Introduction
The drilling-fluid system is one of the well-construction processes that
remains in contact with the wellbore throughout the drilling operation.
Advances in mud technology have made it possible to implement a sustainable system for each interval in the well-construction process. As a result,
the associated problems have been reduced significantly. Reduction of the
cost of the drilling fluid, which is an average of 10% of the total tangible
costs of well construction, is a great challenge. Mud performance can affect
overall well-construction costs in several ways. In addition, failure to select
and formulate the mud correctly will create many problems. This chapter
addresses the problems related to drilling-fluid system and proposes the
solutions. However, there are some problems which are not directly related
to the mud system. These problems are discussed in another chapter. An
identified problem well caused by drilling-fluid can be considered as partially solved. Therefore, identifying any problem turns out to be a crucial
task. The logical relationship of cause and
effect must be well organized to
the identified related problems. Mixing their logical relationship may lead
to hindering further problem analysis tasks.
3.1 Drilling Fluids and its Problems with Solutions
A correctly formulated and well-maintained drilling system can contribute
to cost containment throughout the drilling operation by enhancing the
rate of penetration (ROP), protecting the reservoir from unnecessary damage, minimizing the potential for loss of circulation, stabilizing the wellbore during static intervals, and helping the operator remain in compliance
with environmental and safety regulations. Drilling fluids can be reused
from well to well, thereby reducing waste volumes and costs incurred for
building new mud. Although currently reusing doesn’t diminish costs at
any appreciable manner, as more operators practice this recycling, the economics of recycling will improve. In addition, the introduction of environment-friendly additives is amenable to recycling and minimization of
environmental footprints. To the extent possible, the drilling-fluid system
should help preserve the productive potential of the hydrocarbon-bearing
zone(s). Minimizing fluid and solids invasion into the zones of interest
is critical to achieving desired productivity rates. The drilling fluid also
should comply with established health, safety, and environmental (HSE)
requirements so that personnel are not endangered and environmentally
sensitive areas are protected from contamination. Drilling-fluid companies work closely with oil-and-gas operating companies to attain these
mutual goals.
Drilling fluid (also called drilling mud) is an essential part in the rotary
drilling system. Most of the problems encountered during the drilling of
a well are directly or indirectly related to the mud. To some extent, the
successful completions of a hydrocarbon well and its cost depend on the
properties of the drilling fluid. The cost of the drilling mud itself is not
very high. However, the cost increases abruptly for the right choice, and
to keep proper quantity and quality of fluid during the drilling operations.
The correct selection, properties and quality of mud is directly related to
some of the most common drilling problems such as rate of penetration,
caving shale, stuck pipe and loss circulation, and others. In addition, the
mud affects the formation integrity and subsequent production efficiency
of the well. More importantly, some toxic materials are used to improve
the specific quality of the drilling fluid that are a major concern to the
environment. Among others, this addition of toxic materials contaminates
the underground system as well as the surface of the earth. Economically, it
also translates into long-term liability as stricter regulations are put in place
with increasing awareness of environmental impacts of toxic chemicals.
Therefore, the selection of a suitable drilling fluid and routine control
of its properties are the concern of the drilling operators. The drilling and
production personnel do not need detailed knowledge of drilling fluids.
However, they should understand the basic principles governing their
behavior, and the relation of these principles to drilling and production performance. They should have a clear vision of the objectives of any mud program, which are: (i) allow the target depth to be reached, (ii) minimize well
costs, and (iii) maximize production from the pay zone. In a mud program,
factors needing to be considered are the location of well, expected lithology, equipment required, and mud properties. Hence this chapter refers to
the author’s textbook Fundamentals of Sustainable Drilling Engineering for
details in the basic components of mud, its functions, different measuring
techniques, mud design and calculations, the updated knowledge in the
development of drilling fluid and future trend of the drilling fluid. It is
important because acquiring this knowledge will lead to an understanding
of the real causes, and solutions related to drilling-fluid system.
