This paper presents a methodology for designing setting depths of surface and intermediate casing strings in oil wells using the concept of kick tolerance. The methodology was implemented in a microcomputer program written in FORTRAN. The results are plotted to the microcomputer screen through a graphical interface written in C that calls Windows API functions.
The proposed algorithm uses an iterative procedure to define the shallowest casing shoe setting depth employing the kick tolerance concept. It compares the curve of fracture pressure equivalent density with the pressure generated inside the wellbore during a well control operation to estimate the casing shoe setting depth. The kick tolerance concept is applied in two ways:
for the well containment and
during the gas kick circulation.
In situation (a), the calculation procedure considers the wellbore pressure just after the well closure. In (b), a two-phase flow kick simulator is applied to predict wellbore pressure generated during the gas kick circulation from the well.
Two examples are elaborated to show the application of the proposed methodology and the computer program. The results are presented graphically to illustrate the use of the graphical interface.
The selection of casing setting depth is one of the most important tasks in preparing the well plan. Many aspects, such as lithology, overpressured formations, shallow gases, lost circulation and troublesome zones, directional well profiles and regulations should be considered in this selection. Recently, well control considerations have been included in this selection process through the application of the kick tolerance concept. It made the drilling execution safer and more economical. This motivated this study. Thus, the main objective of this paper is to discuss the effect of these well control considerations on the selection of casing shoe setting depth. The other aspects listed above will not be considered here.
Kick tolerance can be understood as the capability of the wellbore to withstand the state of pressure generated during well control operations (well closure and subsequent gas kick circulation process) without fracturing the weakest formation. To account for the kick tolerance in the casing shoe setting depth selection, there important data are required:
the formation pressure at the final depth of the well phase,
the maximum kick volume that can be taken during the drilling operations, and
the fracture equivalent density curve for the area.
Figure 1 shows how the selection process is performed. With the total depth of a drilling phase set and the formation pressure at that depth known, the shallowest casing setting depth can be established considering the fracture equivalent density curve and pressure inside the wellbore during well control operations. This design process is called bottom-to-top method.