Offshore Brazil Research Articles

A comparison of machine learning methods to predict porosity in carbonate reservoirs from seismic-derived elastic properties

Porosity prediction from seismic data in carbonate reservoirs is challenging because the common presence of heterogeneities in carbonates makes it difficult to establish a clear physical relationship between reservoir properties and elastic responses. Regarding the strong nonlinearities underlying the relationship, machine learning is considered to be a good alternative to traditional methods. We compare several representative supervised machine learning algorithms (light gradient boosting machine [LightGBM], extreme gradient boosting, categorical boosting, random forest, multilayer perceptron, and convolutional neural network) in terms of predictive accuracy and runtime for crosswell blind tests and seismic prediction in a heterogeneous carbonate reservoir, offshore Brazil. The machine learning models are trained with the porosity and elastic parameters (P-impedance and [Formula: see text] ratio) from the smoothed and standardized logging data. Then, we apply the trained model on the inverted elastic properties to predict the porosity profile from seismic data. In the crosswell blind tests for the studied reservoir, LightGBM clearly stands out from the compared machine learning methods with the highest predictive accuracy and the shortest runtime, showing potential for fast and reliable porosity prediction from seismic data. In addition, we analyze the geologic factors (clay content, oil saturation, and relative depth) that possibly affect the predictive accuracy. We find that, with the constraints from clay content and fluids saturation, the performance of porosity prediction from elastic parameters can be further improved to a certain degree.

Challenges for seismic velocity modelling of rafts and impacts for pre-salt depth estimations

Seismic velocity models have significant importance in subsurface studies, notably when applied in structurally challenging areas. In some parts of the Campos Basin, offshore Brazil, the pre-salt reservoir's overburden shows complex structures, mainly due to raft tectonism that positions laterally, resulting in interspersed salt domes, carbonate rafts and siliciclastic sediments. This work used an extensive well database in the Marlim Complex to analyse the raft seismic velocities and their related impacts on pre-salt reservoir models. Based on well data, in combination with detailed seismic interpretation, seven alternative velocity scenarios were proposed for the rafts. The geological motivations for each scenario are discussed with the aim of developing constrained depth models for pre-salt reservoirs. The depth forecast results could be tested by drilled wells, and the resulting models are quantitatively compared in terms of depth predictions and gross-rock volumes. The results show that the topography of the target pre-salt reservoirs can vary considerably, even in scenarios where well and geological constraints are considered. This can impact pre-salt geological characterization and field development.

The characteristics of natural open fractures in acoustic borehole image logs from the pre-salt Barra Velha formation, Santos Basin, Brazil

Natural open fractures have been reported in the Barra Velha Formation (BVE) in the pre-salt lacustrine carbonate reservoirs in the Santos Basin offshore Brazil. These fractures contribute to excess permeability i.e., well test permeability is significantly larger than core permeability or log derived permeability, and they are therefore important for the fluid flow in the reservoirs during production and injection. In this paper we summarize the characteristics of natural open fractures in acoustic borehole image logs (BHI) in 19 wells from the BVE reservoirs and discuss their distribution, occurrences and formation. The data quality is overall good and consistent so that results can be compared between wells. The P21 fracture intensity, which is fracture trace length per area (bore hole wall) was used, instead of Terzaghi corrected P10 fracture intensity which do not distinguish between minor and major fractures. Mean P21 fracture density shows a variation of 2 orders of magnitude between interpreted wells. Data binning was used in the analysis of P21 fracture intensity vs petrophysical parameters to reduce the effect of sampling volume being much smaller than the representative elementary volume (REV) of the fracture pattern. P21 fracture intensity shows a positive correlation with silica content, which varies in the carbonates, whereas the porosity is less important for fracture intensity. The occurrence of fractures is also associated with vug-fraction estimated from the acoustic BHIs. Fractures are also frequent in fault damage zones, above and below cavities. The main orientation of the fractures is NNW-SSE, that is in the same direction as the dominating fault trend, and this indicates a structural control on fracture formation. Natural open fractures are rare in high porosity rocks with a shrubby or grainy texture, and also in intervals with a conglomeratic appearance in the BHI image logs.

