Realtime Drilling Geomechanics Aids Safe Drilling through Unstable Shales and Channel Sands of Wara Formations, Minagish Field, West Kuwait

Abstract

As part of any successful development plan of any hydrocarbon field, drilling boreholes safely is a key factor to make the entire process safe, economic and environmentally friendly. One of the main factors that dictates whether a borehole is going to be drilled safely or not is to understand the geomichanical behavior of the different formation to be penetrated. A definition of geomechanics could be stated as the science that studies the relationship between each of; in-situ stresses, rock mechanics, and the drilling fluid properties. In Kuwait and during the course of efforts to develop Wara channel sands in Minagish Field to the west of the country, Kuwait Oil Company (KOC) realized that continuing to drill development wells using conventional drilling practices is not any more an easy task. Considerable non-productive time has been recorded due encountering events such as shale carvings and pack off leading to stuck pipe. In addition, partial to total lost circulation were faced while drilling through Mutriba Formation which added to the complexity of problem. This study involved gathering data from offset wells to build a mechanical earth model for the area where the new well is going to be drilled. The main objective of having the model built is to perform wellbore stability analysis (WBS) and compute the quantitative mud window values to insure stable and safe borehole drilling. As the case of any study, performing reliable WBS analysis requires accurate modeling of earth stresses and rock mechanical properties. This process is primarily based on sonic logs (compressional and shear slowness), formation bulk density and lithology distribution. The study started with an audit of the available data sets in the region to select the best offset wells and generating empirical correlations to fill- up any missing and/or poor-quality data zones. Initially,7offset wells were identified, based on the geological distribution and data availability.Out of them, only four wells were found to have compressional slowness and three with bulk density measurements. However, it is worth mentioning that no shear slowness measurements were available in any of the offset wells in the region. Due to this, a correlation based compressional-shear relationship from nearby wells was proposed for the pre-drill study. The mechanical properties were characterized using the tri-axial core test results available from Wara and Burgan Formations. Empirical correlations were developed to obtain static mechanical properties from the dynamical mechanical ones and log responses. In addition, horizontal stresses in the region were constrained with formation integrity test data to have better control on the model. Finally, after the WBS model was built,it was compared to the available caliper data from the offset wells for calibration purposes. The resulted pre-drill geomechanics model was used to advise on the drilling parameters (mud weight) to be used in drilling the new development well. Moreover, and being the first realtime drilling geomechanics (RTDG) job in in Kuwait, an LWD sonic was used while drilling to supply the pre-drill model with realtime compressional and shear slowness measurements. Having the model updated in realtime with data from the formation at the borehole location resulted in optimizing the mud weight window limits by the geomechanics engineers as the well was being drilled. Following these mud weight recommendations based on the updated pre-drill model resulted in a smooth landing and horizontal sections in which all the wiper trips until the final pull out of hole were smooth.