Drilling: Special Considerations in the Design Optimization of High Rate, Multistage Fractured Shale Wells
Typical shale well completions involve massive, multistage fracturing in horizontal wells. Aggressive trajectories (with up to 20°/100 ft dog legs), multistage high rate fracturing (up to 20 stages, 100 bpm), and increasing temperature and pressure of shale reservoirs result in large thermal and bending stresses which are critical in the design of production casing. In addition, where cement voids are present and the production casing is not restrained during fracturing, thermal effects can result in magnified load conditions. The resulting loads can be well in excess of those deemed allowable by regular casing design techniques. These loads are often ignored in standard well design, exposing casing to the risk of failure during multi-stage fracturing.
In this work, the major factors influencing normal and special loads on production casing in shale wells are discussed. A method for design and optimization of shale well production casing design is then introduced. The constraints on the applicability of different design options are discussed. Load magnification effects of cement voids are described and a method for their evaluation is developed. It is shown that thermal effects during cooling create both bending stress magnification and annular pressure reduction due to fluid contraction in trapped cement voids. This can result in significant loads and new modes of failure that need to be considered in design. The performance of connections under these loads are also discussed. Examples are provided to illustrate the key concepts described. Finally, acceptable design options for shale well production casing are presented.
The results presented here are expected to improve the reliability of shale well designs. They provide operators with insight into load effects that need to be considered in design of production casing for such wells, and with options for adequate design.
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