Problems Related to the Mud System

 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 sustain￾able 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 par￾tially 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 dam￾age, minimizing the potential for loss of circulation, stabilizing the well￾bore 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 eco￾nomics of recycling will improve. In addition, the introduction of envi￾ronment-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 compa￾nies 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 per￾formance. They should have a clear vision of the objectives of any mud pro￾gram, 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 lithol￾ogy, 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 well￾bore. The fluids also maintain a predetermined hydrostatic pressure to 

balance the formation pressure. The same drilling fluid is usually recon￾ditioned and reused. When comparatively low-pressure subterranean 

zones are encountered during a drilling operation, the hydrostatic pres￾sure 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 authori￾ties and researchers defined the lost circulation in a diversified manner. 

According to oilfield glossary it is defined as “the collective term for sub￾stances added to drilling fluids when drilling fluids are being lost to the 

formations downhole”. Howard (1951) defined it as follows: “loss of cir￾culation is the uncontrolled flow of whole mud into a formation, some￾times 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 cir￾culation is impossible. However, limiting circulation loss is possible if cer￾tain 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 produc￾tion 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 prob￾lem 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 under￾ground 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 circula￾tion zones. Many kinds of materials can be used as LCM. They include low￾cost 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 nar￾row 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 tech￾nical 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 sub￾salt, 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 forma￾tions 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, consider￾ations 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 prevent￾ing intrusion and salt deposition in the wellbore. The rheology consider￾ation 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 com￾plicated 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 con￾centrated “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 sys￾tems lost circulation materials (MEMS-LCM). A typical usage of this tech￾nology 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 ten￾dency of induced fractures (manifested through pressure pulsation), pres￾sure 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 circu￾lation 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) bridg￾ing 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 forma￾tions. Examples of such agents are ground peanut shells, walnut shells, cot￾tonseed 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, shal￾low 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 mecha￾nism 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) for￾mation, (iii) highly permeable zone, (iv) improper drilling conditions, (v) 

induced fractures caused by excessive downhole pressures and setting inter￾mediate 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. High￾permeability 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 pos￾sible if certain precautions are taken, especially those related to induced 

fractures. There are some preventive measures that can reduce the lost cir￾culation 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 antici￾pated, 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 forma￾tion integrity test should be performed to limit the possibility of loss of 

circulation.

Leak-off test (LOT): Conducting an accurate leak off test is funda￾mental 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 weak￾est points in any interval. However, extending a LOT to the fracture￾extension 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 inter￾val. 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.