Advanced Cement Mapping Techniques in Complex Wellbore Environment Aiding InformedWorkover Decisions

Y.D.Kaushik, Sunit Roy, R.S.Chauhan, S.C.Joshi, Oil and Natural Gas Corporation ltd.Rajvardhan Singh*, Vibhor Verma, Ajit Kumar, Schlumberger Asia Services ltd.

Rsingh32@slb.com

Keywords

Flexural Wave, Ultrasonic, Third Interface

Introduction

Cement placement is a critical component of a well’sarchitecture for ensuring mechanical support of the casing,providing protection from fluid corrosion, and, mostimportantly, isolating permeable zones with differentpressure regimes to prevent hydraulic communication.Many cement evaluation techniques based on Sonic andUltrasonic measurements provide a good understanding ofmechanical integrity of casing and cement.

With increasing operational complexities and challengingscenarios like high pore pressure and low fracture pressureformations, application of heavy drilling muds and lightcement slurries respectively becomes imperative.Conventional methods of cement evaluation reach theirlimitations in such environments due to low contrast ofacoustic impedance between cement slurry and annularfluids or cement contamination due to heavy muds.Traditional cement bond log (CBL) and ultrasonic pulse-echo techniques are sometimes used together to diagnosezonal isolation but encounter difficulties when attemptingto evaluate cements with low density and highcontamination of mud. Ambiguity can result because thesetechniques rely on a significant contrast in acousticimpedance between the cement and displaced fluid toidentify solids.

Challenge

It's quite common to make cement squeeze decisions toensure hydraulic isolation of formation zones based onCBL-VDL logs acquired after primary cementingoperation. More than often it was observed that CBL andVDL logs, however reading medium to high and suggestingpoor cement, could not yield a successful slurry squeezeacross affected areas with minimal to no Injectivity.Furthermore, unsuccessful attempts are usually made incasing cut and pull operations owing to dubious CBLmeasurement across sections with barite segregation. Also,during Interpretation of cement quality using ultrasonictechnique, there is always uncertainty associated inevaluation and correct classification of microdebonded

material behind casing as liquid or solid from Ultrasonicmeasurement only.

From cementing operation point of view cement placementin slim hole (5” casing in 6” borehole) is bound to pertainchallenges. The annular volume between the casing andborehole to be filled with cement is quite low and thismight cause improper removal of mud across certainsections by cement, thereby resulting in casing eccentricityand mud contamination of cement during the cementingjob. In order to evaluate the cement bond quality andascertain the efficiency of casing centralizers for propercement placement, a complete cement evaluation is muchneeded.

Flexural wave based Cement mapping

An advanced method of cement evaluation based onFlexural wave propagation was introduced recently. Thetechnique combined the pulse echo measurement with anew ultrasonic technique where an ultrasonic transducerinduces flexural wave in the casing and resulting signal isdetected at two set of separate transducers acting asreceivers. The attenuation calculated between the tworeceivers provided an independent response that was pairedwith the pulse echo measurement to distinctly identifymaterial present immediately behind casing.

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Advanced Cement Mapping – Flexural Wave

Fig.1: Flexural Wave excitation representation and resonancemode of the casing. Solid-Liquid-Gas Map Showing thecorrelation of Flexural Attenuation and Acoustic Impedance.

By measuring radially beyond traditional cement evaluationboundaries, Flexural based technique confirmed zonalisolation, pinpointed any channels in the cement, andensured confident squeeze or no-squeeze decision.Furthermore, analysis provided a better insight to casingeccentricity problems and associated cementing challenges.

Case Studies

Considering the challenges associated with slim holecement placement and heavy mud, Cement mappingthrough flexural measurement was recommended and runfor the First time in ONGC, India in field AB and field CD.

CD, ONGC

In this area, wells are generally drilled with heavy mud(~15 ppg or higher) in order to contain the formationpressure and drill as deep as around 4500m. As a generalpractice, sonic logs apart from basic logs are acquired toevaluate the hole size consistencies and stress regime in thewells in order to produce efficiently after placement ofsuitable completions and cementation across different holesections.

