real time lwd sonic semblance image and the step change inlwd

8/10/2019 Real time LWD Sonic Semblance Image and the Step Change inLWD

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Real-Time LWD Sonic Semblance Image and the Step Change in LWDSonic ApplicationsA Case Study from Latin America (Mexico)Medhat Mickael, SPE, Craig Barnett, SPE, Chris Maranuk, SPE, Rebecca Nye, SPE, Mohamed Diab, SPE,Javier Armando Carreira, Weatherford; Victor Arreola Morales, Erika Nino Batista, and Jose Edilberto Chi, Pemex

Copyright 2013, Society of Petroleum Engineers

This paper was prepared for presentation at the SPE Annual Technical Conference and Exhibition held in New Orleans, Louisiana, USA, 30 September2 October 2013.

This paper was selected for presentation by an SPE program committee following review of information contained in an abstract submitted by the author(s). Contents of the paper have not beenreviewed by the Society of Petroleum Engineers and are subject to correction by the author(s). The material does not necessarily reflect any position of the Society of Petroleum Engineers, itsofficers, or members. Electronic reproduction, distribution, or storage of any part of this paper without the written consent of the Society of Petroleum Engineers is prohibited. Permission toreproduce in print is restricted to an abstract of not more than 300 words; illustrations may not be copied. The abstract must contain conspicuous acknowledgment of SPE copyright.

Abstract

The need for reliable logging-while-drilling (LWD) sonic data in real-time has been of great interest to many operators since

inception. However, providing such information reliably requires many challenging tasks. Unlike wireline tools, where

waveforms are sent and processed on the surface in real-time, the waveforms of an LWD sonic tool cannot be sent in real-

time due to bandwidth limitations. This requires the data to be processed downhole and only the processed data to be sent in

real-time. Traditionally, windows were used to define the range of the real-time compressional velocity. However, a majordrawback to this approach is the requirement of knowing the compressional arrival range beforehand and the possibility of

receiving a different arrival. For example, fast shear could be confused with slow compressional data. Sending a complete

semblance image in real-time is the best way of capturing all arrivals. However, data must be inelegantly compressed tomaintain the arrival times yet reduce the bandwidth to a minimum. A new sonic LWD tool has been developed to provide

high-quality real-time sonic semblance images, creating opportunities for obtaining this information while drilling. Anoffshore case study comparing the real-time sonic images to the recorded memory data in terms of resolution and data quality

will be investigated. The paper will discuss the uses and applications of the sonic semblance image and its real-time value.This proven data acquisition process is crucial in allowing the end user to access all the formation evaluation data, thus

improving real-time decision making.

Technological advances in LWD services have had a direct impact on the ability to evaluate a reservoir and provide

production planning on a foot-by-foot basis. The amount of downhole data processing and data compression required plays a

critical role in delivering real-time sonic data while drilling. High-resolution, real-time semblance images can be obtained inreal-time, and are expanding the uses and applications over traditional delta-t data acquisition.

Operating companies have formed an outdated opinion that real-time LWD sonic data is unreliable and requires post-

processing. With newer technology, high quality semblance images can be provided in real-time and, as a result, there is a

shift in the way operating companies use the data for analysis and planning. The offshore environment has always presented a

difficult challenge for acoustic tools in general; however, with the scientific advances in LWD tools, continuouscompressional and shear arrival from the real-time semblance images can prove useful. This case study will also provide

basic background information on the lithology in the region, highlighting why sonic data in particular was required in real-

time.


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Background

The drilling environment is challenging for most formation evaluation logging sensors but especially for LWD acoustic tools.

Drilling- and flow-induced noise, LWD sensor geometry and size limitations, power requirements, and tool decentralization,

are key issues that deteriorate the quality of the acoustic signals. However, one of the major sources of noise in LWD sonictools is the internal transmitter signal traveling through the tool body. Waves traveling directly through the acoustic tool

collar from the transmitter to the receivers are called tool-mode arrivals, and are unwanted signals that contaminate and mask

the formation arrivals. The more efficient the tool in muting these waves, the higher the quality of the acoustic wavesrefracted from the formation. Fig. 1depicts the acoustic waves and associated noise in an LWD sonic tool.

Another challenge with LWD acoustic tools is the power, frequency, directionality, and ruggedness of the transmitter. The

frequency of an acoustic transmitter is a function of the geometry and dimensions of the piezoelectric material. Wirelinemonopole transmitters typically have a shape of a large ring that is tuned to a frequency between 10 and 20 KHz. The

dimensions of the ring are chosen to produce a high output typically on the order of 3,000-4,000 Pascal of pressure when

measured at 1 m from the transmitter. Ring-shaped transmitters do not lend themselves to the LWD tool geometry. Moreover,these rings are fragile and non-directional.

