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Reservoir Geophysics in Brazilian Pre-Salt Oilfields Paulo Johann, Alvaro Martini, Alexandre Maul e João Paulo Nunes/Petrobras Copyright 2012, Offshore Technology Conference This paper was prepared for presentation at the Offshore Technology Conference held in Houston, Texas, USA, 30 April--3 May 2012. This paper was selected for presentation by an OTC 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 Offshore Technology Conference and are subject to correction by the author(s). The material does not necessarily reflect any position of the Offshore Technology Conference, its officers, or members. Electronic reproduction, distribution, or storage of any part of this paper without the written consent of the Offshore Technology Conference 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 OTC copyright.

Abstract This paper focuses on the impact that reservoir geophysics has had on the production development of the Brazilian marine carbonate pre-salt fields starting from the first oil discovery in 2005 to 2011. The evolution of three main knowledge areas of reservoir geophysics technology will be detailed, namely: acquisition, processing and interpretation, all oriented towards reservoir characterization and monitoring. Seismic acquisition technology has experienced an increase in “information density” (seismic traces per square kilometer). In the seismic processing domain, the improvement of seismic algorithms and methodologies has allowed for better seismic data quality, resolution and imaging. In particular, the algorithms/techniques of 3-D multiple suppression and 3-D depth migration have significantly evolved in recent years. In the seismic interpretation area, geological context-oriented seismic attributes algorithms/methodologies have made possible better reservoir characterization in the deep and ultra deep-water Brazilian offshore basins. Introduction Petrobras is one of the world’s leading companies in deep and ultra-deep water exploration and production. The company currently operates approximately 25% of the ultra-deep water production facilities in the world, enjoying global recognition for its vast and unmatched expertise in this area. Within this context, which still poses enormous scientific and technological challenges, we saw the discovery of the microbial carbonate rocks province of the Brazilian pre-salt play, which is already averaging a daily production in excess of 100,000 b/d and which can potentially double or even triple the present Brazilian reserves level of 15 billion barrels, representing, at this moment, a reserves-to-production ratio of at least 18 years. The Brazilian microbial carbonate reservoirs province which is presently being put in production offers rare opportunities for the Brazilian technological development and for national and international companies that are involved in thematic areas, especially in subsurface sciences and technologies such as geophysics, geology and reservoir engineering. Given the magnitude of the occurrence of these oil reservoirs - ranging from the Santos sedimentary basin, at the south, extending north through the Campos basin to reach the Espirito Santo basin, about 200 km wide by 800 km long, comprising an area of approximately 149,000 km2 (Figure 1) - substantial financial investments will be required to allow exploitation of these deposits, especially with the drilling of wells and the installation of production facilities at ultra-deep water environment and buried reservoir rocks, more than 5,000 m below sea level. The scientific and technological opportunities are immense, starting with the improvement of the geological knowledge of these microbial carbonate rocks, their reservoir characteristics and their fluids types. This knowledge will be improved over the years by the drilling of new wells, collection of core samples, performance of geophysical logging and formation testing in these wells in addition to long-duration production tests (LDT), and analysis of the data obtained from the pilot and final production systems. Particularly in the area of reservoir geophysics, opportunities are associated with improved geophysical characterization of these microbial carbonate reservoirs, which are located at depths greater than 5,000 m below sea level, and their future potential geophysical monitoring, after the onset of secondary recovery projects such as water and/or gas injections. The discovery and production of this province of microbial carbonate reservoirs in the Brazilian pre-salt is impacting the Brazilian oil industry as a whole, including both, large and small companies, and also international ones which, directly or indirectly, work in Brazil. Not only the industry will have to respond, but also research centers and universities throughout the country will need to be strategically integrated together with the Brazilian government to promote the necessary industrial revolution that will materialize the production of these findings.