Well CD-1 was drilled with 15.7 ppg KCL-PHPA-Polyolmud and completed with 5-1/2”, 20 ppf casing. Due toheavy nature of mud within and outside the annulus (MUDAI~2.99 MRayl), it was critical to map and differentiate theliquids from solids in the annulus and also remove theuncertainties related to fluid identification coming fromstandalone CBL or Ultrasonic measurements. A SLG plotwas created in order to locate the flexural and AI datapoints to characterize the material behind casing. SLG plotclearly identified high density points lying in the solidregion with lesser density of liquid filled region, hence

suggesting prominent presence of cement. HydraulicCommunication map indicated no possibility of fluid/solidmovement or injectivity behind casing, hence bringing theconfidence that injectivity may not be found during anyworkover operation. However, cement squeeze decisionwas taken to validate the results from two measurements.No cement injectivity was observed during squeeze acrossthe zone of interest.

11th Biennial International Conference & Exposition

Advanced Cement Mapping – Flexural Wave

Fig.2: Flexural and pulse echo technique combination - Solid-Liquid-Gas Map Showing the Distribution of Flexural andAcoustic Impedance Points across the Log Interval. Third Interfaceand Annulus Material 3-D Visualization is also observed.

AB, ONGC

The well AB was drilled with 12.8 ppg WBM mud andcompleted with 5”, 18 ppf casing. Due to existing slim holeconditions and heavy nature of mud within and outside theannulus (MUD AI~2.3 MRayl), it was imperative toeliminate the uncertainties associated with correctclassification of microdebonded material behind casingsimultaneously evaluating 5” casing eccentricity if any.High confidence data was acquired against zone of interestto validate and provide information about cement behindcasing. Advanced Solid Liquid Gas map showed high solidfraction across the zone of interest while indicating patchesof liquid across few sections. Hydraulic communicationplot indicates no possibility of liquid channel behindcasing. Overcoming Sonic and Ultrasonic limitations,Flexural measurement could clearly distinguish materialsbehind casing. Third Interface Echo observed at few placeshelped analyzing casing eccentricity across the dual casingsections.

Fig.3: Isolation Scanner* - Borehole Cross-section depicting innerand outer casing and amount of eccentricity observed between twocasing

Fig.4: Flexural based -Cement Evaluation Composite Plot (SingleCasing) – Microdebonded image from Ultrasonic measurement isclearly identified as liquid in the SLG Map. 1: Third interfacewaveform Along the 30m Log Interval; 2: Waveforms WrappedOpen at a certain azimuth along 5 sampling intervals; 3: BoreholeCross-section depicting inner and outer casing; 4: Echo Waveformshowing First and Third Interface Echo

Conclusion

Successful ultrasonic cement imaging and third interfaceecho analysis was performed in slim hole condition andhighly attenuative environments where conventionalmethods are limited to ambiguous CBL-VDLmeasurements.

Advanced cement mapping clearly distinguished micro-debonded cement as low AI solids or liquids which is alimitation in Ultrasonic measurements. (For all casingsizes).

Third interface echo analysis provided a better insight tocasing eccentralization and cement placement tounderstand the challenges associated with goodcementing job. (Also recommends cut point depth forcasing, if any)

References

Smaine Zeroug and Benoit Froelich, “Ultrasonic Leaky-lamb Wave Imaging Through a Highly ContrastingLayer”, Internal SLB Document.

Al-khalifah, T. and Tsvankin, I., 1995, Velocity analysisfor transversely isotropic media; Geophysics, 60, 150-1556.

Backus, G. E., 1962, Long-wave elastic anisotropyproduced by horizontal layering; J. Geophysical Res., 67,4427-4440.

AcknowledgementAuthors are thankful to ONGC Mumbai Bassein Asset andONGC Tripura Asset for timely support and coordination.

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