Stratigraphic and sedimentary constraints on presalt carbonate reservoirs of the South Atlantic Margin, Santos Basin, offshore Brazil

Stratigraphic and sedimentary constraints on presalt carbonate reservoirs of the South Atlantic Margin, Santos Basin, offshore Brazil

3D Displacement and Stress Fields of Compacting Reservoir: Alternative Solutions

ABSTRACT. We have presented alternative solutions for the displacement and stress fields outside and inside of a 3D right rectangular prism under constant pressure. These solutions are obtained by integrating the well-known nucleus-of-strain solution over the volume of the prism. They are based on the similarity between the gravitational potential yielded by a volume source under a density variation and the thermoelastic displacement potential yielded by a volume source in a half-space under a pressure variation. This similarity enables the use of closed expressions of the gravitational potential and its derivatives. We use our solution for approximating the displacement and stress fields due to a reservoir with arbitrary shape and under arbitrary pressure changes. We discretized the reservoir as a grid of 3D right rectangular prisms juxtaposed in the horizontal and vertical directions. Each prism has homogeneous pressure; however, pressure variations among different prisms are allowed. This parametrization of the reservoir yields a piecewise constant distribution of pressure in the subsurface. We validate the resultant displacement and stress fields due to the reservoir by numerical simulations including a reservoir with arbitrary geometry and under arbitrary pressure distribution, based on a production oil field in offshore Brazil.

Abrolhos Magmatic Province petrogenesis and its link with the Vitória-Trindade Ridge, Southeast Brazilian Margin, South Atlantic Ocean

The Abrolhos Magmatic Province (AMP) is an example of an igneous province with about 63,000 km2 located at the Continent-Ocean Boundary (COB), into the Southeast Brazilian Margin. The AMP emerges as five small islands (Santa Bárbara, Redonda, Siriba, Sueste, and Guarita) which integrate the Abrolhos Archipelago located about 55 km offshore Brazil. The studied rocks belong to a transitional basalt series of alkaline affinity, with evolved rocks with high TiO2 contents. This work presents new detailed fieldwork mapping, petrographic, and whole-rock chemistry data, besides Sr–Nd isotopic compositions from AMP rocks. Mapped magmatic rocks in the Abrolhos Islands have been described as extrusive rocks, but we point out that they are shallow intrusions, mostly sills, and should be grouped into diabase units based on fieldwork and textural features. The AMP lithogeochemical and isotopic data suggest the operation of differentiation processes more complex than simple fractional crystallization or AFC. Our model mixing calculations based on new and compiled Sr-Nd-Pb isotopic data suggest a mixture with a prevailing depleted asthenosphere mantle source (represented by DMM; ca. 80%) metasomatized by an enriched mantle I (EMI) component (≤ 10%) and a possible HIMU-type constituent (up to 10%) in the AMP genesis. The assimilation of subducted slabs of the oceanic crust associated with the HIMU signatures is possibly linked to the Brasiliano Event due to the range of the AMP Nd TDM model ages, from 407 to 767 Ma. A viable mechanism for the EMI end-member contribution could either be a physical detachment of the South American subcontinental lithospheric mantle during the breakup of the Gondwana or lithospheric delamination of the South American plate caused by an edge-driven convection mechanism. The AMP and the Vitória-Trindade Ridge (VTR) show an eastward decreasing age pattern from the older ca. 60 Ma Abrolhos Province to the younger Martin Vaz and Trindade Islands (ca. 4 – 0.1 Ma), located ca. 1200 km away from the Brazilian coastline. The volcanic alignment between the VTR and AMP, along with the overlap of geochemical and isotopic data of their different igneous rocks, cannot be a random feature but instead represent the sampling of similar shallow mantle reservoirs, thus suggesting a cogenetic relationship. Finally, a possible petrogenetic link between the AMP and VTR magmatism is discussed.

