By Jessica McDanieland Tunde Abass

HOUSTONZonal isolation in wellcementing is critical to the long-termsuccess of any oil or gas well. Withoutproper zonal isolation, gas migration, andsubsequently, sustained casing pressuremay result.Several factors are important to ensure

proper zonal isolation in any drilling andcompletion campaign. These factors,which ultimately determine the cost ef-fectiveness of the oil/gas well program,include drilling practices, cement typeand composition, mud removal, and ce-ment placement techniques.This article focuses on cement sheath

durability and its role in reducing gasmigration for the life of the well, specif-ically in the Marcellus Shale play. Gasmigration can occur at any point duringa wells lifetime. However, the risks andmechanisms of migration can be dividedinto two periods: short and long term.Short-term gas migration is the period

from the time a cement job begins untilthe cement is finally set. This is no morethan a few hours, during which gas mi-gration may occur as a result of severalcement parameters, all of which relate tomaintaining adequate hydrostatic pressureand preventing gas from entering the ce-ment matrix.The cement must maintain enough

hydrostatic pressure during placementand initial setup to combat any fluids orgases moving from the formation to thesurface. This is controlled through adequateslurry density; good fluid loss control;various gas control additives; and quickliquid-to-gel transition time, once the ce-ment has been placed.

Long-term gas migration is the periodstarting when cement sets through theend of the wells lifetime. Gas migrationduring this period can occur in as little asa week and as late as several years aftercementing.Once the cement is set, there are many

variables in the environment, in well op-erations, or even in the properties of thecement itself that can contribute to gasmigration. Many cementing companiesare addressing short-term gas migrationalready. However, the detailed study oflong-term zonal isolation is limited. Thisarticle will highlight the causes of gasmigration in the long term, as well as

some important aspects of cement designand placement that can help prevent zonalisolation issues.

Damaging ForcesDrilling a natural gas well in the Mar-

cellus play can take up to four weeks.After setting the conductor, the next stringto be cemented is the surface pipe. Aftertwo to three days of drilling, the inter-mediate casing is set at around 2,000feet. Then, after a standard wait on cementof eight hours, well operations commencedrilling to the production zone. This lastsanother two to three weeks.During that period, the cement sheath

between the intermediate and surface cas-ing is exposed to daily well-construc-tion-related stresses. Drilling involveschanges in pressure, bit trips, and vibration.These stresses are cyclic and have thepotential to damage the cement sheath,which must maintain an adequate seal incritical areas of the well to control anypotential gas or fluid flow.Hydraulic fracturing is another oper-

ation that may affect the cement sheath,especially in the production string. Casingwill expand as it is pressured, applyingtension to the cement sheath. Typically,a hydraulic fracturing operation occursin several stages, which causes this stressto cycle between high and low pressure.This type of operation may cause the

cement sheath to de-bond from the casing,or stress the cement sheath to the pointof cracking. Such failures could causegas migration through the cracked cementmatrix, resulting in sustained casing pres-sure.Another cause for failure is in the

downhole environment surrounding thecement sheath. Chemical attack is very

Cement Sheath Must Endure Long Term

To ensure long-term zonal isolation, itis important to test and optimize ce-ments mechanical properties, which in-clude Youngs modulus, Possions ratio,tensile strength and anelastic strain.

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detrimental to a cement system, and de-grades the cement matrix, causing in-creased permeability and decreased com-pressive strength and overall integrity.Eventually, this will cause gas migrationthrough the cement matrix, resulting insustained casing pressure at the surface.There are many cement additives that

can reduce or eliminate the effects ofcertain chemical attacks. If cement sheathfailure and gas migration are linked tochemical attack, these additives can bevery useful in preventing sustained casingpressure.

Cement PlacementPrior to cementing an intermediate

casing in the Marcellus play, there usuallyis a water-based mud in the well. Drillingfluids are necessary to lubricate the drillbit, transport cuttings and maintain well-bore stability. However, in cementing,mud is just in the way.Mud removal is very important to the

long-term prevention of gas migration.Any mud left in the wellbore or on thecasing could potentially result in a poorcement bond. However, if a channel ofmud remains in the wellbore, it will dryeventually, leaving a channel in the cementthat will allow formation fluids or gas toflow through.Good mud removal can be achieved

by following these guidelines:

Ensure the mud is adequately con-ditioned prior to cementing.