3.1.1 Lost Circulation
During drilling of hydrocarbon wells, drilling fluids are circulated through
the drill bit into the wellbore for removal of drill cuttings from the wellbore. The fluids also maintain a predetermined hydrostatic pressure to
balance the formation pressure. The same drilling fluid is usually reconditioned and reused. When comparatively low-pressure subterranean
zones are encountered during a drilling operation, the hydrostatic pressure is compromised because of leakage into the zones (Figure 3.1). This
phenomenon is commonly known as “lost circulation.” So, lost circulation
is defined as the uncontrolled flow of mud into a “thief zone” and presents
one of the major risks associated with drilling. However, different authorities and researchers defined the lost circulation in a diversified manner.
According to oilfield glossary it is defined as “the collective term for substances added to drilling fluids when drilling fluids are being lost to the
formations downhole”. Howard (1951) defined it as follows: “loss of circulation is the uncontrolled flow of whole mud into a formation, sometimes referred to as a “‘thief zone.’” It is also defined as “the reduced or
total absence of fluid flow up the annulus when fluid is pumped through
the drillstring (Schlumberger, 2010). The complete prevention of lost circulation is impossible. However, limiting circulation loss is possible if certain precautions are taken. Failure to control lost circulation can greatly
increase the cost of drilling, as well as the risk of well loss. Moreover, lost
circulation may lead to loss of well control, resulting in potential damage
to the environment, fire and/or harm to personnel. The risk of drilling a
well in areas known to contain potential zones of lost circulation is a key
factor in planning to approve or cancel a drilling project. The successful
management of lost circulation should include identification of potential
“thief zones”, optimization of drilling hydraulics, and remedial measures
when lost circulation occurs.
The problem of lost circulation was apparent in the early history of the
drilling industry and was magnified considerably when operators began
drilling deeper and/or depleted formations. The industry spends millions
of dollars a year to combat lost circulation and the detrimental effects it
propagates, such as loss of rig time, stuck pipe, blowouts, and frequently,
the abandonment of expensive wells. Moreover, lost circulation has even
been cited as the cause for production loss and failure to secure production tests and samples. On the other hand, controlling lost circulation can
lead to plugging of production zones, resulting in decreased productivity.
The control and prevention of lost circulation of drilling fluids is a problem frequently encountered during the drilling of oil and gas wells. During
the drilling of wells, fractures that are created or widened by drilling fluid
pressure are suspected of being a frequent cause of lost circulation. Of
course, natural fractures, fissures, and vugs can create lost circulation even
during underbalance drilling, in which fluid pressure doesn’t play a role in
lost circulation.
There are four types of formation and/or zones that can cause loss of
circulation: (i) cavernous or vugular formations, (ii) unconsolidated zones,
(iii) high permeability zones, and (iv) naturally or artificially fractured
formations. Circulation loss can take place when a comparatively high
pressure zone (subterranean) is encountered, causing cross flows or underground blowouts. Whenever the loss of circulation crops up, it is noticed by
the loss of mud, and the loss zones are classified according to the severity of
the loss: (i) “Seepage” with less than 10 bbl/hour loss, (ii) “Partial Loss” for
10 to 500 bbl/hour loss, (iii) “Complete Loss” for greater than 500 bbl/hour
loss. The lost circulation problem requires corrective steps by introducing
lost circulation materials (LCM) into the wellbore to close the lost circulation zones. Many kinds of materials can be used as LCM. They include lowcost waste products from the food processing or chemical manufacturing
industries. Figure 3.1 shows some examples of LCM as listed here.
