PETROPHYSICS, VOL. 56, NO. 6 (DECEMBER 2015); PAGE 577–591; 16 FIGURES; 1 TABLE
Presalt Carbonate Evaluation for Santos Basin, Offshore Brazil Austin Boyd1, Andre Souza1, Giovanna Carneiro1, Vinicius Machado2, Willian Trevizan2, Bernardo Santos2, Paulo Netto3, Rodrigo Bagueira4, Ralf Polinski5, and Andre Bertolini5
ABSTRACT The Presalt carbonate wells of Lula Field, Santos Basin, Offshore Brazil, are currently producing high quality, 28 to 30° API oil at an average rate of 30,000 BOPD. With reservoir pressures over 8,000 psi, and a downhole oil viscosity of 1 cP, the proli¿c Àow rates from these high permeability, lacustrine carbonates have shown no signi¿cant decline in over ¿ve years of production. These heterogeneous, layered carbonates with variable reservoir quality typically have oil columns greater than 200 m and one of the key challenges is to identify the highpermeability intervals to optimize the completion strategy. Since the ¿rst discovery well in Lula ¿eld in 2007, NMR logs have been extensively used to aid in identifying the high-permeability intervals. Laboratory NMR experiments have been performed by Petrobras
on Presalt core samples, oil samples and oil-based mud ¿ltrate at downhole conditions to better understand the NMR response in these oil-wet carbonates. The laboratory measurements have been valuable for understanding the effects of varying wettability and varying surface relaxivity due to the presence of heavy minerals, and how each can affect the NMR T2 response. Complementing the NMR analysis, acoustic rock physics and new algorithms for quantifying vuggy porosity from ultrasonic image logs are now used to aid in identifying high-permeability zones. Finally, advanced formation-tester analysis is used to analyze both horizontal and vertical permeability over larger intervals to aid in upscaling the formation properties for reservoir simulation.
INTRODUCTION The initial discovery well for Lula ¿eld, RJS-628, was drilled in 2005, and ¿eld production started in 2010. There are currently 15 producing wells in the ¿eld and production is at 450,000 BOPD; including associated gas, production is averaging 30,000 BOEPD/well. These tremendous Àow rates come primarily from vuggy carbonates, which can occur in a variety of lacustrine facies, such as coquinas, stromatolites and travertine shrubs, as well as reworked deposits. Porosity averages 12% but can exceed 25%, with permeabilities ranging from 50 mD to >1 Darcy (Pereira et al., 2013). Such porosity preservation is remarkable considering the depth of these carbonates (4.5 to 6 km), where the median porosity (P50) would be expected to be only 5 p.u. (Ehrenberg et al., 2009). This enhancement of reservoir quality is likely aided by diagenesis from
Fig. 1—Lula Field average daily oil production from January 2010 to September 2015 (ANP, 2015).
geothermal convection which can result in sweet spots with up to 10% increase in porosity and one order of magnitude
Manuscript received by the Editor October 2, 2015; revised manuscript received November 24, 2015. 1 Schlumberger Brazil Research and Geoengineering Center, Rua Paulo Emidio Barbosa, 485, Quadra 7B - Parque Tecnológico do Rio, Ilha da Cidade Universitária, Rio de Janeiro, RJ, Brazil 21941-907; [email protected]
; [email protected]
com; [email protected]
2 Petrobras, Centro de Pesquisa e Desenvolvimento Leopoldo Américo Miguêz de MelloAv. Horácio de Macedo no. 950, Ilha do Fundão, Rio de Janeiro, RJ, Brazil, 20031-170; [email protected]
; [email protected]
; bernardo. [email protected]
3 Petrobras, Av. Republica do Chile, 330, 9th Àoor, Centro, Rio De Janeiro, RJ, Brazil, 20031-170; [email protected]
4 Instituto de Química – Universidade Federal Fluminense (Chemistry Institute – Fluminense Federal University) Outeiro de São João Batista, s/nº, Campus do Valonguinho, Centro – Niterói – RJ – Brasil, 24020-141; [email protected]
5 Schlumberger Serviços de Petróleo LTDA, Av. Presidente Wilson, 231, Rio de Janeiro, Brazil, 20030-012; [email protected]
slb.com; [email protected]
Presalt Carbonate Evaluation for Santos Basin, Offshore Brazil
increase in permeability (Jones and Xiao, 2013). One of the major challenges in terms of formation evaluation is to identify the high-permeability intervals and which wells are capable of producing at high Àow rates. In Fig. 2, a stromatolite core-slab photo from a Presalt well clearly shows the high quantity of vuggy porosity.
Fig. 2—Core-slab photo of microbialites/stromatolites from Lula Field, Santos Basin (after Formigli et al., 2009).
