SPE 171839-MS

Sourceless Well Placement with Real-Time Acoustic Measurements Omar Al Mutwali, Saeed Dama, Salem Al Jabri, Bader Al Dhafari, Abdul Salam Al-Mansoori, Hassan AbouJmeih, Amr Serry, Sultan Budebes and Sultan Al Hassani, ADMA OPCO; Paul Cooper, Wael Fares, Ramy Essam, and Ahmet Aki, Halliburton

Copyright 2014, Society of Petroleum Engineers This paper was prepared for presentation at the Abu Dhabi International Petroleum Exhibition and Conference held in Abu Dhabi, UAE, 10–13 November 2014. 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 been reviewed 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, its officers, 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 to reproduce 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 Sourceless well logging (logging without using traditional chemical sources) has become more attractive in certain locations

because of evolving government regulatory and HSE requirements. LWD sonic tools provide opportunities to operators for

both well placement and formation evaluation.

For the first time in the UAE, an LWD acoustic tool was added to a bottomhole assembly (BHA) to acquire sourceless

porosity measurements to help stay in the porous sub-layers in the target reservoir. In comparison to more traditional nuclear

measurements, LWD sonic has a superior depth of investigation that is less affected by standoff and a driller-friendly BHA

free from stabilizers.

Based on the evaluation of offset data, which indicated excellent correlation of sonic vs. density/neutron measurements,

we decided to provide shear porosity in real time owing to the sequence stratigraphy and corresponding energy partition. A

post-job comparison with density/neutron data acquired in a wipe run was also conducted to verify the sensitivity of real-time

acoustic porosity measurements for both well-placement and formation-evaluation purposes. Sonic-derived porosities were

found to be instrumental in the real-time decision making needed to keep the well the in higher porous sub-layers.

Current developments, including real-time azimuthal sonic together with considerations of integrating acoustic and NMR

measurements in both petrophysical modeling and field applications, show promise in providing reliable sourceless porosity

estimation in these formations.

This case history delivered a BHA free from radioactive chemical sources. Safe drilling objectives as well as maximized

productivity per unit lateral length were achieved despite the potential risks associated with the faults that were observed in

the pilot hole.

Introduction With a global focus on exploring oil and gas with minimum environmental impact, the regulations in the region are very

strict on adhering to zero harm to the environment. One of the most stringent laws in place is with regards to the

abandonment of radioactive sources in the well in the event of a stuck pipe. ADMA OPCO is an active player in helping

service companies develop and improve on their “sourceless” alternative technologies to reduce potential risks associated

with stuck pipe.

Real-time acoustic measurements were used for well-placement purposes in Field A, which is one of the giant fields

located in offshore Abu Dhabi. Oil was discovered in 1958, and production began in 1962. Down-flank water injection

started in 1973 followed by crestal gas injection in 1994. The Arab reservoir in Field A was formed from regressive cycles of

sedimentation divided into four highly heterogeneous sub-reservoirs (labelled in ascending order from A to D (Fig. 1)). The

principal oil-producing reservoirs are Arab zone C and Arab zone D, whereas Arab zones A and B still remain undeveloped.

Based on the core description, the Arab reservoir section is mainly composed of three lithologies, namely, anhydrite (purple

shading), dolomite (green), and limestone (light blue), as shown below in Fig. 1.

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Fig. 1—Field A – Arab Reservoir Typical Lithology, Petrophysics, and Sequence Stratigraphy, 1:500-ft Scale.

Pre-Job Planning and Well Design The original deviated pilot hole, shown as a black line in Fig. 2, was drilled at a 47° inclination with wireline gamma ray,

density, neutron, and both induction and laterolog wireline resistivity tools. The original hole was side-tracked at 63°

inclination with LWD tools. Azimuthal deep resistivity (ADR) and gamma data were acquired in real time, and another wipe

run was performed with ADR, gamma, azimuthal litho-density (ALD), and compensated thermal neutron (CTN) sensors,

which are shown as a blue line in Fig. 2. After the evaluation of both the original and mother holes, resistivity-forward pre-

job modelling was fine-tuned using StrataSteer® 3-D software, and it was decided that ADR be used in Drain 1 Layer 2 and

azimuthal-focused resistivity be used in Drain 2 Layer 1 to benchmark the suitability of laterolog resistivity, as opposed to

induction-based resistivities for Rt determination across these thin reservoir sub-layers.

