correctly. In well No. 34 of the Norouz field, the outcast caliper log and the diameter of the well were recorded, as well as those parts of the wellbore which were unstable. The results showed that the Modified Lade criterion can be viewed as most accurate equation and Hoek-Brown, Mohr-Coulomb and Mogi-Coulomb criteria regarded as the most useful equations, predicting the unstable points. The findings demonstrated that the only modified Lade criterion defines the unstable points adequately. In the other case in shale formation, drilling operations in 4800 to 6200 feet depth with the mud weight of 5.10 pound per gallon in Indonesia, the instability was not reported. According to Mohr-Coulomb criterion, the wellbore was meant to be instable while these criteria show the suitable mud weight is found 1.2 lb/gal more than current weight in which there is the possibility of fluid lost. Using Mogi-Coulomb criterion through the analyses, the wellbore was found stable at the deviation of more than 35 degrees. But according to modified Lade criteria, the well is found stable in any dip in azimuth, which coincided with the case in real terms.
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