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This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper OTC 30626, “Brazil Presalt, Santos Basin: Feasibility Study for the Application of Borehole Gravity To Improve Reservoir Monitoring,” by Zhijun Du, Adrian Topham, SPE, and Jeremy C. Lofts, SPE, Silicon Microgravity, et al., prepared for the 2020 Offshore Technology Conference, originally scheduled to be held in Houston, 4-7 May. The paper has not been peer reviewed. Copyright 2020 Offshore Technology Conference. Reproduced by permission. The complete paper uses a feasibility study to present the potential application of a three-axis gravimeter borehole measurement in the Libra presalt reservoir in the Santos Basin offshore Brazil. The authors’ findings suggest that an annual survey with a limited well stock could be effective in monitoring this type of reservoir and that a wireline-deployed three-axis gravity tool is likely to provide significant additional surveillance to constrain a reservoir production strategy through better appreciation of the direction of water movement. Introduction The giant Brazilian presalt reservoirs are in ultradeep water (greater than 2000 m) and buried at a depth surpassing 5000 m. In order to conduct reservoir monitoring and achieve successful reservoir management, borehole-based geophysical technologies are preferred because the measurements can be made closer to the reservoir mass. The development of a borehole gravimeter suit-able for use in deep reservoirs, however, poses significant technical challenges, and breakthroughs have been limited by the sensor form factor (size) and measurement stabilization. By use of microelectromechanical-system (MEMS) vibrating-beam technology, the authors introduce a borehole gravimeter that enables the recording of gravitational acceleration at very high sensitivity. The new resonant MEMS gravitational sensor is designed to sense mass in the subsurface such that time-lapse wireline-based surveys can be conducted to build a picture of fluid movements at relatively large distances from a wellbore, thereby enabling time-lapse or 4D gravity monitoring. The innovation of the three-axis gravimeter also allows the acquisition of directional information about the spatial movement of fluid, even when acquired from only a single borehole. The principles of measuring gravity in the borehole environment are provided in the complete paper. Libra Field. The field is one of the largest presalt oil discoveries in the Santos Basin. It was discovered in May 2010 with recoverable resources of approximately 9 billion bbl of oil. The first appraisal well was completed and tested in February 2015. At the time of writing, nine appraisal wells have been drilled. A high-density 3D seismic program was acquired over the Santos and Campos basins, and advanced seismic processing technology has also been applied. Reservoir Simulation. The geophysics for reservoir characterization and simulation of these carbonate reservoirs is still in its early stages, and little published knowledge about their properties exists. Nonetheless, since their discovery, 2D and 3D seismic technologies have been applied intensively for imaging the region structures and delineating the reservoirs.
Topham, Adrian (Silicon Microgravity Ltd) | Lofts, Jeremy (Silicon Microgravity Ltd) | Du, Zhijun (Silicon Microgravity Ltd) | Espie, Tony (BP PLC) | Zett, Adrian (BP PLC) | Krahenbuhl, Richard (Colorado School of Mines) | Li, Yaoguo (Colorado School of Mines)
In the attempt to fill the scale gap between pulsed neutron and 4D seismic geophysical techniques to monitor underground reservoirs for ongoing oil/gas productionor CO2capture and storage, we introduce an emerging 3-axis borehole gravity technology that enables the recording of gravitational acceleration at high sensitivity, targeted at5 µGal.
This is made possible using an innovation in resonant Microelectromechanical systems (MEMS) vibrating beam technology. This technology is designed to sense gravitational field produced by mass density changes in the subsurface, such that time-lapse wireline-based surveys may be taken to image fluid movements as far as 100s of meters from a wellbore and thereby enable time-lapse or 4D gravity monitoring. The innovation of 3-axis gravity measurement allows the acquisition of directional information about the spatial movement of fluid, even when acquired from just a single borehole.The cost effectiveness of a 4D wireline gravity survey compared to 4D seismic survey is highly attractive especially in the later stages of production surveillance programs, or as a complementary survey.
We will first introduce the technology, and then present its application through a feasibility study aimed at the monitoring of CO2 in a deep storage reservoir in Canada. We model and predict the gravity variation in a 4D gravity surveylikely to be seen due to density changes during a period of CO2 injection at the storage site. Survey feasibility modelling and a workflow are presentedthat together provide important information forplanning and acquiring successful 4D gravity surveys, including the optimal time intervalbased on the planned injection rateand the optimal well location to use.
The study will illustrate the general use for of time-lapse 3-axis gravity in monitoring reservoirs for optimising production over time while additionalexamples will be shown to further demonstrate application within oil/gas production reservoirs.
Boyd, Austin (Schlumberger Brazil Research and Geoengineering Center) | Souza, Andre (Schlumberger Brazil Research and Geoengineering Center) | Carneiro, Giovanna (Schlumberger Brazil Research and Geoengineering Center) | Machado, Vinicius (Petrobras) | Trevizan, Willian (Petrobras) | Santos, Bernardo (Petrobras) | Netto, Paulo (Petrobras) | Bagueira, Rodrigo (Instituto de Quimica - Universidade Federal Fluminense ) | Polinski, Ralf (Schlumberger Servicos de Petroleo LTDA) | Bertolini, Andre (Schlumberger Servicos de Petroleo LTDA)
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 prolific flow rates from these high permeability, lacustrine carbonates have shown no significant decline in over five 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 high-permeability intervals to optimize the completion strategy.
