عنوان مقاله [English]
During or after underground drilling such as oil well drilling, instabilities are observed in wells, in such a way that 90% of these instabilities are related to Shale formations. On the other hand, Shale formations are found in 75% of all drillings in Iran, causing various difficulties such as complete or partial collapse of wells before reaching the reservoirs. Shale porosity may vary from small (a few %) to quite high (up to 70%). Even with highest porosities, permeability remains very small. In this study, by considering geophysical log data and using experimental relations (presented by researchers) in Shale rock, physical and mechanical properties of Shale layer in Asmari formations are calculated and after that by using finite difference method (FLAC2D software), stability of wellbore in Shale formation has been surveyed.
The modeling was done in 3 modes of drilling. The first mode is drilling without pressurizing mud into the well, the second, drilling at a higher pressure than balance condition i.e. drilling mud pressure to be higher than the pore pressure. The third mode is underbalance condition in which the drilling mud pressure is lower than the pore pressure. The results of numerical modeling showed that the most displacements and plastic surface around the well occurred when the drilling mud was not pressurized. The displacement underbalance and over balance condition significantly decreased and the minimum amount of displacements and plastic surface around the well occurred at higher than balance condition.
Methodology and Approaches
In this study, analysis of stability of wellbore and determination of the optimum mud pressure for wellbore stability has been carried out by elastoplastic method. If this value is less than the critical value defined by Normalized Yielded Zone Area (NYZA) the wellbore is considered to be stable.
Results and Conclusions
The results showed that the NYZA parameter is inversely proportional to the density of drilling mud, so that increasing drilling mud pressure and its density; results in more stable well bore and lower NYZA of the well. Based on NYZA method using numerical technique the critical value for wellbore stability is 1 and it can be realized from the surveyed well that the mud pressure over 22 MPa is within the scope of stability zone. Optimum mud pressure range obtained for the surveyed well is 33.02-33.71MPa for balance drilling and 35.09-35.78MPa for after balance drilling. All these data are obtained from Shale formation of one of the oil wells in Marun Field at a depth of 3915 meters.