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Fig. 2—Well Plan and Design Showing Original Deviated Pilot and Side-Track Mother Holes Together with Two Horizontal Drains.

Two horizontal drain wells were drilled from the mother hole. Drain 1 was drilled in Layer 2 (shown as a red line in Fig.

2), and Drain 2 was drilled in Layer 1 (shown as a green line in Fig. 2).

Acoustic measurements were not available in the original deviated pilot and side-track mother holes in this case-study

well; however, wireline density, neutron, and acoustic logs from two offset wells were analyzed. A preliminary comparison

of estimated porosities across Arab formations and computations for a synthetic density were performed to provide an insight

into the potential use of acoustic measurements for well-placement purposes.

The acoustic porosities using Wyllie, Raymer-Hunt-Gardner, Modified Chapman, and shear porosity transforms were

compared to density porosities. Acoustic porosities were also input to derive synthetic bulk densities, which were then

compared to the measured bulk-density data for offset wells 1 and 2.

The Wyllie compressional porosity, shear porosity, density porosity, synthetic bulk density, and bulk density for both

wells are displayed in Figs. 3 and 4.

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Fig. 3—Density Porosity vs. Wyllie and Shear Porosities and Synthetic Bulk Density Using Wyllie Porosity vs. Measured Bulk

Density Across Arab A, B, C, and D Reservoirs in Offset Well 1.

Figure 4: Density Porosity vs. Wyllie and Shear Porosities and Synthetic Bulk Density Using Wyllie Porosity vs. Measured Bulk Density Across Arab A, B, C, and D Reservoirs in Offset Well 2.

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Real-Time Well Placement of Drain 1 The real-time LWD data was transmitted from the rigsite to the client’s headquarters via satellite technology. The data

integration, processing, and presentation took placed in the Real Time Operating Center (RTOC) for proactive decision

making. The synergy between geologists, petrophysicists, and geoscience subject-matter experts all in one room helped

validate the incoming real-time data.

Shear-slowness (DTS) measurements were used to derive porosity in real time because of the higher energy partition

compared to compressional-slowness (DTC) measurements, as per the offset data. Real-time acoustic processing was

performed using the DTC and DTS semblance picks transmitted, as presented in Fig. 5 below.

Fig. 5—Well Placement of Drain 1 with Real-Time DTC and DTS Semblance Peaks.

The shear porosities are then calculated in real time for well-placement purposes. The shear porosity equation used is:

The shear matrix slowness input was derived from the dolomite, limestone, and anhydrite constituents assigned to the

targeted reservoir sub-layers based on offset petrophysical data. Real-time gamma ray together with velocity ratio

(DTS/DTC) are also used to quality control the real-time data across tight intervals and anhydrite marker beds.

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The comparison of real-time shear porosities to density and neutron logs acquired in a wipe mode is presented in Fig. 6.

Fig. 6—Real-Time Shear Porosity in Comparison to Memory Bulk Density and Neutron Limestone Porosity Measurements Acquired after Drilling in a Wipe Mode.

After drilling the well, a wipe run was recorded using the density neutron and sonic tools. Post-petrophysical processing

was performed to compare the real-time shear porosity to the density-neutron-based formation volumetric interpretation, the

results of which are presented in Fig. 7.

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Fig. 7—Real-Time Shear Porosity Comparisons to Memory Density-Neutron-Based Porosities from Post-Petrophysical Interpretation.

The statistical analysis for the derived effective porosity (PHI) and the real-time shear porosity (DTSPOR) is presented in

Fig. 8.

Fig. 8—Statistical Analysis of Density, Neutron-Recorded Data-Derived Effective Porosity (PHI) vs. Real-Time Shear Porosity

(DTSPOR).