Historically, mud losses have been dealt with by dumping some mica
or nut hulls down a wellbore. There are numerous reports of ‘throwing
in everything available’ to stop the extreme cases of mud loss. However,
as the drilling operation becomes increasingly sophisticated and great
feats are achieved in terms of drilling in difficult terrains and deep wells,
simplistic solutions are no longer applicable. The industry is accelerating
its activities in deepwater and depleted zones, both of which present narrow operating limits, young sedimentary formations, and high degree of
depletion overbalanced drilling. These newfound prevailing conditions are
susceptible to creating fractures and thus lead to lost circulation. Among
others, drilling through and below salt formations presents a host of technical challenges as well. The thief zone at the base of the salt can introduce
severe lost circulation and well control problems. This often results in loss
of the interval or the entire well. The lost time treating severe subsalt losses
can last for several weeks, with obvious cost implications, especially for
deepwater drilling operations. Salt formations are common for oil-bearing
formations that can be termed pre-salt if older or subsalt if younger. The
oil-bearing formations of below salt in the Gulf of Mexico are mainly subsalt, whereas those in offshore Brazil are a mix of subsalt and pre-salt. The
difficulty in managing a drilling operation through a salt formation lies
in the fact the salt composition varies greatly. For instance, for the Gulf
of Mexico, the salt formation contains mainly NaCl. On the other hand,
the offshore Brazil salt formations have predominantly MgCl2
, which is
far more reactive than NaCl. Salt formations are typical of other formations that are equally plastic and mobile can also be encountered during
drilling. Controlling losses in this zone has proven to be extremely difficult
as it involves matching the composition of the mud with that expected
downhole, in order to minimize leaching of the in-situ salt into the drilling
mud – a process that would create imbalance in the fluid system. Also, the
plasticity of the salt may cause shifting. Therefore, the mud weight should
be as close to overburden gradient, otherwise salt may shift into wellbore,
leading to pipe being stuck. Very few lost circulation remedies have been
successful, especially when using invert emulsion drilling fluids. Typically,
a salt formation should be drilled with salt-tolerant water-based drilling
fluids or with invert emulsion fluids. Deeper salt zones can be drilled with
oil-based fluids that can be replaced with water-based mud after the salt
formation has been passed. Such formations are available in the Bakken
basin of the United States. In drilling through salt formations, considerations of density, salinity, and rheology are of paramount importance. The
density consideration relates to maintaining bore stability. The salinity
relates to preventing leaching from the salt formation as well as preventing intrusion and salt deposition in the wellbore. The rheology consideration relates to cleaning the salt cuttings and keeping them afloat during
the return of the mud.
When dealing with induced fractures the problem is even more complicated because the shape and structure of induced formation fractures
are always subject to the nature of the formation, drilling and mechanical
effects, as well as geological influences over time. When the overbalance
pressure exceeds the fracture pressure, a fracture may be induced and lost
circulation may occur. By incorporating a lost circulation material (LCM)
in the fracture to temporarily plug the fracture, the compressive tangential
stress in the near-wellbore region of the subterranean formation increases,
resulting in an increase in the fracture pressure, which in turn allows the
mud weight to operate below the fracture pressure.
LCM are often used as a background treatment or introduced as a concentrated “pill” to stop or reduce fluid losses. The main objective when
designing an effective treatment is to ensure that it is able to seal fractures
effectively and stop losses at differential pressure. The differential pressure
is caused by the elevated drilling fluid pressures compared to the pore fluid
pressure in regular drilling operations or drilling fluid pressures exceeding
the wellbore fracture pressure. The design of the LCM treatment hinges
upon particle size distribution (PSD) as the most important parameter
(Ghalambor et al., 2014; Savari et al., 2015). Al Saba et al., 2017) compared
various PSD methods and proposed one that is the most accurate. Table 3.1
lists these methods. The most recent selection criteria are the most accurate
and they stipulate that D50 and D90 should be equal or greater than 3/10th
and 6/5th of the fracture width, respectively. Al Saba et al. (2017) reported
that nutshells can plug fractures with relatively low concentrations whereas
graphite and calcium carbonate are effective only at higher concentrations.
In general, there is a general fascination for sphericity and roundness of
LCM, needs to be taken into consideration when analyzing PSD. As such,
artificial LCM have gained popularity.
Recent advances in LCM have been in developing an array of materials with
a range of sizes, shapes, and specific gravities. The new generation of these
materials involve smart materials, such as the one patented by Halliburton
(Rowe et al., 2016). Rowe et al. introduced Micro-electro-mechanical systems lost circulation materials (MEMS-LCM). A typical usage of this technology would involve drilling at least a portion of a wellbore penetrating
the formation with a drilling fluid that comprises a base fluid. This can be
followed by several cycles of MEMS-LCM, and another set of LCM, wherein
the MEMS-LCM and the LCM are substantially similar in size, shape, and
specific gravity. After this cycle, measurements can be made to determine
concentrations of the MEMS-LCM in the drilling fluid before circulating
the drilling fluid through the wellbore and after the MEMS-LCM treatment,
thus finalizing the concentration of the next phase of MEMS-LCM.