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In order to meet this challenge, the technological system will have to seek effective, secure and environmentally sustainable technologies. In addition to the technologies it will be necessary to train human resources in different branches of science and technology that support the oil industry. To that end, PROMINP (Programme to Promote the National Oil and Natural Gas Industry) was created and indicated that by 2013 Brazil would need to qualify 207,000 professionals for the oil industry, of which about 15,000 would have to be engineers. An innovative supply chain, strongly anchored in R&D projects, is being developed around the exploration and production of the microbial carbonate rocks of the Brazilian pre-salt deposits. This new province of the Brazilian pre-salt inspired the creation of a business cluster for E&P research in Rio de Janeiro, called the “Technology Park of the Fundão Island”, where the Federal University of Rio de Janeiro and the Petrobras Research Center (CENPES) jointly act as anchors for R&D projects in partnership with companies who are installing their research centers such as Schlumberger, Baker, Halliburton, FMC and others. In addition to this technology park, there are a hundred Brazilian universities with research projects in the oil and gas area in different thematic networks. These universities offer graduate courses in geology, geophysics and petroleum engineering to train professionals in Brazil.

Reservoir Geophysics Reservoir geophysics can be characterized by a variety of technical activities from feasibility studies for new acquisitions, data acquisition, processing and interpretation of geophysical data aimed at the characterization and monitoring of oil and gas reservoirs up until the making of decisions based on the knowledge derived from these geophysical interpretations. This work will present the methodological sequence of the various activities that comprise Reservoir Geophysics. These activities can be ordered as follows: technical feasibility studies of new data acquisitions, reservoir-oriented data acquisitions, data processing, data inversions, seismic attributes and facies analyses, special data processing, interpretation of the geometry and architecture of geological bodies, structural interpretations and studies for geophysical monitoring. In addition, it will present some technological innovation studies which are being conducted through internal research and/or in partnerships with companies that operate in the Brazilian market. In each case we can illustrate the progresses that have been attained for the improvement of geophysical characterization and future monitoring of the pre-salt reservoirs in Brazil.

The Microbial Reservoirs of the Brazilian Pre-Salt The microbial carbonate reservoir rocks in the fields discovered so far are found at a total depth of some 5,000 - 6,500 m below sea level, which includes some 1,900-2,400 m of water and an additional 2,000 m layer with different types of saliferous rocks. These rocks have porosities ranging between 9 and 12%, and average permeability of 100 mD with variations between different wells and oil fields within the pre-salt province. These microbial carbonate reservoirs introduce multiple challenges from the point of view of geophysical characterization since the geological context varies in terms of the shapes of both the overburden and the saliferous rocks which follow these microbial deposits. In some of these discoveries, a 2,000m thick autochthon salt layer shows little halokinetic movement. In other instances, the deposits found are beneath allochthonous salt layers, meaning intense halokinetic activity, what poses considerable technological challenges to the imaging of these microbial rocks below these salt layers due to the distortions inherent to seismic wavefront propagation (Figure 2). Another major technical challenge for seismic imaging is the presence of seismic multiples between layers (interbeded multiples), given the seismic contrast between the clastic rocks and the flanks of the saliferous rocks. These seismic multiples superimpose themselves on the pre-salt seismic horizons causing major distortions in their imaging (Figure 3).

Feasibility Study for Data Acquisition The investment for acquiring new reservoir geophysical data is preceded by technical and economic analyses in order to determine the relevance and potential impact of this new information in project decision making. Normally, the oil fields are discovered with the use of geophysical data which are inadequate for the reservoir characterization and monitoring objectives. The ignorance about the existence of the oil field in the exploratory phase of the sedimentary basin and the large areal extent of the basins under investigation do not allow higher investments to be made in acquiring expensive high density data at this stage of the exploratory effort. In this context, soon after the discovery of a new deposit, depending on its size and the anticipated return on the investments required for the projects, it is recommended that technical and economic studies be immediately launched to define the feasibility of acquiring new geophysical data oriented towards the characterization and future monitoring of the reservoirs that comprise the new deposit. In the case of the Brazilian pre-salt, such technical and economic feasibility studies for the acquisition of new data have been customized to match the specific characteristics of each project, namely, its dimensions, its geological complexities and its inherent operational difficulties (Johann et al, 2011).