The objective of running acoustic measurements on LWD was to have a porosity indicator while drilling for geosteering

operations. The secondary objective was to validate/compare the calculated acoustic porosity measurements with the neutron

porosity measurements for formation evaluation. After processing and comparing the more reliable memory data for each

tool, it can be seen from the statistical analysis that mean values are in agreement, however, with a variation in the

distribution of the recorded measurements.

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New Developments As discussed, real-time sonic-derived porosities have been successfully used to keep the well in the higher porosity sub-

layers. There are several complementary technologies and techniques that can be used to provide a more complete well-

placement and formation-evaluation solution without the need for chemical nuclear sources in the well.

The shear-slowness measurements used for well placement in this case study were average measurements from around

the borehole. To enhance the utility of sonic information for steering, an azimuthal sonic service, such as the XBAT, can be

used. In contrast to standard LWD sonic tools, an azimuthal tool bins the compressional and shear velocities as the tool

rotates, providing an image of these quantities around the borehole. An azimuthal sonic tool was run in an offshore field to

demonstrate the azimuthal sensitivity of the measurement, along with an azimuthal density tool (ALD) and an azimuthal

resistivity tool (AFR) for comparison. A section of the log from this run is shown below in Fig. 9.

Fig. 9—Azimuthal Sonic Data from Well C in Offshore Abu Dhabi.

It is clear that the wellbore is approaching a faster unit on the high side, as indicated by lighter colors on either side of the

azimuthal compressional and shear-slowness tracks in the middle of the interval displayed, as confirmed by the azimuthal

density and azimuthal high-resolution resistivity images.

As shown in Fig. 8, the real-time shear porosity and nuclear porosity have a similar mean, with some variation in the

overall distribution of data points. While these acoustic measurements allowed for accurate well placement, additional

measurements can be considered for enhanced porosity determination. NMR measurements provide a mineralogy-

independent porosity measurement, which is particularly useful in the complex lithologies found in the Arab reservoir of

Field A.

Work has been done (Serry et al. 2013) to evaluate NMR total porosity as an alternative to porosity derived from nuclear

sensors. A section of a log containing conventional and NMR log data for a well penetrating the Arab sub-reservoirs A, B, C,

and D is shown in Fig. 10.

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Fig. 10—Conventional and NMR Data from Well in Arab Reservoir.

In Fig. 10, the fifth track shows neutron porosity, density, and NMR total porosity. We see good agreement between the

nuclear sensors and the LWD NMR result. Future work will focus on integrating azimuthal acoustic and NMR measurements

to complement each other in an enhanced sourceless porosity determination through both petrophysical modeling and field

applications to provide reliable sourceless porosity estimation in both carbonate and clastic reservoir environments. Conclusions While sonic porosity still has its uncertainties from a formation evaluation stand point, it proved to be a key driver in the

successful well placement for both drains in this case-study well. With the increasing demands on running “sourceless”

alternatives while drilling, sonic measurements could be considered as a viable alternative for replacing density and neutron

porosity measurements for geosteering applications.

The development of azimuthal acoustic measurements, NMR measurements, and further integration with near-real-time

advanced cutting analysis is delivering promising results for a comprehensive formation evaluation solution without the need

for chemical radioactive sources.

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Acknowledgements The authors thank the management of ADNOC, ADMA-OPCO, and Halliburton for their support and encouragement to

publish this work, as well as for reviewing the manuscript and providing helpful comments and suggestions.

References Serry, A.M., Al-Mutwali, O., Al-Mansoori, A.M., et al. 2013. Source-less Porosity and Permeability Estimation with NMR Logs While

Drilling in a Carbonate Reservoir: A Case Study. Paper SPE 13MEOS presented at the SPE Middle East Oil and Gas Show and

Exhibition held in Manama, Bahrain, 10–13 March.

Serry, A.M., Budebes, S.A, Aboujmeih, H., Aki, A. and Bittar, M., 2014. What is Rt? Logging-While-Drilling and Wireline Resistivity

Measurements Spotlighted: An Offshore Case Study. Paper presented at SPWLA 55th Annual Symposium, 18‒22 May.