OPTIMIZING THE COST OF THE CASING DESIGN
This chapter addresses the optimization theory that results in assuring the selection of the cheapest combination casing string (Fig. 5.1).
After calculating the loads that the casing will be subjected to, the engineer is faced with the decision of selecting an appropriate casing grade, weight and thread, such that these properties meet or exceed the calculated load conditions. This is not an easy task because many casings qualify and, therefore, the question arises: which casing is the best choice? The answer is: the one that can withstand all loads at the absolute (or ultimate) minimal cost possible. Finding this casing string is not straight-forward because the type of casing selected affects the calculated loads, which are a result of the wall thickness, and leads to an implicit solution. Sometimes simple cases may be solved by trial and error.
In the case of directional wells, the problem is further complicated because the loads and the trajectory length for a fixed surface location and target are a function of factors including drilling costs, risk assessment and casing program costs. Therefore, different spatial configurations will alter the final casing cost. Considering all well costs, might not be the critical issue but it is the specific topic addressed in this book.
The following questions must be answered here:
1. What is the absolute minimal cost of a combination casing string, given external loads, design factors, and casing supply?
|
В: 9 5/8" N80 43.5 lb/ft LTC Example 2: A: 9 5/8" N80 40.0 lb/ft LTC B: 9 5/8" N80 40.0 lb/ft BUT |
Fig. 5.1 : Casing nomenclature.
2. What is the quantitative effect of certain decisions made by the casing
designer (value of the design factors or number of sections) on the cost of
casing?
3. How significant, given specific loads, is the conflict between the minimum weight and the minimum price criteria for selecting casing?
4. How do the external casing loads in directional wells affect casing cost?
5. What is the correlation between the directional well profile and its minimum cost?
6. W’hat is the effect of the borehole friction factor (also referred to here as
friction factor, and pseudo friction factor) on casing design in directional
wells?