One condition of paramount importance in sealing induced fractures
(i.e., to change shape and size as per wellbore pressure changes) is having
the LCM reaching the tip of the fracture. Related to the breathing tendency of induced fractures (manifested through pressure pulsation), pressure buffering is another condition that should be fulfilled for effective
sealing. Preferably, to stop the breathing tendency in a robust manner, the
pills should be able to increase the fracture gradient at a level sufficiently
high to avoid reopening the fracture during the subsequent drilling phases.
Table 3.2 shows several LCM with their characteristic concentrations.
Figure 3.2 shows partial (Figure 3.2a) and total lost-circulation zones
(Figures 3.2b, and c). In partial lost circulation, mud continues to flow to
surface with some loss to the formation. Total lost circulation, however,
occurs when all the mud flows into a formation with no return to surface.
A series of lost circulation decision trees is developed to address lost circulation problems for the deepwater prospect (Figure 3.3).
In general, there are three types of basic agents used in the petroleum
industry to control the loss of circulation problem. These are: (i) bridging agents, (ii) gelling agents, and (iii) cementing agents. These agents are
either employed individually or in a blended combination. The bridging
agents are the ones that plug the pore throats, vugs, and fractures in formations. Examples of such agents are ground peanut shells, walnut shells, cottonseed hulls, mica, cellophane, calcium carbonate, plant fibers, swellable
clays ground rubber, and polymeric materials. Bridging agents are further
classified based on their morphology and these can be: (i) flaky (e.g., mica
flakes and pieces of plastic or cellophane sheeting), (ii) granular (e.g.,,
ground and sized limestone or marble, wood, nut hulls, Formica, corncobs
and cotton hulls), and (iii) fibrous (e.g., cedar bark, shredded cane stalks,
mineral fiber and hair). Gelling agents and cementing agents are used for
transportation and placement of the bridging agent at the appropriate place
in the circulation loss zone. Highly water absorbent cross-linked polymers
are also used for loss of circulation problem, as they form a spongy mass
when exposed to water.
The LCM are evaluated based on their sealing properties at low and high
differential pressure conditions. In addition, effectiveness of the sealing to
withstand all kind of pressures during drilling is tested. LCM are classified
according to their properties and application, such as formation bridging
LCM and seepage loss LCM. Often more than one LCM type may have to
be used to eliminate the lost circulation problem.
These drilling problems are encountered both in onshore and offshore
fields when the formation is weak, fractured and/or unconsolidated. Drilling
for oil and gas in deep water encounters further challenges, brought about
by a host of reasons. Some potential hazards are shallow water flow (SWF),
gas kicks and blowouts, presence of unconsolidated sand formations, shallow gas, gas hydrate lost circulation, sea floor washout, borehole erosion,
etc. These problems are not only hazards on their own; they can also cause
a significant increase in the total drilling cost. Consequently, alleviation of
the scope and capacity of these hazards and challenges is imperative for
safe and economic completion of deep water wells, so that work can be
done systematically with the least amount of risk.
3.1.1.1 Mechanics of Lost Circulation
Lost circulation frequently occurs in cavernous limestone or in gavel beds at
relatively shallow depths and under normal pressure conditions. In this type
of lost circulation, the mud will flow into the cavities at any pressure more
than the formation fluid pressure without disturbing the reservoir rock.
This type of lost circulation is prevalent in the cap rock of pier cement-type
salt domes. Lost circulation under these conditions is essentially a filtration
problem which can be corrected if the large pore spaces can be plugged.
However, the lost circulation due to abnormal pressures differs in mechanism from the foregoing one. In this case, mud fluid is not lost by filtration
into large pore spaces in the reservoir rock. The loss of whole mud can take
place only through formations in which the pore sizes are so large as to
cause the concept of permeability to lose its generally accepted meaning.
Lost circulation occurs only when the mud weight is approaching the weight
of the overburden (15 to 18 lbs per gallon). Loss of circulation in this case
results from tensile failure of the sediments along lines of weakness, rather
than from mud filtration into existing pore spaces. That formation failure
does occur as evidenced by the conditions under which circulation is lost.