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These studies have potentially considered EM acquisitions, borehole geophysics and, especially seismic in its many forms, including high-density, multi-azimuth, circular acquisitions, and also ocean bottom nodes (OBN) or cables (OBC), etc.. all depending on the stage and the geological complexity of the deposit being investigated. In recent years, these technical feasibility studies have experienced significant evolution especially in the geophysical modeling area aimed at defining the best method, technique, data-acquisition directions and all the related parameterization. This evolution originates from dramatic advances in computational resources and new algorithms. It is becoming routine the practice of geophysical modeling studies, data pre-acquisition, 3D modeling with the use of algorithms which include 3D seismic wave equation modeling, with seismic velocities volumes including algorithms with seismic tomography for velocities which take into account anisotropies in some of their versions. The technical and economical feasibility studies for new geophysical acquisitions are considered the beginning of what can be termed the geophysical cycle, which comprises several technical activities starting with these initial studies and ending in the decision making phase of the subsurface projects with the support of the interpretation of these geophysical data. This geophysical cycle, measured in time, unfortunately is still very long, taking many months, in some cases, years, thus reducing the value of the geophysical projects from the viewpoint of the production development area or even of the field production area itself. Figure 4 show examples of modeling for high density and circular geophysical data acquisition in the Santos basin pre-salt area during seismic evaluation and design phase (illumination study).

Reservoir- Characterization-Oriented Data Acquisition The geophysical data acquisitions campaigns that have been made for the Brazilian pre-salt includes borehole geophysics acquisitions - walkaway VSP type, radial and 3D - high-density seismic (Figure 5), circular seismic and ocean bottom cable seismic.

Data Processing for Reservoir Characterization The processing of the geophysical data is an extremely important step due to the fact that the geophysical industry has experienced a dramatic progress with the advent of new algorithms and the availability of increasingly powerful computational resources. Figure 6 below shows an example of improvement of the seismic data applying anisotropic seismic migration algorithms, TTI type (tilted transversely isotropic), where the velocity model construction was performed with the use of borehole geophysical data information in addition to the key horizons of the processed area. Also, it will be important to develop algorithms for the suppression of internal multiples in layers adjacent to the flank of the saliferous rocks of the Brazilian pre-salt (Figure 3). Given the complex geology of the saliferous rocks above the microbial carbonate reservoirs of the Brazilian pre-salt deposits, efforts to develop and apply innovative seismic deep imaging techniques are currently under way and include the use of different algorithms and different parameterizations for pre-stack migration techniques like Kirchoff, Gaussian Beam, RTM, Wave Equation, etc., noting that in each new project we have made improvements in the definition of the reservoir-characterization-oriented seismic image. This is one of the very promising knowledge areas for the years ahead which will help enhance the geophysical characterization of the microbial carbonate reservoirs of the Brazilian pre-salt deposits.

Data Inversion for Reservoir Characterization The data used for the discovery of oil fields during the exploratory phase of the sedimentary basins are mostly seismic data in amplitude. These data show some distortions when seeking detailed characterization of the geological layers of the reservoirs in their production development phase and, later, during their production phase. These imperfections are associated with constructive and destructive interferences of the seismic signal as a result of the variations of the geologic layers that comprise the reservoirs, such as their thicknesses, different types of intercalated lithologies, different types of fluids, etc.. To minimize the effect of these distortions in the geophysical images we use geophysical data inversion techniques, which transform the seismic data on amplitude to seismic impedance data. These impedances can be acoustic and/or elastic depending on the objectives and, particularly, on situations where the elastic impedances really add value towards the geophysical characterization of the reservoirs. Figures 7 to 9 below feature examples of inversions of geophysical data for the pre-salt reservoirs of Brazil.

Analysis of Attributes and Seismic Facies The attribute analysis and seismic facies analysis phase is one of the crucial steps for the interpretation of geophysical data. Prior to the analysis of attributes it is necessary to critically review the methodology of extraction of these attributes, as well as the selection of which attributes could in fact add value to the process of geophysical characterization of the reservoirs, whether stratigraphic or structural. The geophysical industry offers the interpreters numerous technologies and methodologies to help them investigate which attributes and/or facies are best suited to achieve the understanding of the geological features which characterize the reservoirs being studied.

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In this area of seismic attributes, studies have been made for increasing the use of the pre-stack attributes, potentially revealing other aspects of the seismic data during the geological interpretation of these data and before their filtering by the stacking operation. Figures 10 to 12 characterize different types of seismic attributes and facies applied to the Brazilian pre-salt deposits.