Design an optimized spacer andensure enough volume is used.

Centralize the casing. Rotate and reciprocate the casing

while pumping, if possible. Pump at an adequate rate to ensure

optimal fluid movement.In addition to these guidelines, main-

taining the designed density of the cementwhile pumping is critical. Cement pumpedat the designed density has cement, waterand additives in the correct ratios, whichprovides adequate fluid-loss control,enough available pump time, and goodcompressive strength development.Cement pumped above or below that

designed density will have properties dif-ferent from original design expectations,potentially resulting in a poor quality ce-ment sheath. Preventing gas migrationlies in controlling the cement propertiesbefore and after setting.

Cement PropertiesCement systems typically are tested

while liquid for specific properties andinitial compression strength development.These tests are run in field laboratoriesall over the world for the hundreds of ce-ment jobs run daily. Standard cementproperties include thickening time, com-pressive strength development, rheology,

fluid loss, and free fluid.These properties indicate whether a

cement system can be pumped and forhow long. They also indicate the stabilityof the system and its ability to developcompressive strength.These properties also can be used in

conjunction with testing for short-termgas migration to determine whether asystem is likely to allow gas flow whilethe cement is still liquid. However, noneof these properties indicate whether long-term gas migration can be an issue.Outside of compressive strength, ce-

ment systems rarely are tested for theseproperties after the cement has set. Yet,these tests can indicate the ability of acement to maintain zonal isolation duringthe life of the well, and indicate any po-tential for long-term gas migration. In-creasing a cements ability to handlestresses is very important.To achieve this, the cements mechan-

ical properties need to be optimized.These include Youngs modulus, Possionsratio, tensile strength, and anelastic strain.Youngs modulus is a measure of

how stiff a material is. The larger thenumber, the stiffer the material. The typ-ical range for cement is 140,000 to 1.4million psi. For cement to perform well,this number must be optimized. If it istoo low, the cement may be of poorquality; if it is too high, the cement be-comes too brittle.Possions ratio is another value that

when optimized can help increase thecement sheaths resistance to failure. Asa cement specimen is stressed axially, itwill deform both axially and radially.The ratio of the deformation is Poissonsratio. A higher Poissons ratio means thematerial can withstand greater deformationbefore failure. This is a desirable trait ina cement system exposed to stress.

Cement Sheath StressesCement systems have the ability to

perform very well in compression. How-ever, in practice, cement sheaths are ex-posed to other forces, including tension.Any time pressure is applied to the casingand it expands, the cement is being pulledin tension. Cement systems have a rela-tively low tensile strength, compared withother materials such as steel casing. How-ever, cement can show improved tensilestrength through design changes and in-cluding certain additives.The different stresses cement sheaths

are exposed to in the wellbore are often

FIGURE 1100

90

80

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60

50

40

30

20

10

0Field Success Rate Tensile StrengthPoissons RatioYoungs Modulus Anelastic Strain

System 1System 2

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cyclic forces. A material usually canmaintain its integrity when exposed to asmall force one time. However, when ex-posed to this same force many times overa long period, such as during drilling orhydraulic fracturing, the cement sheathsintegrity may become compromised.This scenario is simulated during the

anelastic strain test, which uses a com-pressive load cycling between a low andhigh force compressing the cement sample.Sample deformation is measured overtime, resulting in a curve showing thepercentage of deformation versus timeor cycles.The smaller the anelastic strain number,

the higher performing the cement sheathis. This information will enable the engi-neers designing a cement job to select asystem that shows the most recovery andleast total deformation.These properties can all be correlated

to determine the ability of a cement towithstand outside forces. As the integrityof the cement increases, the cement sheathwill be able to maintain complete zonalisolation while enduring cyclic stressesfrom drilling, fracturing, and other sources.