The last challenge with LWD tools is the sensitivity and directionality of the receiver(s). Sensitive and directional receivers

are needed while drilling to reduce the sensitivity to axial noise (signal transmitted through the collar either from the bit or

from mud flow) and enhance the sensitivity to signals at a right angle from the receiver surface (these are typically indicative

of formation arrivals).

LWD Tool Design and Configuration

The new tool employs a unipole configuration with a single, directionally focused, transmitter and one array of six directional

receivers which are azimuthally aligned with the transmitter (Fig. 2). The inter-receiver spacing is 6 inches, and the spacingfrom the transmitter to the first receiver is 6 feet.

Like traditional monopole sonic tools, this tool measures the velocity of mud head waves associated with compressional and

shear waves refracted through the formation at the critical angle parallel to the borehole. However, unlike radially symmetric

monopole tools, this azimuthally focused sensor is able to differentiate the slowness of refracted compressional and shear

waves from different azimuthal directions around the borehole. This is accomplished by using X-Y magnetometers to trackthe orientation of the transducers as the drill string rotates, and sort the measured waveforms into 16 azimuthal sectors.

Other key elements of the measurement include:

Unique, powerful and directionally focused transmitter that produces 50% higher output than a typical wirelinetransmitter.

Effective isolation section between the transmitter and receivers that reduces the tool mode signal by more than 98%.

Good acoustic isolation of the receivers themselves.

Piezoelectric receiver material that has a very high degree of directional sensitivity.

Data Processing

The downhole tool acquires waveforms in 512 channels from the six receivers and fires the transmitter at an adjustable

repetition rate ranging from 50 to 200 ms. The transmitter can be fired at this rate for 1 to 20 times and the waveforms from

all fires can be stacked to improve data quality. The waveforms can also be stacked as a function of tool face if an azimuthal

measurement is required. The stacked data are used as input to the semblance processing and a semblance projection image iscomputed and saved to memory every 5, 10, or 20 seconds. The compressional slowness (DTC) and shear slowness (DTS)

values can be computed downhole within a certain range or picked from the coherence peaks of the semblance projection

image. Fig. 3shows the downhole sonic data processing technique:

Waveforms are acquired from the six receivers.

The frequency response of the data is determined and a frequency filter is applied to remove low-frequency drilling noiseand high-frequency shock.

The data is filtered and up-sampled from 512 to 2,048 channels.

Time-slowness semblance map is computed with a mud-line collider to reduce processing time and eliminate aliases.


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Semblance projection is computed, then sent to surface and saved to memory. The peaks in the semblance projection plotare the different arrivals received from this sample. In this specific case, a compressional arrival is seen at 72 s/ft and a

shear arrival is seen at 144 s/ft.

There are two options for real-time:

1. Use the traditional gated approach where a slowness window is set on the semblance projection plot and the highestcoherence peak within the window is sent to the surface in real-time. In the example shown, if a window is set between

50 and 90 s/ft, the compressional arrival of 72 s/ft would be sent to the surface.

2. Send the whole semblance projection plot to the surface as a compressed image. Fig. 4shows a log plot of the semblanceimage first measured by the sonic tool downhole (the far left track) versus the image plotted by the surface computers

(the far right track) after compression, transmission and surface decoding. This plot demonstrates the ability of thesystem to transmit and decode at the surface what was actually measured downhole. The compression technique

incorporated in the downhole tool firmware is compatible with the rigs WITS software, allowing the data to be batch-

sent to remote offices as the well is being drilled. Fig. 5shows the compressional and shear slowness that has been

traced along the path of highest coherence on the semblance image.

Case Study Tumut Field, Mexico

The accuracy of reserve estimates is of significant importance for the operator. Pemex, being Mexico's state-owned

petroleum company and the sole supplier of all Mexicos commercial gasoline, means erroneous estimates can have

detrimental economic effects on the country. Consequently, accuracy of the formation evaluation data is of substantialimportance. In the evaluation of the Tumut field the operating company required the most precise, yet economical, way of

acquiring the data needed to evaluate the reserves and design the field development phase. The client also required highly

accurate real-time formation evaluation data for making critical decisions during the drilling process. In order to optimize theformation evaluation program in the Tumut field development, the client decided to embark upon a detailed comparison

study of a variety of data types acquired during the Tumut-2 well evaluation program.