Since the first discovery well in Lula field 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 filtrate 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.
Abstract Objectives/Scope The advantages of measuring gravity in the borehole environment have been well established in the literature and through first-generation instruments. These measurements can be very effective for directly imaging mass distributions at-depth in the subsurface and at large-distances from well bores. To date, a breakthrough has been limited by the sensor form factor (size) and measurement stabilization. Newly emerging MEMS three-axis microgravity technology, deployable by wireline, is showing the potential for a host of applications and capable of realizing the long-coveted advantages. For reservoir surveillance, a primary application is to perform more pro-active, frequent flood front monitoring. With its large volume of investigation, the proposed three-axis borehole gravity measurements would complement as well as fill the existing gap between traditional methods such as Pulsed Neutron and 4D seismic. Further applications extend to saturation monitoring, by-passed pay, and thin-bed identification. In conjunction with a collaborative program to develop a three-axis gravity sensor that is now being incorporated into a 54-mm diameter wireline tool with a targeted sensitivity ≈5 μGal (microGal), we have carried out extensive numerical studies to understand the signal strength of such measurements produced by the dynamic processes in different types of reservoirs, and demonstrate the capabilities and limitations of borehole gravity and its potential use within a revised reservoir surveillance plan. Methods, Procedures, Process We show examples of forward modelling data from reservoirs with varying fluid displacement mechanisms. Reservoir porosity and saturation data are used to model the predicted three-component (i.e., vector) gravity anomaly (gz, gx, and gy) responses along the wellbore in a variety of wells as the fluid-water front progresses through the field and the modelling included both producing wells and injector wells. The paper will present a description of a forward modeling workflow, simulation studies based on real reservoir data and the validating measurements. Results, Observations and Conclusions The paper examines the results of the forward modelling and compares the results with the target sensitivity of the new three-axis borehole gravity sensor. The results will show that a wireline deployed three-axis gravity tool with a noise floor of ≈5 μGal will provide additional important surveillance to constrain reservoir models. It will also provide vital information to help reduce uncertainty when actively managing waterfront movement (sweep), secondary recovery and for detecting early breakthrough of water; and for monitoring and adjusting strategy when producing through reservoir depressurization. The described workflow is seen as very important for any future survey that planning to understand the time-lapse gravity signal and the feasibility of time-lapse gravity surveillance under different reservoir conditions. Novel/Additive Information A three-axis borehole gravity tool with a form factor enabling it to be deployed through cased hole and into deviated and horizontal wells is completely novel and has not been presented previously. A workflow that understands survey feasibility and optimal survey-time intervals is novel. A systematic and comparative study of three-axis borehole gravity responses through modelling of water flood in a set of reservoirs located on different continents is novel and has limited previous work.
Abstract This study summarizes the pre-salt hydrocarbon play elements in Brazilian offshore and identifies additional exploration potential within the BMS-50/52 and BS-1_South 3D seismic surveys areas within the Santos Basin and of the BC-200 merged and reprocessed PSDM survey located within the deep to ultra-deep waters of the Campos Basin. Several large opportunities have been identified based on seismofacies recognition and preliminary seismic interpretations of 3D Multi-Client PSDM surveys. The BMS-50/52 and BS-1_South 3D surveys are within the pre-salt play polygon of the Santos Basin. The pre-salt exploration trends identified in the these surveys are: a) Rift/Pre-rift play in the central-west, with hydrocarbon accumulations in siliciclastic reservoirs of the Paleozoic (pre-rift) to the Lower Cretaceous rift succession in fault traps; b) Sag/Rift Limestone Edge Play (Sagitário trend), comprising structural or paleo-topographic traps beneath the base salt in microbial platform limestone, with occasional isolated microbial buildups; c) The Carcará North/Itaipava Sag-Rift Limestone Play which is a light-oil trend that includes the Carcará discovery in BMS-8, with hundreds of square kilometers of closures at the base salt and relief reaching 350 to 400 meters. In this trend, reservoir facies are expected to have excellent reservoir properties and are interpreted as very large microbial buildups, oriented along fault zones which are located at the edges of the previous rift horsts. 3D seismic interpretation of the pre-salt section in the BC-200, deep to ultra-deepwater area in the Campos Basin has allowed the delineation of several pre-salt plays: i) Structural Play in coquinas (Late Rift to Sag); ii) Stratigraphic/Combination Play in coquinas (Late Rift to Sag); iii) Microbial Build-up Play on rift shoulders (Sag microbialites) and; iv) Microbial Build-up Play on volcanic complexes, potentially large limestone units beneath the salt nucleated on top of large volcanic complexes in distal settings.