ZONAL ISOLATION IS CRITICAL todelivering a successful oil or gas wellcompletion as well as being a key tosafety. Cementing operations are partic-ularly challenging in difficult formationsand complex well configurations.

A session at the 2002 IADC/SPE DrillingConference in Dallas, 26-28 Feb, focuseson how to provide effective isolation. DT Mueller chairs the session.

R E D U C I N G F A I L U R E R I S K

One of the main objectives of a primarycementing job is to prevent formationfluids from migrating into the annulus.To achieve this objective, the cementsheath should withstand the stressesinduced by the various operations andmaintain integrity during the life of thewell.

However, most of the cementing designprograms in the industry today considerthe slurry properties and not the cementsheath properties.

IADC/SPE paper 74497, “Improve theEconomics of Oil and Gas Wells byReducing the Risk of Cement Failure,”reports on a design procedure devel-oped for estimating the risk of cementfailure as a function of cement sheathand formation properties and well load-ing.

Examples of the loading are cementhydration, pressure testing, well com-pletion, and production.

K Ravi, Halliburton Energy Servicesand M Bosma, Shell International,prepared the paper for the Conference.

The design procedure is based on a rig-orous finite element analysis and simu-lates the sequence of events from thetime the well is drilled through cementhydration, well completion, hydraulicfracturing and production. Cement fail-ure modes simulated are de-bonding,cracking and plastic deformation.

An appropriate nonlinear materialmodel, including cracking and plasticity,is used in the analysis. The cement isassumed to behave linearly as long asits tensile strength or its compressiveshear strength is not reached.

The material model adopted for undam-aged cement is a Hookean model bound-ed by smear cracking model in tensionand an appropriate model such as Mohr-Coulomb in the compressive shear. Theshrinkage/expansion of cement isincluded in the material model.

S A L T Z O N E C E M E N T I N G

Salt zone cementing is still a challenge.IADC/SPE paper 74500 presents aunique theoretical and experimentalresearch project developed to supportsalt zone cementing operations in Cam-pos basin, offshore Brazil.

The approach includes the developmentof a numerical simulator that repro-

duces the mass transfer phenomenainvolved in the displacement of a cementslurry in front of a salt formation. Theformulation is based on the solution of awell-known diffusion-convection equa-tion for the flow of a fluid through anannular section, being the external wallsoluble.

Experimental work includes the deter-mination of salt mass transfer rates tocement slurries flowing through a large-scale flow loop where the internal wallwas formed by real halite cores. Masstransfer coefficients are then fed intothe computer model.

“Dynamic Simulation Of Offshore SaltZone Cementing Operations,” was pre-pared for the Conference by A L Mar-tins and C Miranda, Petrobras.

The results obtained by running thesimulator allow proper slurry composi-tion design and the definition ofhydraulic parameters required for mini-mizing salt dissolution.

S P A C E R R H E O L O G Y

IADC/SPE paper 74498 presents a newmethod to design the optimum composi-tion of a spacer fluid used for mudremoval in a cement job.

The method integrates two steps: deter-mining the ideal spacer properties fromthe knowledge of well conditions, mudand cement properties; and designingthe fluid composition to achieve theseproperties. “Optimization of SpacerRheology Using Neural Net Technology,”was prepared by B E Theron, D Bodinand J J Fleming, Schlumberger.

The first step is based on well-acceptedrequirements for spacers, including fric-tion pressures and density hierarchycriteria. The second step is based on aneural network model developed froman extensive database of fluid proper-ties, where all key variables weremoved—composition, spacer density,and temperature.

A software module simulates and dis-plays the mud-removal performance ofthe proposed design.

Several years of field experience demon-strate this integrated method hasadvantages in two areas, according to

34 D R I L L I N G C O N T R A C T O R January/February 2002

Cementing operation key to effective zone isolation“...most of the cementingdesign programs in the indus-try today consider the slurryproperties and not the cementsheath properties.”

IADC/SPE 74497

the authors. First, the display of mud-removal performance in a graphical wayenables better and faster job design.Second, the method achieves fluids withthe required properties in significantlyless time than conventional methodsbecause fewer fluid samples are mixed.

R E M O T E L O C A T I O N S

Innovative solutions for drilling prob-lems can be developed offsite, concur-rently with the drilling operations, forimplementation in remote areas havingno local support.

IADC/SPE alternate paper 74501, “Sup-port Requirements and Innovative Solu-tions for a Remote Location with Diffi-cult Cementing Challenges,” describes aproject in Kazakstan. It was preparedby B M Piot, Schlumberger; K F Lamband E L Biessen, Texaco; and AZhukin, R Schafers and A Ferri,Schlumberger.

Drilling and cementing problems inKazakstan’s remote Magistau areabrought the innovative implementationof a new slurry technology based onengineered particle size distribution,which was designed to solve the prob-lems resulting from gas influx and fluidlosses during cementing.

North Buzachi is an undevelopedonshore field 180 km north of Aktau. Inthe exploratory phase both vertical andhorizontal well technology were effec-tive, and there were no drilling prob-lems. Local technical support for thecementing operations consisted of acoordination engineer in Aktau and lab-oratory capability from 1,000 km away.

The appraisal phase was based on con-ventional procedures used in theexploratory phase. Wells drilled in newparts of the reservoir experienced unan-ticipated fluid losses and gas influx.These problems continued despite con-tinuous improvements devised withavailable materials.

To solve these problems, the servicecompany’s area client support laborato-ry engineered several new designs forremoving mud and controlling lossesand gas influx at the same time.

A solution based on the worst-case sce-nario was used, and a cement programwas designed that improved mudremoval and incorporated a preblendedlow-density slurry, the authors report.

The flexibility of the new systemenabled selection of a very low densitythat would prevent losses in the weakestparts of the reservoir while preventinggas influx. The design also enabled den-sity increases of 1 lbm/gal withoutaffecting slurry properties.

The new slurry was used in the last 4wells of the campaign. Density was var-ied on site without logistical difficultiesor impairment of performance. Theresults from these 4 wells were the bestobtained in the campaign, as shown bythe cement bond logs.

F I L T E R C A K E R E M O V A L

Removal of the filter cake formed duringdrilling is essential for a successfulcementing job.

Today, the use of synthetic base fluidsmakes it important to evaluate the effi-ciency of the washers/spacers in remov-ing filter cake and to guarantee the wet-tability inversion of the formation fromoil to water wet.

IADC/SPE alternate paper 74502, “ANew Method of Evaluating the FilterCake Removal Efficiency,” describes anew method of evaluating the filter cakeremoval performed by different washersusing a core. The process is followed byx-rays and the removal efficiency is cal-culated using x-ray tomography.

This method makes it possible to quan-tify the remaining filter cake comparedto the original filter cake.

The paper was prepared for the Confer-ence by C R Miranda, Petrobras SA; J CLeite, Petrobras Research Center;and R T Lopes and L F de Oliveira,Federal University of Rio de Janeiro.

According to the authors, the processmakes its possible to follow the period ofcontact of the wash with the formation.

It also makes it possible not only toselect the most appropriate wash for agiven drilling fluid, but also to predictthe necessary contact time between thewash and the formation to achieve anappropriate filter cake removal. n

January/February 2002 D R I L L I N G C O N T R A C T O R 35