I N NOVATIVE DR I LLI N G TECH N IQU E S
Amid growing uptake of MPD in
deepwater, controlled mud level
technology deserves another look
By taking opposite pressure management
approach to SBP method, CML achieves similar
benefits while widening deployment window
BY ANTHONY SPINLER, ENHANCED DRILLING
The oil and gas industry is constantly
evolving, driven by the need for more
efficient and sustainable exploration and
production methods. EC-Drill and its con-
trolled mud level (CML) systems have
grown from an initial concept of two sys-
tems used in the North Sea to a technology
that has drilled almost 100 offshore wells
and, within a few months, has gone from
one CML-capable rig in the Gulf of Mexico
(GOM) to having its third.
This article delves into the world of
CML, exploring its benefits and underly-
ing principles, showing the potential to
reshape the future of offshore drilling.
Similar benefits as SBP
but achieved differently
Managed pressure drilling (MPD) is one
of the most prominent components of the
offshore market today. Drilling contrac-
tors have stated that approximately 80%
of deepwater rig tenders require MPD
capability. In almost all cases, the speci-
fications are toward surface back pressure
(SBP) technology, which is well estab-
lished on land and offshore. Drilling engi-
neers and executives may have heard of
CML but must familiarize themselves with
the breakthroughs driving adoption.
CML, introduced in 2010, is an MPD
technique that combines innovative pump
technologies and advanced drilling prac-
tices to optimize the extraction of oil and
gas reserves from the initial liners to
completions. Since the wellbore is open,
similar to conventional drilling, it enables
well-known and established drilling and
well control procedures to improve drilling
30 efficiency, increase recoverable reserves
and reduce the environmental footprint.
Pressure in the wellbore is controlled by
changing the equivalent mud weight in
real time by adjusting the height of the
mud column in the riser; this provides
greater flexibility during drilling opera-
tions and enables operators to navigate
through challenging geological formations
with improved accuracy with less pres-
sure-induced risk.
CML offers several advantages similar
to SBP, including enhanced wellbore sta-
bility, reduced nonproductive time, better
control over wellbore pressures and the
ability to mitigate kicks and prevent blow-
outs. But it does so in the opposite way to
SBP. The EC-Drill CML pump systems pro-
vide more than MPD, including pre-BOP
and pre-riser installation for cuttings
transfer and riserless mud recovery. Top-
hole sections that cannot use SBP can still
utilize CML equipment, providing addi-
tional environmental and cost savings at
applicable water depths.
Underbalanced vs overbalanced
MPD, predominantly known and used
as SBP since 1968, offers a revolutionary
approach to drilling through reservoirs.
Unlike conventional drilling, SBP uses
mud weight less than the formation pres-
sure, causing the well to be hydrostatically
underbalanced when not applying back
pressure. As such, reservoir fluids can, and
are, sometimes allowed to flow into the
wellbore as it operates on the pore pres-
sure side of the formation. This technique
is particularly beneficial in unconvention-
al reservoirs, tight gas formations and
mature fields. SBP starts with an under-
balanced mud weight and increases the
pressure through a rotating control device
(RCD) and choke, thus increasing the pres-
sure from above at the rig floor. In doing
so, the pressure returning from the bottom
of the hole increases when SBP is applied.
CML utilizes a balanced mud weight for
the drilling window and lowers the height
of the mud column to reduce the effec-
tive bottomhole pressure. Therefore, the
hydrostatic pressure profile increases with
depth – the opposite of SBP. The wellbore
pressure will then match the operating
window’s typical trend and allow for lon-
ger sections, especially in deepwater envi-
ronments that would otherwise require
frequent casings.
CML is conventional drilling where
the balanced mud weight is utilized and
only available offshore. In deepwater, the
EC-Drill system places an approximately
16.5-ton subsea pump at roughly 1,200 ft
(366 m) below the water surface, and the
pump’s output maintains the desired riser
level to establish the required bottomhole
pressure (BHP). Since CML does not insert
an RCD into the wellbore, it performs MPD
during all drilling operations: drilling, trip-
ping, liner and casing running, measure-
ment, cleaning, completions, or whatever
product or tool customers want to put
down the hole.