Characterization of volcanic reservoirs; insights from the Badejo and Linguado oil field, Campos Basin, Brazil

The Badejo and Linguado oil fields are hosted within non-conventional volcanic reservoirs which produced commercial hydrocarbons from the Lower Cretaceous Cabiúnas Formation, Campos Basin, offshore Brazil. Despite over 30 years of production, limited characterization of the nature and reservoir properties of the volcanic reservoirs have been presented to date. A comprehensive reappraisal of the Cabiúnas Formation volcanic reservoirs is presented in this study incorporating extensive existing and new data including core descriptions, laboratory petrophysical analyses, petrography, QEMSCAN, SEM, wireline, and microtomography results from five cored intervals of wells spanning the main reservoirs of the Badejo and Linguado fields. Volcanic facies analyses of the sequences reveal a predominance of subaerial effusive basic volcanic rocks interbedded with sediment and in several cases comprising peperites revealing intricate lava-sediment interaction products. Four volcano-sedimentary units are identified, showing alternations between low (compound pahoehoe lava dominated Units 1 and 3) and high (tabular rubbly pahoehoe lava dominated Units 2 and 4) effusion rates. Paleoenvironmental conditions also varied between units with extensive oxidation present in Unit 1 inferred to relate to extended periods of subaerial exposure and weathering in an arid environment. Overlying Units 2–4 reveal an increase in humidity evidenced by an increase in the presence of well sorted fine-grained sediment interlayers, peperites, and non-marine ostracods. Both original facies and alteration reveal key controls on reservoir properties. Extensive weathering and alteration of Unit 1 caused pervasive filling of original porosity (vesicles and fractures) and resulted in reservoir degradation. Lesser weathering of overlying Units 2, 3 and 4, resulted in improved reservoir properties which can be clearly linked to volcanic intra-facies including vesiculated and autobrecciated lava flow tops which commonly reveal extensive oil staining. This study reveals the intricate interplay between primary volcanic facies and subsequent alteration history in dictating volcanic reservoir properties in a successful offshore oil field development.

The properties of faults in the Brazil pre-salt: A reservoir characterization perspective

Subsurface characterization of faults and fault systems is a central piece to describe the subsurface permeability architecture of a carbonate reservoir during field development. Data integrity and quality are often compromised due to the challenges (mud losses and stuck pipe) that are posed when drilling into a fault zone. Core is frequently not recovered or in very poor condition (rubble) that make quantitative description of a fault zone very challenging. For these reasons wellbore trajectories tend to avoid fault zones leading to a sampling bias. As a result, many authors default to using field analogs to fill in the knowledge gap in fault properties that are required to build robust subsurface reservoir models. In this work, we select a subset of wells from the pre-Salt, offshore Brazil, where faults have been either intersected by a well or investigated by a well test. We use case studies to illustrate how observations made across a multidisciplinary dataset can be brought together to provide insights into likely ranges of fault properties. We discuss subsurface observations of fault zones in the pre-Salt in the context of analogs to propose end members of fault architecture that can have an impact on reservoir performance. Results suggest that these faults act as conduits for fluid flow. Their internal structure is characterized by a high density of fractures that often show evidence of dissolution enhancement. In the case of early syndepostional structures, early karst processes are responsible for enhancing fracture porosity and leading to the development of cavernous porosity. Faults developed during burial have a lesser degree of diagenetic overprinting, resulting in an overall lower pore volume. However, permeability (and its degree of anisotropy) in both cases is several orders of magnitude greater than that of the matrix. We have used numerical forward modeling of pressure derivatives to evaluate fault permeability values that yield 10's of darcy permeability associated with such features.

Influence of ocean dynamics on the route of argonauts in the Southeastern Brazil Bight

Argonauts (Cephalopoda: Argonautidae), also known as ‘paper nautilus’, are epipelagic octopuses that are believed inhabit the open ocean despite being rarely encountered alive. Previous studies on the early-life stages of cephalopods have reported unexpectedly high abundances of argonauts in continental shelf areas of the Southeastern Brazilian Bight (SBB). Based on this finding, we explored the potential ocean dynamics driving both seasonal and interannual argonaut paralarvae occurrence in the SBB shelf. A historical archive ofplankton samples (n = 884) collected during 22 survey cruises between 1974 and 2010 covering the northern portion of the SBB was used. Spatiotemporal analysis combining environmental variables and horizontal and vertical abundance maps indicated that high frequency of argonaut paralarvae mostly occurred during summer with a high interannual variability. Horizontal and vertical maps revealed that they were mainly transported from the open ocean into the shelf by the onshore intrusion of water masses promoted by meanders and eddies of the offshore Brazil Current. Furthermore, argonaut occurrence in the southern shelf areas was remarkably influenced by the coastal jet originating from upwelling in the northern SBB. These results suggest that argonauts may be used as indicators of mesoscale processes. The present study provides a novel hypothesis for the route of argonauts from the pelagic realm to the continental shelf.

