Chemical and mechanical model to analysis wellbore stability

Wellbore instability problems mainly contribute to increased nonproductive time (NPT) through drilling operation developments in the Southern Iraq oilfield. This work aims to study the impact of drilling fluid chemicals on shale formation based on lab tests. At the same time, a mechanical earth model (MEM) constructs for the wellbore failure analysis, which simulates the mechanical effect on the shale formation. The Lab tests showed that the shale formation is of many types, including illite, kaolinite, palygorskite clay mineral, montmorillonite, and non-clay minerals quartz and calcite, where kaolinite was a dominant clay mineral of the shale. The shale samples swelled slightly when sodium silicate was used with polymer mud, but they had significant swelling when potassium chloride was applied with polymer mud. The MEM showed that the leading cause of those problems is using unsuitable mud weight (10.4 ppg) during the drilling of the Tanuma formation. Thus, the model indicates that an equivalent mud weight of 11 ppg for vertical wells will be required to stabilize the Mishrif and Tanuma shale formations. This work was unique in studying chemical and mechanical effects to analyze the wellbore stability and develop the optimum solutions.