Comparison of different failure criteria in prediction of safe mud weigh window in drilling practice

Wellbore stability is one of the most significant problems observed during drilling practice. Determination of safe mud weigh window is hence critically important in order to avoid instability problems. Mechanical properties of formations along with magnitude and direction of ins-situ stress are necessary parameters for evaluating the possibility. However, important role of an efficient failure criterion should never be neglected. In this paper three failure criteria known as Mohr-Coulomb, Hoek-Brown and Mogi-Coulomb were used for prediction of safe mud weigh window. Mechanical earth models were built using wireline logs and to get a rational conclusion, three wellbores drilled in sandstone, carbonate and gas shale reservoirs were taken into consideration. The results of this study indicated that Mohr-Coulomb criterion underestimate the strength and estimates the highest minimum mud pressure required to stabilize the wellbore. This is due to the fact that this criterion is linear and neglects the effect of intermediate stress. The Hoek–Brown criterion on the other hand due to having a non-linear equation is able to provide better estimation compared to that of the Mohr-Coulomb. However Mogi-Coulomb criterion is the best failure criterion for determination of safe mud weight window and this is due mainly to considering the effect of intermediate stress

Comparing U-statistic and nonstructural methods for separating anomaly and generating geochemical anomaly maps of Cu and Mo in Parkam district, Kerman, Iran

© 2015, Springer-Verlag Berlin Heidelberg.In applied geochemistry, obtaining quantitative descriptions of geochemical patterns and identifying geochemical anomalies are important. To identify and separate geochemical anomalies, several statistical methodologies (nonstructural and structural) are presented by researchers. In this study, four nonstructural methods including threshold assessment method based on median and standard deviation, median absolute deviation (MAD), P.N product and Sinclair’s method are selected first and then U-statistic is considered as a structural method to compare their performance. Subsequently, the best method is used to assess prospective areas of Parkam district. Results show that P.N and Sinclair’s methods are not always efficient. MAD method reduced the background well and roughly increased the correlation factor of points. However, U-statistic method includes both mentioned advantages meaning in addition to reducing outlier data effect, it regularizes anomalous values and also their dispersion is reduced significantly. It is possible to determine anomaly areas according to anomalous samples positioning so that denser areas are more important. Finally, lithogeochemical map of study area is generated for copper and molybdenum. In this map, the Cu mineralization which is delineated by this method is closely associated with the defined potassic alteration zone (according to alteration map of the study area), and also, the delineated Mo mineralization is exactly associated with the phyllic alteration and is spatially conformable with the zone defined for it

Assessment of prospective areas for providing the geochemical anomaly maps of lead and zinc in Parkam district, Kerman, Iran

There are several statistical methodologies presented for separating anomalous values from background leading to determination of anomalous areas. These methods range from simple approaches to complicated ones and include nonstructural and structural methods, subtraction separation method and so on. Structural methods take the sampling locations and their spatial relation into account for estimating the anomalous areas. The U-statistic method is one of the most important structural methods. It considers the location of samples and carries out the statistical analysis of the data without judging from a geochemical point of view and tries to separate subpopulations and also to determine anomalous areas. In the present study, several nonstructural methods including assessment of threshold based on median and standard deviation, median absolute deviation (MAD) and P.N product are used and U-statistic is considered as structural method to assess prospective areas of Parkam district. Results show that MAD method reduced background well and P.N method increased correlation of points. However, U-statistic method plays the role of both mentioned advantages meaning in addition to reducing outlier data effect, it regularizes anomalous values and also their dispersion is reduced significantly. It is possible to determine anomaly areas according to anomalous samples positioning so that denser areas are more important. Finally, lithogeochemical map of study area is provided for lead and zinc