Special Processing All seismic data processing activities in addition to and beyond the standard time or depth seismic migrations are classified as “special geophysical processing”. These processing activities may be complementary to the most classic flows, such as the suppression of multiples using innovative strategies, or the conducting of seismic inversions or alternative processing flows for the extraction of differentiated seismic attributes. Multi-component data processing with the aim of characterizing preferential directions of faults and, in specific cases, of fracture corridors have been used in some fields of the pre-salt deposits.

Stratigraphic and Structural Interpretations The interpretation of geophysical data for the purpose of characterization of structural and stratigraphic elements of the geology of carbonate reservoirs, is a challenge in the oil industry, especially in the case of this set of microbial carbonate rocks, due to the fact that they were recently discovered and show inherent peculiarities, and also for being geographically distributed over an immense area spread out in three sedimentary basins. The geological stratigraphic interpretations have been technically supported by seismic attributes and seismic facies analyses and, as mentioned in the previous section, by adapted or customized technologies and methodologies for use in these new microbial carbonate reservoirs of the Brazilian pre-salt. Differently from the turbidite reservoirs dominant in other Brazilian oil fields, these carbonate reservoirs have, on average, lower permeabilities, causing their recovery factors to be low during their production phases. The characterization of faults and/or geological fracture corridors for these carbonate reservoirs becomes of great importance both for the pursuit of increased recovery factors and to improve the management of water injection, which is the dominant secondary recovery method in Brazilian reservoirs. The surface seismic data with depth migration and extraction of attributes focused on the structural mapping has been the conventional path for seismic structural characterization of the deposits. In addition to these routine procedures, new technologies and methodologies have been sought for the extraction of structural geological features, such as the use of pre-stack seismic attributes. Walkaway as well as 3-D VSP images were also used as support for these structural interpretations in the pre-salt of Brazil. Figure 13 show an example of attributes for the structural interpretations for the pre-salt reservoirs of Brazil.

Studies for Seismic Monitoring Studies for seismic monitoring in carbonate rocks have defied the geophysical industry because of the difficulty of obtaining time-lapse seismic signal in reservoirs with high incompressibility. Geophysical modeling studies which have been conducted seeking the oil-water and oil-gas fluid substitution have shown low impedance contrast values, particularly in the first case. In the case of the microbial carbonate reservoirs of the Brazilian pre-salt, given their more favorable porosity and permeability characteristics as compared to more traditional carbonate reservoirs, and the fact that the injection of gas is the secondary recovery method intended for use, it is estimated that the geophysical monitoring of these reservoirs have potential for application. To date, the 4-D geophysical modeling have pointed to moderate values of impedance contrasts before and after the oil-gas substitution, thus microbial carbonate reservoirs will require a practical field experiment, a pilot seismic acquisition and processing oriented towards 4-D signal before carrying out a larger acquisition and processing campaign to cover the entire area under investigation.

Technological Innovations Among the many geophysical technological innovations applied to the reservoirs of the pre-salt we can mention the data acquisition campaigns which include VSPs, circular acquisition, multi-component seismic, as well as research efforts in geophysical modeling aimed at the definition of best techniques for future seismic acquisitions in situations of allochthonous saliferous layers with great halokinetic movement, which distort the propagation of the seismic waves in the imaging of the microbial carbonate reservoirs deposited below these layers of salt. Research has been conducted in the development of seismic attributes focused on the extraction of geological stratigraphic and structural features (Figure 11). In the area of seismic data inversion, research will be carried on with recently acquired data originated from high-density surveys, 3D VSPs, circular acquisitions and multi-component surveys.