Study Reviews ESP Performance in High-Viscosity Applications

_ This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper OTC 31839, “Understanding ESP Performance Under High-Viscosity Applications and Emulsion Production,” by Luiz Pastre, SPE, Jorge Biazussi, and William Monte Verde, University of Campinas, et al. The paper has not been peer reviewed. Copyright 2022 Offshore Technology Conference. Reproduced by permission. _ Despite its common use as an artificial lift method for heavy-oil developments, electrical-submersible-pump (ESP) system performance in high-viscosity applications is not fully understood. Failure to understand challenges to equipment performance in such conditions can lead to operational inefficiency and equipment failures. The complete paper presents results of single-phase and multiphase tests performed by researchers at the University of Campinas. It also presents operational data, lessons learned, and examples of failures gathered over 10 years of ESP operation in the Peregrino field, a highly viscous, heavy-oil field offshore Brazil operated by Equinor. Peregrino Field The field was discovered in 1994 in the southern part of the Campos Basin offshore Brazil. The discovery lies in shallow waters approximately 120 m deep in Block BM-C-7. Although a large amount of recoverable oil in place (300 million– 600 million bbl) was confirmed, the technology to properly develop the field was not available. A detailed development plan was created to optimize Peregrino heavy-oil production while considering drainage strategy and well design. Recovery and production are maximized in Peregrino by extending the reservoir section length. As a result, every well in Peregrino is drilled with a long horizontal section inside the reservoir and completed with openhole gravel-pack lower completions. This strategy resulted in the longest wells in Brazil, with a record of 8613 m measured depth (MD) total length, with 2272 m MD in the horizontal section. To improve the effectiveness of sand control and avoid solids production, gravel is pumped to fill the annular space between the 8½-in. openhole section and the 5½-in.-outer-diameter screens. Inside the reservoir, water flows at a much higher velocity than viscous crude does. Therefore, inflow control devices are deployed in the lower completion to limit water flow into the wellbore at selected wells. To make gravel-pack operation feasible in such a configuration, sacrificial screens are installed in the bottom of the lower completion string. An artificial lift method is required to produce the heavy and viscous oil from the field and maintain commercial rates when pressure support and production declines. ESPs proved to be the most-efficient method for this application.

Iterative sequential robust optimization of quantity and location of wells in field development under subsurface, operational and economic uncertainty

Determination of optimal quantity and location of wells is a crucial step in any field development project. Multiple challenges exist in the real-world projects, such as gigantic multidimensional search space and complex parameterization of the problem. To address these challenges, we present an iterative sequential optimization framework that consists of multiple stages for optimizing number and location of producers and injectors. Once the initial guesses have been established, the first optimization stage begins in order to determine the optimal number of producers and injectors. Next, a two-stage process is used to determine the best location for each well. By optimizing the number and location of wells, well type is optimized implicitly and does not need to be included in the optimization process as an individual decision variable. To consider the dependency and coupling between parameters, the entire optimization process is repeated iteratively up to a stopping point. The proposed approach is highly flexible and can be extended to include other potential decision variables of field development projects. We test the proposed methodology on UNISIM–II–D, a realistic benchmark case that represents pre-salt reservoirs in deep offshore Brazil, which are light-oil fractured carbonates with large volumes of CO2-rich associated gas. In our application, all producers and injectors are equipped with internal control valves (ICVs), all produced gas is reinjected into the reservoir, and water-alternating-gas with CO2 (WAG-CO2) is used to increase oil recovery and ensure that the produced CO2, as a harmful greenhouse gas, can be safely discarded underground. We use the iterative discrete Latin hypercube (IDLHC) optimization algorithm. To achieve a robust decision, we consider subsurface, operational and economic uncertainties by considering nine representative reservoir models and three economic scenarios. Based on the results, the proposed framework can provide an appropriate solution containing optimal quantity of wells, their locations and types in realistic cases with large search spaces and complex parameterizations.

