Extended Well Testing (EWT) phase has started on the Libra field, which is operated by Petrobras in partnership with Total. EWT is a standard practice in the Brazilian pre-salt. The objective? Evaluate the future performance of the field and the reservoir behavior in order to better determine the specifications of the definitive production system of the field. We talk to Didier Plisson, head of the Libra integrated development study team of the Deepwater Product line (GEDI OBO Brazil).
Could you briefly present the Libra field and its potential?
Didier Plisson: Libra field has been discovered offshore in 2010 in Brazil’s Santos basin, in the so-called pre-salt polygon, where Petrobras and ANP (the National Agency of Petroleum, Natural Gas and Biofuels) have made the main pre-salt discoveries. It overlaps slightly with the Campos basin, historical basin for the Brazilian offshore oil production.
It’s a region of giant pre-salt fields, such as Lula, Sapinhoá, Buzios, Iara and Libra, containing several billion barrels in resources and quite recently discovered. The reservoirs generally lie under 2,000m of salt. We didn’t drill under the salt very often before 2005 or 2006, as the pre-salt structures were not yet identified at that time as potential reservoirs.
The Libra field is offshore at about 180km off the coast of Rio de Janeiro in a deep water environment (water depth of 2,000m), with sea conditions quite harsh.
We’re dealing with a pre-salt field of lacustrine carbonates, with a lot of heterogeneity and a lot of different facies, leading to some difficulties to appreciate correctly the reservoir and in particular the potential of the production. One of the risks we’ve identified on reservoirs like this is that of an early gas breakthrough: producers and injectors could communicate much too quickly, so you risk recycling your gas after few months or years, thus limiting production.
Libra fluids are complicated: the Libra’s gas initially contains 44% of CO2, whereas the neighboring pre-salt fields have 20%. The Gas-to-Oil Ratio (GOR) is about 430 volume by volume, nearly twice as high as neighboring fields.
So Libra is a field of extremes compared to the others of the area: twice as much gas, but twice as much CO2, in a heterogeneous reservoir which can be complex to manage and produce.
Which appraisal and development strategy has been chosen?
D.P: To cover the North-West panel of Libra – called now Mero since its recent declaration of commerciality -, four large FPSOs are planned, each of them with a capacity of 150 000 barrels of oil per day and a gas reinjection capacity of 12 SMMm3/d at 550 bar. The gas facilities will be similar in scale to the AKPO FPSO, operated in Nigerian deep water. About 16 wells are scheduled per FPSO (8 producers and 8 Water-alternate-Gas injectors).
To explore and appraise Libra, the “Pre-salt Golden rules” of Petrobras are applied. Those rules, heritage of the experience of Petrobras in the pre-salt, have been defined to prepare the development of pre-salt fields and to de-risk the main Geosciences uncertainties. Petrobras usually drills 3 wells and performs an Extended Well Testing (EWT) on each future FPSO location. This EWT phase -or Dynamic data acquisition phase- lasts between six months and a year with a dedicated fit-for-purpose FPSO. During that time, the production and the well pressures are monitored, to try to assess the complexity of the field: are there any barriers? Are there any permeability drains?
Those are the objectives of the new built and chartered Pioneiro de Libra EWT vessel.
What’s so special about this ship?
D.P: Petrobras usually works with two EWT vessels, the Cidade de Sao Vicente and the Dynamic Producer. These are basically medium-size FPSOs that can only produce few wells and re-inject or export the produced gas. They are designed to handle 20% of CO2 and few volumes of produced gas, and are therefore not suitable for Libra’s 44% CO2 requirement.
That’s why the partners had to charter a new-built specific FPSO, the Pioneiro de Libra (PDL), under a Lease & Operate contract. It’s a FPSO based on a converted hull of tanker to which a turret has been added. The lines, umbilicals and so on pass through the turret. This FPSO has a daily capacity of 50,000 barrels of oil and 4 MMm3 of gas. For Libra, 1 to 2 production wells and one gas injection well will be connected for each of the EWT sequences.
At the end of November 2017, the Pioneiro de Libra achieved first oil, starting the first EWT phase, and for the next four years will perform long-term well-tests in three different areas of the field.
Is this a new approach for Total?
D.P: Petrobras is the champion of the pre-salt. As they claim from their EWT experience, it allows them to reduce the number of appraisal wells compared to a conventional appraisal approach. It’s obvious that, when developing a heterogeneous field like this, it’s important to acquire dynamic data to ensure the proper development of the field by investing the proper CAPEX in the proper area. Those dynamic data will help to derisk the reservoir development, allowing to adjust the number of wells and the pattern of the wells.
We at Total are discovering this method, which we’ve never experienced before. We are learning a lot from it, and it’s a major first for the Group to be a partner on a project like this. It’s perfectly conceivable to use this technique on other of our fields, as it offers a way to explore the reservoir and acquire data on a large scale, which is something that we’ve never done before.
Technical Responses to New Challenges
In-House Security Expertise for FPSOs