Page 110 - Practical Well Planning and Drilling Manual

Section 1 revised 11/00/bc 1/17/01 2:56 PM Page 86

[ ]
1.4.17 Well Design

to do is to compute the combined effect of all the forces acting on the
casing and compare the resultant stress to the minimum yield strength
of the material. In order to carry out a triaxial analysis you have to
have completed your design using the previously stated criteria. The
casing design itself defines many of the loads; for instance the length
and thickness of the casing will be the main determinant of tensile
stress and this must be known before the analysis can be carried out.
Triaxial analysis is a complex procedure and, if necessary, should
be done using a computer program. For this reason it is not covered in
detail. In the vast majority of cases, the axial and biaxial procedures
mentioned above will be sufficient. However, if you do have such a pro-
gram available, the lower safety factors necessary may allow lighter
grades/weights and give worthwhile cost savings.

1.4.17. Design for Casing Off Massive Salt Formations

Massive salts have two properties that may cause serious problems
during the life of the well. Salts may flow plastically when field stress-
es exert a force on the salt. Also, significant enlargement of the well-
bore will occur through salt if unsaturated, water-based mud is used.
Salt mobility is affected by several factors, such as water content,
impurities, grain size, temperature, overburden pressure, and mud
density. Salt can be so mobile that the bit can get stuck in it during
drilling if the salt has closed around the bit gauge. Increasing mud den-
sity is one way of slowing this down, but salt will never be “pushed
back out” by increasing mud density.
From a casing design viewpoint, salts can exert large forces on the
casing. First, if we regard plastically deforming salt as a hydraulic
fluid, it will impose overburden pressure uniformly on the circumfer-
ence of the casing. Thus, collapse design for massive salt assumes a 1
psi/ft collapse pressure. However, the fact that the salt may flow in a
particular direction can also cause problems of ovalled or even
sheared off casing sometime after the well has been completed. Also,
if the salt moves unevenly, then point loading will occur; the cement
must be competent to prevent this as no casing can resist the uneven
loading that will result.

86