This paper summarizes the challenges and issues that encountered while drilling the unstable shale. The objective of comparing the wells which had issues and which smoothly drilling through is to identify the main factors of drilling practice to drill the shale safely. Through geo-mechanic assessments, the wells that were drilled towards the maximum horizontal stress, and penetrated the shale with angle less than 86–87° will have better wellbore stability than other wells towards minimum horizontal stress and with higher angle. However, the mud properties also contribute to stability of the shale. This paper assesses several factors to identify most influencing factors for wellbore stability during drilling shale intervals, and possible to calibrate the geo-mechanic model. This paper uses data of the shale interval which is over pressured, fractured, weak, laminated and reactive due to clay contents. Below the shale is a depleted limestone reservoir due to long-time production history. The geo-mechanic assessment and integration with real time data including logs, mud report and drilling report to compare and summarize mud properties were used in each wells, angle of penetration and drilling events in order to fine tune the stability issue in the shale interval. Geo-steering pre well modelling provided in order to know the expected log responses, predicted shallower or deeper shale lithology and tools to be used, as real time angle of penetration need to be precisely adjusted.
The observation of inclination or penetration angle in unstable shale interval plays important role in the study case. Precisely adjusted inclination is the main key to the successful landing section in target reservoir. Limitation due to the thickness of bottom shale interval to the expected latitude of target reservoir required high building rate tool in order to land the well properly. Land deeper inside thicker reservoir is one approachment to have low angle in overlying shale section. Some close to bit sensor are required in future wells including Near Bit Inclination, Near Bit Resistivity, Near Bit Gamma and Vibration Sensor. It is very important to have good hole cleaning before pulling out of the hole (POOH), in some cases the Equivalent Mud Weight (ECD) from Pressure While Drilling (PWD) tool is high while POOH and occasionally operated without circulation.