EFFECT OF TEMPERATURE AND Z-FACTOR ON CASING DESIGN USING KICK TOLERANCE

The appropriate selection of casing shoe depth is an important aspect in the casing design of an oil well. It significantly impact on the well cost and safety during drilling. As the search for oil moves into challenging territories such as deep water, where there is a narrow window between pore pressure and fracture pressure, the determination of casing setting depth using kick tolerance needs to be more robust. The current industry practice for predicting casing setting depth using kick tolerance assumes a constant geothermal gradient and ideal gas behavior in the calculations.
The focus of this research is to study the effect of geothermal temperature variations and compressibility (Z) factor on the casing setting depth design process. Such study is important in order to evaluate how these parameters affect the selected depths especially for HP/HT wells. The research method adopted to achieve this aim involves developing an iterative excel macro program for casing setting depth prediction using kick tolerance which takes in to account Z-Factors and temperature gradients variations across subsurface formations. Four cases with different combination of geothermal temperature gradients and Z-Factor are studied to evaluate the effects. The setting depth for each case is predicted by comparing the fracture pressure equivalent density with the pressure generated inside the wellbore during influx circulation.
The results from the study shows that variations in geothermal formation gradients and the incorporation of real gas behavior has an impact on the circulation influx volumes and internal pressures generated during well control procedures and hence affects the selection of casing setting depths. The main conclusions from this study are correcting for Z-Factors and varying geothermal gradients gives lower influx volumes during circulation, thereby reducing the risk of fracturing the formation, Z-Factors and varying geothermal gradients have significant effect on the predicted setting depth at high temperatures but little or no effect at low temperatures; accounting for these effects especially in conventional wells makes it possible to drill longer hole section, thereby reducing the casing sizes to be run, hence lowering well cost considerable. This dissertation recommends that these effects be taken into account during the casing design process for safe and cost effective drilling.