Page 136 - Fundamentals of Enhanced Oil and Gas Recovery

124 Pouria Behnoudfar et al.

standards and recommended practices. For well technology and field piping, common
standards are available in relevant literature [88].
There are two basic elements which should be considered in order to design a
well, which are as follows:
• The wellbore, consisting of casing, cement, and wellhead,
• The mechanical completion equipment consisting of valves, tubulars, and packers.

4.9.1 Wellbore Design
The well designs are almost similar for different cases of CO 2 injection, consisting of
surface casing and production casing. The reason of using multiple casings is isolating
groundwater resources from potential sources of contamination and maintaining the
stability of the wellbore. A typical CO 2 wellhead is available in relevant literature [90].
From the mechanical point of view, thickness and weight of a casing are selected
based on maximum potential burst and collapse pressures plus safety factors. The safety
factors are function of injection and production pressures, well trajectory, and reser-
voir conditions. Carbon steel casing is a common casing type used for wells of
10,000 ft or less in depth and usually, in these cases, grades of J-55 and K-55 are typi-
cal. In deep, high pressure and high temperature environments, higher strength grades
should be used, and CRA are used in wells susceptible to H 2 S and CO 2 leaks [90].
For new construction, almost all wells are cased-hole completions. In isolated
cases, depending on reservoir conditions, open-hole completions are still used; how-
ever, they are rare [92,93]. Since cased-hole completions are amenable to a larger vari-
ety of profile management techniques (mechanical isolation, chemicals, squeeze
cementing, etc.) than open-hole completions, they are the more common completion
strategy [91].
It should also be considered that pore pressure variation, which is caused by injec-
tion (or production), changes the in-situ stresses. This fact may lead to some huge
influences on wellbore designs. For instance, fault reactivation is possible and conse-
quently casings will be sheared. In addition, wellbore instability is very common
[94,95]. Besides, induced thermal stresses should be estimated during thermal recover-
ies, in wellbore trajectory design as well as completion design [96].

4.9.2 Cement Technology
Cementing is critical to the mechanical performance and integrity of a wellbore both
in terms of its method of placement and cement formulation used. Chemically, the
degradation of Portland-based cements by carbonic acid (H 2 CO 3 ) is well known and
documented [97]. The basic chemical mechanism is described below [98]:
yields
CO 2 1H 2 O ! H 2 CO 3