Fill‐and‐Spill, Tilt‐and‐Repeat (FaSTaR) cycles: Stratigraphic evolution above a dynamic submarine stepped slope

AbstractThe classic fill‐and‐spill model is widely applied to interpret topographic controls on depositional architecture and facies distributions in slope successions with complicated topography. However, this model implies a constant topographic configuration over the lifespan of a turbidite system. In contrast, the impact on patterns of erosion and deposition above dynamic slopes whose topographic configuration varies spatially over time remains poorly investigated. Here, using high‐resolution 3D seismic reflection data and more than 100 wells from a 40 km long stepped slope system (Campos Basin, offshore Brazil), we document the evolution of a sand‐prone turbidite system active during the Oligocene–Miocene transition. This turbidite system was influenced by vertical and lateral deformation, and we propose a new stratigraphic model to explain the resultant depositional architecture. Two depocentres were identified as steps, with channels on the proximal step, and channel–lobe complexes on the distal step, bounded by sediment bypass‐dominated ramps. Lateral stepping of channels on the proximal step, and oblique stacking of the down‐dip lobe complexes, each cut by through‐going channels, indicate multiple fill‐and‐spill cycles. A persistent north‐east‐ward stepping and thickening on the steps are interpreted to reflect lateral tilting of the seafloor driven by salt tectonics. The dynamic substrate prevented the establishment of a single long‐lived conduit across the proximal step, as recorded in systems with fixed topographic configurations. The filling of through‐going channels with mud at the end of each cycle suggests waxing‐to‐waning sediment supply cycles and periods of sand starvation when the lateral tilting dominated and drove avulsion of the feeder channels towards topographic lows. This study demonstrates that subtle dynamic slope deformation punctuated by discrete sediment supply cycles results in complex stratigraphic patterns with multiple phases, and multiple entry and exit points. Repeated cycles of fill‐and‐spill, tilt‐and‐repeat are likely to be present in other stepped slope systems.

E&P Notes (August 2022)

E&P Notes (August 2022)

Physical modelling of the interplay between salt‐detached gravity gliding and spreading across complex rift topography, Santos Basin, offshore Brazil

AbstractThe Santos Basin, offshore Brazil contains a complex set of salt‐tectonic structures, the origins of which are debated, that is, the Albian Gap and the São Paulo Plateau (SPP). The Albian Gap is a ca. 450 km long, 60 km wide feature characterized by a post‐Albian, counter‐regional rollover overlying depleted Aptian salt, and in which the Albian is largely absent. The SPP, located immediately downdip, is defined by a pre‐salt structural high overlain by ca. 2.5 km thick salt. Another prominent feature is the Merluza Graben, a rift‐related depocentre that underlies the southern portion of the Albian Gap and displays significant (3–4 km) base‐salt relief along its main faults. Two competing hypotheses have been proposed to explain the kinematics of these provinces. One invokes post‐Albian extension in the Albian Gap and kinematically‐linked contraction in the SPP. The other invokes post‐Albian salt expulsion in the Albian Gap and salt inflation in the SPP. Recent studies, however, suggest these processes likely alternate in time and space, contributing nearly equally to the evolution of these domains. We apply 3D physical modelling to (i) test this hypothesis; and (ii) to more generally understand how gravity gliding and spreading over three‐dimensionally variable base‐salt relief control regional salt tectonics. The results show a similar salt‐related evolution and structural styles to those proposed in the most recent studies. They also (i) explain the origin of the ca. 25 km wide diapir precursor of the Albian Gap by early salt inflation against base‐salt steps; (ii) show that normal faults with different polarities and rollover types form due to the interplay between gliding and spreading over different base‐salt domains and (iii) provide a mechanism for the origin of strata encased within salt structures. This improves our understanding of the distribution and origin of salt‐related structural styles in worldwide salt basins.

Acoustic image log facies and well log petrophysical evaluation of the Barra Velha Formation carbonate reservoir from the Santos Basin, offshore Brazil

Acoustic image log facies and well log petrophysical evaluation of the Barra Velha Formation carbonate reservoir from the Santos Basin, offshore Brazil

New age and lake chemistry constraints on the Aptian pre-salt carbonates of the central South Atlantic