GEOLOGY AND DEPOSITIONAL ENVIRONMENT The Presalt reservoirs offshore Brasil were formed between 123 and 113 Ma during the breakup of the supercontinent Gondwana (Cainelle and Mohriak, 1999). During the rift phase, the Itapema organic-rich source rock was deposited as a mudstone followed by coquinas and then transitioned into microbial carbonates and travertine during the Sag phase. These carbonates were capped by an anhydrite layer 30 to 50 m thick and then overlain with up to 2 km of halite and other salt minerals providing a regional seal. This huge evaporitic sequence was able to hold signi¿cant hydrocarbon column heights, which was a key factor to the establishment of one of the most proli¿c petroleum systems
in the world. The high thermal conductivity of the salt section has maintained a relatively low reservoir temperature (40 to 100°F), considering the burial depth, which has minimized thermal degradation of the Presalt oil (Melo et al., 2011). Oil viscosity is about 1 cP and ranges from 28 to 30° API with GOR varying from 140 to 240 m3/m3. Carbon dioxide content ranges from 8 to 18% of the associated gas (Formigli et al., 2009). Based on isotope studies, the CO2 is primarily due to two sources, (a) either originating in the deep mantle and migrating upwards through deep faults or igneous intrusions, or (b) derived from organic matter associated with microbial or diagenetic processes (Santos Neto et al., 2012). Lacustrine carbonates are formed in lakes and the type of deposition is sensitive to salinity, pH, water level, topography and sediment supply variations (Corbett and Borghi, 2013; Riding, 2000). They have a wide range of porosity and pore types and diagenesis is considered to have a signi¿cant control on reservoir quality in addition to depositional texture (Jones, 2013). The complex mineralogy found in the Presalt carbonates is primarily composed of limestone, dolomite and silica with varying amounts of magnesiumrich clays, such as stevensite, and heavy minerals, such as pyrite. In addition to geothermal convection, these minerals can be associated with varying lake-bottom water chemistry during early deposition (Dorobek et al., 2012; Cazier et al., 2013), and isotope studies of core material are ongoing to evaluate which process predominates. Nonreservoir-quality volcanics are occasionally encountered and have been analyzed using the TAS chart system (Li et al., 2008) but the results have not yet been veri¿ed with core data. In addition to lake water chemistry, the water level and lake topography played a signi¿cant role in the carbonate facies development, with three main types encountered in the sag phase. When water level was moderate to shallow, deposition was primarily in-situ constructions made up of microbialites and stromatolites, which would be considered boundstones using the Dunham classi¿cation system (Terra et al., 2010; Dunham, 1962). Shallower water levels and subaerial exposure created conditions for reworked grainstones, while massive laminates are associated with deeper water levels (Dos Santos et al., 2013; Alabi et al., 2013). Coquinas were deposited during the rift phase, when waters were fresh to brackish, and are considered grainstones to rudstones when using the Dunham system, and can have excellent reservoir quality. Figures 3 to 5 illustrate paleogeographic reconstructions of the opening of the South Atlantic during this process, with the Santos basin outlined in white in Fig. 5.
Boyd et al.
Figure 6 shows the South Atlantic opening in terms of the major depositional sequences for each phase.
Fig. 3—Paleogeographic reconstruction, prerift phase, 152 Ma (after Petersohn and Abelha, 2013).
Fig. 4—Paleogeographic reconstruction, rift phase, 122 Ma (after Petersohn and Abelha, 2013).
Fig. 5— Paleogeographic reconstruction, drift phase, 108 Ma. (after Petersohn and Abelha, 2013).
Fig. 6—Schematic geodynamic model showing the opening of the South Atlantic and creation of the salt basins from 170 Ma to present: (a) 170 Ma prerift, eolian sediments; (b) 130 Ma rift, lacustrine, organic-rich shales (source rock), coquinas, conglomerates and volcanics; (c) 120 Ma transitional, microbial carbonates followed by evaporates; (d) 105 Ma initial drift, post-salt platform carbonate sedimentation; (e) present late-drift, clastic turbidite sedimentation (after Cainelli and Mohriak, 1999).
Key Reservoir Facies Terra et al. (2010) described an extensive variety of carbonate facies found in Brazilian basins and classi¿ed them according to existing classi¿cations, such as the Dunham (1962) system and its later modi¿cations by Embry and Klovan (1971). Terra et al. (2010) further adapted or modi¿ed some terms by introducing new names for lacustrine facies not previously described in detail. From this extensive list of facies, Corbett and Borghi (2013) then identi¿ed three key facies in lacustrine carbonates that typically have the
Presalt Carbonate Evaluation for Santos Basin, Offshore Brazil
best reservoir properties tufa and travertine, stromatolites and coquinas. Tufa and Travertine. This facies can have a wide range of layering and geometry and extensive vuggy porosity (Fig. 7). Tufa is associated with geothermal spring waters entering alkaline lakes and precipitating CaCO3, while travertine is associated with geothermal water charged with calcium and bicarbonate in solution reaching the surface and precipitating around vents (Riding, 2000).
Fig. 8—Modern-day stromatolites, Shark Bay, Australia (ANP-IBP, 2012).
Fig. 7—Shrubby travertine with low-angle bedding and vuggy porosity, often visible on borehole image logs (after Corbett and Borghi, 2013).
Stromatolites. This facies is formed by algal mats in shallow saline lagoons or lakes. The algal mats trap carbonate sediment between layers, creating broad circular-to-elliptical bodies (Riding, 2000). The convex bedding associated with the dome shapes can often be identi¿ed on borehole image logs and the vuggy porosity (Fig. 2) makes them one of the most productive reservoir facies in the Presalt. Even when the stromatolites are not physically touching, they are normally laterally and vertically connected by eroded grainstones (Adams et al., 2005). Coquinas. This facies consists of bivalves deposited in fresh to brackish water, more commonly found in the rift phase. Coquinas would be considered grainstones to rudstones using the Dunham system and can have excellent reservoir quality in terms of intergranular porosity and permeability. In addition to these three key reservoir facies, other common facies encountered in the Presalt are ¿nely layered laminites, and spherulites, which are calcite grains usually