The Tumut field is located on the continental shelf in the Gulf of Mexico, within the territorial waters of Mexico where the

water depth is approximately 35 m (114 ft). The field is situated in the Coatzacoalcos geological province approximately 112

kilometers (70 miles) northeast of Dos Bocas Campeche. Fig. 6shows a map with the Tumut field location. The target

reservoir is found in Upper Jurassic Kimmeridgian deposits shown in Fig. 7. The figure also shows that Tumut-2 is thedeepest well located in the field reaching a maximum depth of 4,374 m (14,350 ft) TVD.

During the drilling phase, the real-time data of particular importance was the compressional slowness. This data was acquired

in real-time for monitoring the pressure gradient and identifying several abnormally pressured zones. Many LWD formationevaluation tools can be utilized in the prediction of pore pressure while drilling, but the sonic is the most sensitive, therefore

when it is accurate, it is the best option. The data was also utilized in updating the seismic well ties; correlating formation

tops to seismic horizons and fine-tuning the seismic model in real-time.

Geological Setting

The structure of the Tumut field is an elongated anticline bounded by two reverse faults in the same direction. The central

part of the anticline is affected by the presence of a salt diapir that divides the structure into two blocks. The seismic line in

Fig. 8shows the location of the Tumut-2 well in relation to the compressive reverse faults that bound the field.

The geological column consists of sediments from the Upper Jurassic Kimmeridgian to recent, with carbonate sediments inthe Lower Cretaceous, and the tertiary being represented by interspersed shales and thin alternating layers of sandstone. The

major source rock is of the Upper Jurassic Tithonian age and consists of organic material in the black bituminous shale and

dark gray shaley limestone with abundant organic matter and broad regional distribution.

The reservoir, of Upper Jurassic Kimmeridgian age, is primarily made up of mesodolomite containing oolites and some sand.

The appearance of anhydrite is infilling some of the hollows caused by dissolution, and thin shales are interspersed in the

limestone, bentonite and sandy mudstone. The quartz sandstone is fine to medium, well classified and with calciticcementation exhibiting considerable primary inter-granular porosity. The micro-fractures are mostly sealed by dolomite,

calcite and oil. Using well logs, cores and channel samples the average porosity of the reservoir is estimated at approximately

6% with a typical water saturation of about 15%. The reservoir contains light oil estimated to be 35 degrees API.

This setting provides an ideal structural trap with both flanks bounded by the reverse faults and the salt diaper. The reservefaults that bound the trap tend to create pressure abnormalities resulting in under and over pressured formations. Real-time

sonic data sensitivity is of critical importance to pore pressure changes for this particular application.


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Data Comparisons

Tumut-2 was an appraisal well with the main objective of fully evaluating the reservoir. An auxiliary objective, while drilling

Tumut-2, was to assess LWD and wireline logging tools in order to optimize the field development logging programs in the

future. The sonic assessment involved two categories of comparisons; real-time LWD vs. recorded LWD sonic data andrecorded LWD vs. wireline data. In this case, the clients most critical comparison was made on the compressional slowness

data; due to the necessity of highly accurate real-time data and the historical accuracy issues affiliated with other LWD

acoustic logging tools.

Real-Time versus Recorded Acoustic Data

Real-time and recorded data were both acquired by LWD tools and compared at the end of each run. When the comparisons

were made it was generally accepted that there would be slight difference in the data but overall they should closely mimic

each other. Due to bandwidth limitations, not all LWD data can be transmitted in real-time; therefore some downhole tool

processing must be done and other data simply recorded for analysis post drilling. Acoustic measurements require the mostsignificant downhole processing given the magnitude of waveform data required to create the final compressional and shear

slowness logs. The new LWD acoustic tool has been designed to transmit reliable semblance images in real-time from which

the compressional and shear slowness can be determined, as they would be from the recorded data. This technology was used

in the Tumut-2 well for the data comparisons.

Fig. 9shows the comparison between the real-time and recorded semblance images, from which the compressional and

refracted shear slowness was traced, in Track 3 and 4. Track 2 shows excellent agreement between the traced slownessarrivals from real-time and memory data, which is further demonstrated by the VP/VS ratio comparison in Track 1.

This type of data comparison gave the operating company confidence in the real-time LWD sonic data provided.

