English Edition (6 MB pdf) - Saudi Aramco

as pipe sticking. In some parts of the field, the mud weight
must be increased considerably because of the over pressure
present in the Jilh formation, which increases the difficulty of
drilling the section 1 .
Rotary steerable system (RSS) tools were introduced into
the Saudi Aramco deep gas operations to improve hole quality
and ROP. To enable optimum steerability, stability, ROP, and
bit life, it was necessary to develop a polycrystalline diamond
compact (PDC) bit to match the system.
After several bit optimization cycles and optimum drilling
practices based on the accumulated experience, the latest
iteration of the 12” PDC bit designed to match the powered
rotary steerable systems (PRSSs) achieved multiple drilling
records in most zones of the Ghawar field. Today, it is
possible to drill the entire section in one run, which sig -
nificantly reduces the drilling time and costs of the deep gas
operations in Saudi Arabia.
GEOLOGY OF THE 12” SECTION
The geology of the 12” section, Fig. 2, includes the following
formations.
Base of the Jilh Formation
The base of the Jilh formation consists primarily of carbonates
(dolomitic limestone and dolomite) with streaks of anhydrite
and shale. Because the Lower Jilh is known to be a highpressure
zone, casing is set in the base Jilh dolomite to
increase the mud weight to compensate for the high-pressure
zone. The estimated formation pore pressure is 75 pounds per
cubic foot (pcf) to 80 pcf equivalent mud weight, and it is
drilled with a mud weight of 78 pcf to 90 pcf. The typical
mud weight is 100 pcf.
Sudair Formation
The Sudair formation consists primarily of shale with streaks
of anhydrite, siltstone and dolomitic limestone. The estimated
formation pore pressure is 80 pcf to 82 pcf equivalent mud
weight, and it is drilled with a mud weight of 90 pcf to 93
pcf. The shale in the Sudair formation is reactive to hydration;
bit and stabilizer balling is common in some zones of the field.
Khuff Formation
The Khuff formation is a highly intercalated formation that
consists primarily of carbonates (limestone and/or dolomite
and/or dolomitic limestone) with streaks of anhydrite and
shale. The Khuff formation contains three gas reservoirs
(Khuff A, B and C). Khuff A and B are medium hard
formations, and Khuff C is more consolidated.
WELL PROFILE
Early directional wells used conventional motor systems. The
vertical section was drilled with a performance motor and an
aggressive PDC bit. The bit was then pulled out of the hole,
Fig. 2. Stratigraphic column; formations drilled directionally.
and the build section was drilled with a steerable motor
bottom-hole assembly (BHA). Directional objectives could be
achieved; however, drilling with a conventional motor BHA
requires numerous bit runs and provides low ROPs. Before
the introduction of the PRSS application, the average ROP
when drilling a 12” curve section was approximately 10 ft/hr.
Figure 3 shows the well profile.
In a typical well design, 13 3 ⁄8” casing is set 30 ft into the
base of the Jilh dolomite. A typical well profile requires
vertical drilling approximately 800 ft to 1,000 ft with a
performance motor and an aggressive PDC bit. The curve
section is planned with a dogleg severity (DLS) of
approximately 4°. The 9 5 ⁄8” casing was planned to be set into
the Khuff C carbonate. It required an average of two to four
bit trips to drill the curve section. The formations encountered
while drilling the build section are hard, and because of the
number of hours, bit wear, low ROP, and tool failures,
additional trips were required.
The objectives with the PRSSs were to drill both vertical
and curve sections in one run, maximize the ROP, and
increase the footage drilled per bit run, while meeting
directional requirements.
BIT PERFORMANCE WITH A STEERABLE MOTOR
This section will discuss the bit performance with a steerable
motor in a 12” section of the southern Ghawar field where
the bit optimization process occurred. The hardness of the
rock, transition zones, shale reactivity, and rock abrasiveness
in the southern zones differs slightly between zones, which
represents various bit design challenges.
46 SUMMER 2010 SAUDI ARAMCO JOURNAL OF TECHNOLOGY