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Conclusion The geophysics for reservoir characterization and monitoring of the microbial carbonate reservoirs of the Brazilian pre-salt is still in its early stages, because these discoveries are very recent - all dated from the last five years - and the number of drilled wells drilled - less than 50 - is still too small, considering the areal extension of these reservoirs of approximately 149,000 km2, divided into three sedimentary basins along the southeastern coast of Brazil. Furthermore, these microbial carbonate rocks are little known to the world’s oil industry, what poses great challenges for the community of Brazilian and foreign-company geologists who are working with these reservoirs. This geological complexity is not only related to the permoporous and facies distribution characteristics of these reservoirs, but also to their structural complexities, their relief characterization, faults and geological fractures. The seismic imaging of these reservoirs presents serious difficulties since they are subjacent to rather thick (up to 2,000 m) geological saliferous deposits with varying relief due the halokinetic movements. In this situation, the presently available multiple supression and seismic migration algorithms are unable to correctly image the structural and stratigraphic parts without having the amplitudes affected by distortions in the seismic waves paths. New reservoir-oriented seismic imaging technologies, methodologies and algorithms, associated with new seismic data acquisition technologies will have to be developed by the industry in the years to come to supply the needs for reservoir characterization and monitoring of these microbial carbonate reservoirs. The big challenge will be to keep continuously improving the seismic imaging in parallel with the implementation of high-investment projects, some in the production development phase, others undergoing long-duration production tests, some running production pilots and others already operating permanent production systems. These microbial carbonate reservoirs already represent 5% of the Brazilian production. The geophysical technology, being applied since the inception of the exploratory phase, following during the production development phase of the discoveries until the final production stage of fields which were informed as “commercially viable” to the Brazilian government regulatory body, is the main tool used by geophysicists, geologists and reservoir engineers to reduce subsurface uncertainties, allowing them to make decisions with greater technical support, thus optimizing the return on the financial investment in projects of this nature and with such challenging features: very recent findings, offshore areas in ultra deep waters (> 2,000 m); reservoirs located at great depths below sea level (> 5,000 m) and characterized by rocks about which the oil industry still has insufficient knowledge. But in fact, this huge technical challenge is exactly the great motivator for the geosciences and petroleum engineering professionals in Brazil, due to its strategic importance to a country which now ranks as the world’s sixth largest economy but still has many serious social disparities to overcome.

Acknowledgment The authors thank Petrobras the permission to publish this paper. The cooperation of technical reservoir geophysicist teams of E&P pre-salt province is also appreciated. Reference 1. Azevedo, J.S. G., Brazil: The Next Oil Giant?, Oxford (2011). 2. Johann, P. et al, Advanced Seismic Imaging Impacting Brazilian Offshore Fields Development, OTC 21934 (2011).

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Figure 1 - The pre-salt province, from the Santos sedimentary basin, at the south, extending north through the Campos basin to reach the Espirito Santo basin, comprising an area of approximately 149,000 km2 (Gabrielli de Azevedo, 2011).

Figure 2 - Thick autochthonous salt layer shows little halokinetic movement (left part). In other instances, the deposits found are beneath allochthonous salt layers, meaning intense halokinetic activity, what poses considerable technological challenges to the imaging of these microbial rocks below these salt layers due to the distortions inherent to seismic wavefront propagation.

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Figure 3 – Intrabed seismic multiple from salt layer flancs superimposed on the horizons of the pre-salt reservoirs (left) and horizon slice with seismic circular artefacts due the intrabed multiples (right).

Figure 4 – Seismic evaluation and design of circular acquisition: (a) hit map pre-plot and (b) hit map pos-plot from illumination study.

Figure 5 – High density 3D seismic acquisition with Ramford Sovereign vessel from PGS, with 14 cables of 8 km each one, 50 m between seismic cables. Since 2009 this vessel acquired more then 6,000 km2 of seismic data over pre-salt carbonates reservoirs province.

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Figure 6 - Depth seismic images from pre-salt province: seismic cable with 6 km and 2009 seismic processing (left); seismic cable with 8 km and 2011 seismic processing (data processing from CGGVeritas Brazil).

Figure 7 - Acoustic impedance section with simulated anneling inversion methodology for pre-salt carbonate reservoirs province. Note the excellent external and internal geometry delineated by acoustic impedance (blue/yellow contrast).

Figure 8 - Elastic impedance from pre-salt carbonate reservoirs province. Note the internal elastic seismic facies variations (dark red/orange/green contrast).

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Figure 9 - P and S-velocity ratio section from pre-salt carbonate reservoirs province.

Figure 10 - Seismic amplitude derived attribute to characterize potential microbial construction applied to the Brazilian pre-salt deposits.

Figure 11 - Seismic horizon slice amplitude derived attribute to characterize potential microbial construction applied to the Brazilian pre-salt deposits.

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Figure 12 - Seismic facies from neural network self organization map methodology applied to reservoir characterization of microbial carbonate reservoir pre-salt deposits. Note that better seismic facies are very well correlated with well placement.

Figure 13 - An example of structural interpretation for carbonate microbial pre-salt reservoirs: surface seismic (left) and walkaway VSP (right).