The author estimates that only 30-40% of
operational time offshore can utilize SBP;
with CML, potentially 100% can be done
when the riser is in position. Enhanced
Drilling estimates that EC-Drill operations
in the GOM have shown utilization during
approximately 85% of all drilling opera-
tions (drilling, cementing and tripping)
and 50% of completions.
Smoother drilling
One unique aspect of CML is that it is
similar to conventional drilling, as the
subsea pump does not close the well-
bore. With SBP’s use of an RCD, operations
require multiple calculations: a lowered
mud weight and then applying addition-
al pressure. As SBP operations contin-
ue, the well often requires an entirely
different mud weight and conducting a
SEPTEMBER/OCTOBER 2023 • DRILLING CONTRACTOR
I N NOVATIVE DR I LLI N G TECH N IQU E S
mud rollover. The new lower mud weight
cycles into the well, so back pressure must
increase again to reach the required BHP,
or a higher mud weight must be applied to
transition out of utilizing SBP.
CML eliminates these situations by
applying a balanced mud weight to the
well, starting the BHP closer to the frac-
ture gradient than SBP. Then, the pressure
is lowered to manage the BHP. One could
say that SBP approaches the MPD solu-
tion from the pore pressure side, risking
an influx, while CML approaches the MPD
solution from the fracture pressure side,
risking losses. For operators concerned
about the incredibly costly and environ-
mental expense of losses, CML provides
the tool to manage losses effectively while
lowering the risk of operating from the
pore pressure side of the pressure window.
Further, there is an additional ease of
use for cementing and completion opera-
tions, which require managing increas-
ing pressures as these operations proceed.
Excess pressure in cementing results in
costly losses and a possible poor cement
job, risking catastrophic failure. Excess
pressures in completion operations result
in a damaged reservoir that could impact
everything from initial production to total
recovered reserves. With CML, as the pres-
sure rises, lowering the riser level man-
ages the pressure profile and creates what
looks like a saw tooth: Pressure goes up
during the operation, then the pressure is
reduced by CML maintaining the window
throughout the cementing or completion.
As experienced in several cases with
the EC-Drill system, completion results
reached top-quartile
performance. Effectively finishing both – the alpha
and beta phases of completion operations
– without damaging the formation has
resulted in flow rates exceeding the drill-
ing rig’s ability to flare. As one customer
explained, “Wells estimated to have a 5- to
10-year lifespan are now turning out to be
10- to 15-year wells.” The exact flow rates
are proprietary to the operator.
For example, various drilling operations
require heavier/denser fluids using water-
based drilling fluids or heavy brines like
CaBr2 and ZnBr2. An overpressure of these
fluids can damage everything from the
casing shoe to the reservoir. The ability to
lower the overall BHP by reducing the riser
Figure 1: The EC-Drill CML system can help to eliminate intermediate casing strings
while navigating tight pressure windows in deepwater wells.
fluid level enables standardizing on other
proven technologies and fluids.
An issue offshore is that the casing
string length sometimes exceeds the
water depth, which heavily impacts the
value proposition of SBP-based MPD. Even
if the SBP drills the well, it cannot manage
the pressure while installing these longer
liners/casings. This issue does not affect
CML, which can apply MPD techniques
while installing all casing sizes. In addi-
tion, for operators who strive for a robust
separation between the primary and sec-
ondary barrier, CML provides a clear tran-
sition from the barrier envelope to stan-
dard well control procedures. Additional
steps are not required to roll over the
mud or close the BOP while tripping, as it
already uses a balanced mud. Operators
and drilling contractors can operate with
well-established and proven well control
procedures while also having MPD.
Improving sustainability
Apart from the operational advantag-
es, CML also contributes to environmen-
tal sustainability. By optimizing drilling
operations, these techniques minimize the
number of wells required to extract equiv-
alent hydrocarbons, reducing the overall
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