Petroleum Engineering 405 Drilling Engineering 1 Well Drilling Engineering Casing Design Example Dr DO QUANG KHANH 2 Casing Design Example Example Problem API Design Factors “Worst Possible Cond[.]
Well Drilling Engineering Casing Design Example Dr DO QUANG KHANH Casing Design Example Example Problem API Design Factors “Worst Possible Conditions” Effect of Axial Tension on Collapse Strength Iteration and Interpolation Design for Burst, Collapse and Tension Read: Applied Drilling Engineering, Ch.7 HW # Casing Design Example Design a 5/8-in., 8,000-ft combination casing string for a well where the mud wt will be 12.5 ppg and the formation pore pressure is expected to be 6,000 psi Only the grades and weights shown are available (N-80, all weights) Use API design factors Design for “worst possible conditions.” Casing Design - Solution Before solving this problem is it necessary to understand what we mean by “Design Factors” and “worst possible conditions” API Design Factors Design factors are essentially “safety factors” that allow us to design safe, reliable casing strings Each operator may have his own set of design factors, based on his experience, and the condition of the pipe Casing Design We’ll use the design factors recommended by the API unless otherwise specified These are the API design Factors: Tension and Joint Strength: NT = 1.8 Collapse (from external pressure): Nc= 1.125 Burst (from internal pressure): Ni = 1.1 Casing Design What this means is that, for example, if we need to design a string where the maximum tensile force is expected to be 100,000 lbf, we select pipe that can handle 100,000 * 1.8 = 180,000 lbf in tension Note that the Halliburton Cementing Tables list actual pipe strengths, without safety factors built in Casing Design Unless otherwise specified in a particular problem, we shall also assume the following: Worst Possible Conditions For Collapse design, assume that the casing is empty on the inside (p = psig) For Burst design, assume no “backup” fluid on the outside of the casing (p = psig) Casing Design Worst Possible Conditions, cont’d For Tension design, assume no buoyancy effect For Collapse design, assume no buoyancy effect The casing string must be designed to stand up to the expected conditions in burst, collapse and tension Above conditions are quite conservative They are also simplified for easier understanding of the basic concepts Casing Design - Solution Burst Requirements (based on the expected pore PB = pore pressure * Design Factor = 6,000 psi *1.1 PB = 6,600 psi Depth pressure) Pressure The whole casing string must be capable of withstanding this internal pressure without failing in burst 10