"Maximum Load" Casing Design
Authors: Prentice Charles M. 
Published: December 2007

By choosing at the outset the least expensive weights and grades of casing that will satisfy the burst loading, and upgrading only as called for by the sequence prescribed here, the resulting design will be the most inexpensive possible that can fulfill the maximum loading requirements.

Introduction

With increased drilling in areas where there are lost returns, abnormal formation pressures, and differential sticking problems, there is a great need for stronger, better designed casing. The attainable, optimum condition is to design casing to withstand these problem-imposed loads for the minimum cost. problem-imposed loads for the minimum cost. To properly evaluate the loads imposed on different types of designs, each type should be considered separately. They are:surface casing,intermediate casing,intermediate casing with a drilling liner,drilling liners, andproduction casing.

The loading for burst should be considered first, since burst will dictate the design for most of the string. Next, the collapse load should be evaluated and the string sections upgraded if necessary. Once the weights, grades and section lengths have been determined to satisfy burst and collapse loadings, the tension load can be evaluated. The tube can be upgraded as necessary, and the coupling types determined. The final step is a check on biaxial reductions in burst strength and collapse resistance caused by compression and tension loads, respectively. If these reductions show the strength of any part of the section to be less than the potential load, the section should again be upgraded. By initially choosing the least expensive weights and grades of casing that will satisfy the burst loading, and. upgrading only as called for by the prescribed sequence, the resulting design will be the most inexpensive possible that can fulfill the maximum loading requirements.

Design Procedure

The basic procedure applies, in its entirety, only to intermediate casing. The other types of design require variations that will be discussed later.

Burst

To evaluate the burst loading, the values of surface and bottom-hole burst limits must first be established. The surface burst pressure limit is arbitrary, and is generally set equal to the working pressure rating of the surface equipment used. (5,000 psi is used for illustrations.) The bottom-hole burst pressure can be calculated, and is equal to the predicted fracture gradient of the formation immediately below the casing shoe plus a safety factor. Since the value of fracture gradient is generally expressed in terms of mud weight, the recommended safety factor is 1.0 lb/gal. Thus the bottom-hole burst pressure, defined as the injection pressure, is equal to the fracture gradient expressed as mud weight plus the safety factor of 1.0 lb/gal, converted to pressure. With the end points determined (surface and injection pressure), the maximum burst load line may now be constructed. Since the maximum load will occur when the end points are satisfied simultaneously, the loading will necessarily be provided by kick conditions. A characteristic of kick loading is the existence of two or more fluids in the borehole - the mud being drilled with at the time of the kick, and the influx fluid.

JPT

P. 805