Marcellus Case HistoryCementing operations in the Marcellus

Shale involve several slurry designs. Thesuccess of the cement job depends oncement design, type, composition, andcementing program.Two cement slurry systems were placed

in intermediate casing strings in severalwells in the Marcellus Shale. One systemwas almost twice as successful as theother, which led to an investigation* todetermine the source of success so thatother cement systems could be improved.Short-term gas migration analysis in-

dicated successful results for both systemsand gave credence to the idea that thefailures were the result of long-term gasmigration factors such as cement dura-bility.As a result, testing to determine the

possibility of long-term gas migrationwas inevitable. To validate the hypothesisthat stresses on the casing resulted inweakening the cement bond in the inter-mediate casing, tests were conducted formechanical properties, tensile strength,anelastic strain, impact resistance, andseal durability.Basic cement testingthickening time,

compressive strength, fluid loss, free fluidand rheology testsconducted on bothsystem 1 and 2 showed few differences.These variations were not significantenough to account for the dissimilarityin cement durability.An analysis of set cement properties

further strengthened the argument that

cement design and composition werethe reason for the difference in field re-sults. The differences in compressivestrength between the two systems weretoo low to be considered significant.This led to in-depth testing of mechanicalproperties for Youngs modulus, Poissonsratio, tensile strength, and anelasticstrain.

Figure 1 shows the comparison ofeach of the mechanical properties withthe field results. Youngs modulus, Pois-sons ratio, and tensile strength were allhigher for system 1. However, as seen inFigures 2A and 2B, anelastic strain po-tential calculated from the slope of de-formation versus cyclic loads was lowerfor system 1, resulting in less deformation

FIGURE 2BSystem 2 Anelastic Strain Chart

0.30

Strain Percentage

0.25

0.20

0.15

0.10

0.05

0.001,400400 8006002000 1,000 1,200

Time (seconds)

Anelastic StrainTrend Line

FIGURE 2ASystem 1 Anelastic Strain Chart

0.12

Percentage Axia

l Deformation

0.10

0.08

0.06

0.04

0.02

0.00

-0.02

1,400400 8006002000 1,000 1,200

Time (seconds)

Axial StrainTrend Line

1,600

in the cement sample. Consistently bettermechanical properties, along with thehigher field performance of system 1,may be linked to the durability of the ce-ment system.

ConclusionsAll forces exerted on the cement sheath

for the life of the well need to be consideredwhen cementing a well. These forces dochange over time, varying with the welloperations as well as chemical and geologicactivity. This includes the cement sheathsshort- and long-term performance.While many companies address short-

term gas migration when designing theircement systems, the mechanical propertiesof the cement that affect long-term per-formance may not be a priority. Whentaking long-term performance propertiesinto account results in a better cementsheath, the risk of gas migration decreases.There are three major points to re-

member when attempting to prevent long-term gas migration:

Numerous forces may be placed on

the cement sheath, and all should be con-sidered when designing a cement system.

Optimized cement placement andmud removal will help to prevent chan-neling and promote cement bonding.

An appropriate cement design con-siders mechanical properties that affectlong-term performance, and not only settime and compressive strength.By designing cement for the life of a

well, durability and dependability will in-crease, resulting in a quality cement bondeven in the face of damaging forces.

* Editors Note: Funding for thisproject was provided through the ultra-deepwater and unconventional naturalgas and other petroleum resources researchand development program authorized bythe Energy Policy Act of 2005. This pro-gram is administered by the ResearchPartnership to Secure Energy for Americaunder contract with the U.S. Departmentof Energys National Energy TechnologyLaboratory. Additional information canbe found at www.rpsea.org.

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JESSICA MCDANIEL is a devel-opment engineer at CSI Technologies,where she is responsible for long-term research projects. Previously, sheworked as a field engineer for Schlum-berger, providing cementing servicesfor operators in challenging Arcticconditions as well as offshore. Mc-Daniel has authored SPE papers aboutcement integrity for horizontal wells,and has led RPSEA projects analyzingwellbore integrity, cementing opera-tions, cement systems, and zonal iso-lation. She holds a B.S. in engineeringfrom Michigan State University.

TUNDE ABASS is a developmentengineer at CSI Technologies, wherehe is responsible for several short-term research projects. He has priorexperience as a field engineer withHalliburton, working on horizontalwells in the Eagle Ford Shale. Heholds a B.S. in chemical engineeringfrom West Virginia University, and anM.S. in natural gas engineering fromTexas A&M University-Kingsville.