Recorded vs . Wireline Acoustic Data

Wireline acoustic data cannot be used to predict pore pressure in real-time since they are deployed after the well has been

drilled. Moreover, wireline tools are difficult to deploy in deviated and horizontal wells. However, in order to use LWD as a

replacement for wireline, verification of the accuracy and data quality of LWD measurements is required. Wireline acousticlogs as well as LWD acoustic logs were acquired on Tumut-2 and compared.

Fig. 10shows the comparison between the LWD recorded data and the wireline data. Track 3 shows the compressional

slowness from both wireline and LWD. There is a good agreement in the measured slowness and data resolution between the

wireline and LWD logs. Track 2 displays total acoustic porosity logs computed from each data set using the Wyllie time-average equation (Wyllie et al, 1956). Based on this data, the operator decided that they would be able to solely use LWD

acoustic measurements in the development phase of the Tumut field.

Summary and Conclusions

1. The new LWD sonic tool provides a rugged platform for real-time data acquisition comparable to wireline.

2. The design of the tool greatly reduces drilling and tool noises, providing high-fidelity compressional and shear arrivals.

3. Processing techniques allow for delivery of real-time semblance images, which can be used to ensure quality well sitedata delivery

4. The new sonic tool eliminates the need for post-processing allowing the operator to use real-time data for drillingdecisions.

5. The new LWD sonic tool provides confidence to replace wireline, especially in situations where wireline tools cannot beeasily deployed.


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Acknowledgements

We want to thank Pemex Corporation and Weatherford for allow us to present this paper. We also acknowledge the efforts of

several members of the R&D and Operations staff at Weatherford, especially, Craig Barnett for data analysis, Aun Hussain

for software testing, Alanna Pope for firmware development, John Wright and Lorenzo Rodrigues for electronics design, andLance Pate for mechanical design.

References

Talwani, Manik. 2011. The Future of Oil in Mexico Geology, Production Rates, and Reserves. Rice University, April.

Mickael, Medhat, Barnett, Craig, and Diab, Mohamed. 2012. Azimuthally Focused LWD Sonic Logging for Shear Wave

Anisotropy Measurement and Borehole Imaging. Paper SPE 160133, SPE Annual Technical Conference and Exhibition, SanAntonio, Texas, October 8-10.

Mejia, Miguel Camilo, Casanova, Mauricio. 2012. Integration and Formation Evaluation and Geomechanical Properties in

Real-Time for Horizontal Sections. Convocatoria Congreso Mexicano del Petroleo.

Nye, R. 2013. Benefits of Real-Time LWD Sonic Semblance Images Case Studies from the North Sea and Offshore China.

Paper, Offshore Mediterranean Conference and Exhibition, Ravenna, Italy, 20-22 March.

PEMEX Exploration and Production. 1999. Hydrocarbon Reserves of Mexico. Volume 2.

Wyllie, M. R. J., Gregory, A. R., and Gardner, L. W. 1956. Elastic wave velocities in heterogeneous and porous media:Geophysics, 21, no. 1, 41-70.


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Figures

Fig. 1Attenuator sections function is to reduce the effects of tool mode and drilling noise on the received signal.

Fig. 2Configuration of the azimuthally focused LWD unipole sensor.

Green uni-directional

arrows correspond to the

signal transmitted into

the formation.Blue bi-directional arrows

correspond to drilling induced

noise that is reduced whilepassing through theattenuatorsection andis minimized at the

receivers.

Red uni-directional arrows

correspond to tool-mode

waves travelling through the

body of the collar are reduced

while passing through the

attenuator section and are

minimized at the receivers.


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Fig. 3Downhole sonic processing technique.

Fig. 4Far left track shows the down-hole semblance image before compression, while the far right track shows the

decompressed semblance image on surface.


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Fig. 5Compressional and shear slowness that has been traced along the path of highest coherence on the semblance

image.

Fig. 6Tumut field is located off the coast of Mexico, 112km (70 miles) northeast of Dos Bocas Campeche.

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Fig. 7NW-SE seismic line detailing the modeled seismic horizons in the Tumut field; the section also shows the

location of Tumut 1, 2 and 5 wells.

Fig. 8NE-SW seismic line that shows the location of the Tumut-2 well in relation to the compressive reverse faults

that bound the field.

TUMUT2


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Fig. 9Comparison between real-time LWD acoustic data and recorded LWD acoustic data from the Tumut-2 well.


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Fig. 10Comparison between recorded LWD acoustic data and wireline acoustic data; calculated Wyllie sonic

porosities are also compared.

real time lwd sonic semblance image and the step change inlwd

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