AbstractThe Cretaceous lacustrine carbonates of the offshore Brazilian and West African presalt basins represent some of the most extensive non-marine carbonates discovered in the geologic record. Despite being intensively studied over the past decade, the age of these carbonates and the overlying regional salt sequences is highly controversial. Similarly, the conditions under which these carbonates were deposited remains poorly understood. Here, we provide the first integrated geochronology-thermometry study of these carbonates to develop an improved understanding of when and under what conditions they formed. We utilize carbonate clumped isotope and 87Sr/86Sr geochemistry alongside traditional petrographic techniques to identify samples minimally altered from burial diagenesis that may yield reliable age and lake chemistry constraints. Carbonate clumped isotope apparent temperatures for the studied carbonates range from 36 °C to 91 °C, which we infer to represent a range in sample preservation from minimally altered depositional temperatures through to those that have been overprinted by burial diagenesis. 87Sr/86Sr values of our samples are consistent with those of previous studies for Cretaceous pre-salt carbonates that have not experienced significant alteration from hydrothermal fluids. Through this approach, we measured the first high resolution isotope dilution U-Pb age constraint of 115.83 ± 1.56 Ma (2σ) on a well preserved carbonate. Combined with overlapping lower resolution laser ablation U-Pb ages for time-equivalent stratigraphy on two separate carbonate platforms of 114.46 ± 4.72 Ma and 109.73 ± 9.26 Ma, these ages provide the first robust direct age calibration for pre-salt carbonates deposited on either side of the South Atlantic during the final stages of the break-up of Gondwana in the Early Cretaceous. These ages also provide the first calibration for a combined 87Sr/86Sr-facieslog based relative age framework within the Santos Basin, offshore Brazil. We further utilize δ18O constraints on samples that yield depositional clumped isotope apparent temperatures to constrain the δ18O of the water in these ancient lakes to between 1.9 and 4.9‰Vienna standard mean ocean water. Such heavy values reveal a picture of a hot and arid environment. This is consistent with prior biostratigraphic studies of the carbonates that show a decrease in faunal diversity in these lakes prior to marine ingress and the development of open marine conditions in the South Atlantic Ocean.

E&P Notes (June 2022)

E&P Notes (June 2022)

Tectono‐stratigraphic evolution of the rift and post‐rift systems in the Northern Campos Basin, offshore Brazil

AbstractThe Campos and Santos Basins have been a focus of subsurface studies since the discovery in 2006 of large accumulations of hydrocarbons in the Early Cretaceous rift and post‐rift strata below Aptian evaporites. In this study, regional 2D seismic lines, a 3D seismic survey and well data were interpreted to reconstruct the tectono‐stratigraphic evolution of the rift and post‐rift stages in the northern sector of the Campos Basin. Detailed 3D seismic interpretation and geological modelling were used to subdivide the pre‐salt sedimentary record. This revealed a diachronous strain distribution along the Guriri Fault System (GFS), a roughly NE‐SW striking fault‐bounded horst that was a focus of rift‐related deformation. The syn‐rift succession is interpreted to be the product of two episodes of rifting (RP1 and RP2) with contrasting fault activity patterns and lithostratigraphy. Volcaniclastics and coarse siliciclastics of RP1 were deposited under WNW‐ESE transtension that formed discontinuous half‐grabens followed by extensive erosion and tectonic inversion. Structurally controlled bioclastic rudstones and hybrid deposits characterize RP2 and were deposited in elongated half‐grabens that delimit the GFS horst, which developed under an NW‐SE extension. Deposition under increasingly less tectonically active transitional and post‐rift stages dominated by thermal subsidence gradually led to the healing of the rift‐related structural relief. Selective reactivation of rift faults testifies active tectonic inversion through compression from immediate post‐rift throughout evaporite deposition. These interpretations are put into the context of recent geochronological data of onshore dyke emplacement and the new age constraints for the end of deposition of the pre‐salt sequence. Therefore, we propose an earlier onset on rifting in the Campos Basin, at the Berriasian.

Seismic at Sea - PESA News Issue 165

Shearwater to perform Third Isometrix Survey at Equinor's Mariner field Nova Scotia, Korea North Sea Bottom Node Project Searcher Hunts for Hydrocarbons in ‘Oil Rich’ Oman PGS Wins 3D Seismic Award Offshore Cyprus PGS Partners with ANH and SGC on Colombia Pacific Seismic Project CGG Wins Major 4D OBN Imaging Contract Offshore Brazil High EPC Spend Stimulated Offshore Rig Market in 2021

E&P Notes (May 2022)

E&P Notes (May 2022)