Summary
Casing failure has been found in nearly 20% of production wells in the
Daqing oil field after 45 years of oil-production history. Plastic failure of
casing strings has been recognized in the zones of yielding shales overlying
reservoir pay zones in the field. This type of casing failure is one of the
most costly problems in the field. No research has been conducted to study the
resistance of the casing string to the lateral loads from the yielding shales.
This paper offers results from such research.
Assuming a lateral load uniformly distributed along a casing section from a
clay zone clamped between an upper and a lower sandstone zone, a mechanistic
model was developed in this study. Results of the model indicate that the
resistance of casing string to the lateral loads drops sharply in the early
stage of development of radial deformation. The resistance of the casing string
to the lateral loads is not very sensitive to the longitudinal deformation. The
remaining strength of the casing string also depends on properties of the
casing string.
A calculation example is illustrated in this paper. This work provides an
analytical tool that can be used for optimizing casing programs against lateral
loads from yielding-clay zones to minimize casing failure and field-operation
cost.
Introduction
The Daqing oil field is located in the central region of the Songliao basin
in northeastern China. It was discovered in 1959 and is still considered to be
China’s largest oil field. Vertically, it consists of more than 140
freshwater-bearing oil zones at depths from 2,000 to 4,000 ft.
Waterflooding was initiated in 1961 for reservoir-pressure maintenance. The
water-injection-pressure gradient reached peak values greater than the
overburden-pressure gradient in some areas in the late 1980s. The high-pressure
water injection introduced water to faults and the overlying shales, which is
believed to be responsible for casing damage and casing failure in the oil
field.
By the end of 2002, 47,104 wells had been put into oil production and water
injection, among which 9,115 wells were found to have casing-damage problems.
The damaged wells accounted for 17.33% of the total number of wells. The casing
damage was found to be in the form of deformation against the water-sensitive
shales and shear failure near faults. Field studies indicate that a large
portion of deformed casing in the field can be attributed to the lateral load
from shale formations. Evidence has been found to support the following theory
of casing damage in the field: The injected fresh water goes up along the
channels in the cement columns and reaches overlying shales, which are
sensitive to fresh water. The shale swells, yields, and creeps near the
wellbore, causing lateral force acting on the casing. Also, the injected water
lubricates the fault face, which accelerates the relative movement of formation
rocks separated by the fault face. This movement induces lateral force acting
on casing.
Currently, the radial deformation, δ, of casing can be measured in
the field, but the side load, q, from the formation cannot be quantified
(Yu and Zhang 1996). Before this paper, no research had been conducted to study
the resistance of the casing string against the lateral loads from the yielding
shales and creeping formation.
A mechanistic model has been developed in this study, assuming a lateral
load from a section of formation uniformly distributed along a casing section.
Results of the model indicate that the resistance of the casing string to the
lateral loads drops sharply in the early stage of development of radial
deformation. The resistance of the casing string to the lateral loads is not
very sensitive to the longitudinal growth of deformation. The remaining
strength of the casing string also depends on properties of the casing
string.
An application example is illustrated in this paper. This work provides an
analytical tool that can be used for optimizing casing programs against lateral
loads from yielding formations to minimize casing failure and field-operation
cost.
© 2008. Society of Petroleum Engineers
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History
- Original manuscript received:
22 August 2006
- Meeting paper published:
20 February 2007
- Revised manuscript received:
7 May 2008
- Manuscript approved:
7 May 2008
- Version of record:
10 December 2008
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