The usual condition is a sudden and complete loss of returns which may
occur while drilling, circulating, or while out of the hole to run an electrical
survey. There are several situations that can result in lost circulation such as
(i) formations that are inherently fractured, (ii) cavernous (i.e., hollow) formation, (iii) highly permeable zone, (iv) improper drilling conditions, (v)
induced fractures caused by excessive downhole pressures and setting intermediate casing too high, (vi) improper annular hole cleaning, (vii) excessive
mud weight, and (viii) shutting in a well in high-pressure shallow gas.
Induced or inherent fractures or fissures may appear as horizontal at
shallow depth or vertical at depths greater than approximately 762 m.
Excessive wellbore pressures are developed due to high flow rates (i.e.,
high annular-friction pressure loss) or tripping in too fast (i.e., high surge
pressure). This can lead to mud equivalent circulating density (ECD).
Induced fractures can also be caused by improper annular hole cleaning,
excessive mud weight, and shutting in a well in high-pressure shallow gas.
Equations (3.1) and (3.2) show the conditions that must be maintained to
avoid fracturing the formation during drilling, and tripping in, respectively
Cavernous formations are often limestones with large caverns. This
type of lost circulation is quick, total, and the most difficult to seal. Highpermeability formations are potential lost-circulation zones, which are
shallow sand with permeability greater than 10 Darcies. In general, deep
sand has low permeability and presents no loss circulation problems. The
level of mud tanks decreases gradually in non-cavernous thief zones. In
such situations, if drilling continues, total loss of circulation may occur.
Partial loss of returns is common in the case of mud loss by filtration.
However, this is a rare occurrence under abnormal pressure conditions.
The mechanics of lost circulation of this type are probably most closely
duplicated in nature by igneous intrusions. In both cases, the formation
falls under extreme pressure. The only difference is in the source of the
pressure
Preventive Measures
The complete prevention of lost circulation is impossible because some
formations, such as inherently fractured, cavernous, or high-permeability
zones, are not avoidable when encountered during the drilling operation if
the target zone is to be reached. However, limiting circulation loss is possible if certain precautions are taken, especially those related to induced
fractures. There are some preventive measures that can reduce the lost circulation which can be listed as: (i) crew education, (ii) good mud program
i.e., maintain proper mud weight, (iii) minimize annular friction pressure
losses during drilling and tripping in, (iv) maintain adequate hole cleaning
and avoid restrictions in the annular space, (v) set casing to protect weaker
formations within a transition zone, (vi) updating formation pore pressure
and fracture gradients for better accuracy with log and drilling data, and
(vii) study wells in area to be drilled. The rule of thumb is that if anticipated, treat mud with LCM.
If loss of circulation happens, there are some actions that need to be
followed: (i) pump lost circulation materials in the mud, (ii) seal the zone
with cement or other blockers, (iii) set casing, (iv) dry drill (i.e., clear
water), and (v) updating formation pore pressure and fracture gradients
for better accuracy with log and drilling data. Now, once lost-circulation
zones are anticipated, preventive measures should be taken by treating the
mud with LCM and preventive tests such as the leak off test and formation integrity test should be performed to limit the possibility of loss of
circulation.
Leak-off test (LOT): Conducting an accurate leak off test is fundamental to prevent lost circulation. The LOT is performed by closing in
the well, and pressuring up in the open hole immediately below the last
string of casing before drilling ahead in the next interval. Based on the
point at which the pressure drops off, the test indicates the strength of
the wellbore at the casing seat, typically considered one of the weakest points in any interval. However, extending a LOT to the fractureextension stage can seriously lower the maximum mud weight that may
be used to safely drill the interval without lost circulation. Consequently,
stopping the test as early as possible after the pressure plot starts to break
over is preferred.
During the LOT, the leak-off test pressure, and equivalent mud weight
at shoe can be calculated using the following equations.
Formation integrity test (FIT): To avoid breaking down the formation, many
operators perform a FIT at the casing seat to determine whether the wellbore
will tolerate the maximum mud weight anticipated while drilling the interval. If the casing seat holds pressure that is equivalent to the prescribed mud
density, the test is considered successful and drilling resumes.
When an operator chooses to perform an LOT or an FIT, if the test fails,
some remediation effort such as a cement squeeze should be carried out
before drilling resumes to ensure that the wellbore is competent.
During the FIT, the formation integrity test pressure, and equivalent
mud weight at shoe can be calculated using the following equations.
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