Heavy Oil Exploration, Appraisal and Reservoir Development in Campos Basin: Successes but Still Many Challenges Ahead

Celso C. M. Branco, Antônio C. C. Pinto, Jéferson Kinzel, and Jéferson Dias
Petrobras

1. Introduction

Exploration efforts offshore Brazil has been indicating important heavy oil discoveries in reservoirs that are often located in deep waters; present potential reserves are more than 4 billion barrels. In this paper, the terminology “offshore heavy oil” (OHO) is applicable to oils with an API below 20, with viscosities at reservoirs conditions over 20 cp and surface (“dead” oil) viscosities over 500 cp. Although not as heavy as inland heavy oil fields, where thermal methods are usually employed, development of these fields imposes extra technical difficulties as water flooding might be the only feasible economical alternative for increasing recoveries. The process must be used in association with long horizontal production and injection wells carefully placed in order to take advantage of the geological setting (1).

OHO water flooding projects may face several reservoir related problems during their lifetime, such as early water breakthrough and increasingly fast water rates. The fact can be very easily interpreted through the concept of water oil fractional flow derived from the theory for two phase flow in porous media. This result in poor sweep and the need for offshore production facilities prepared to deal with large liquids flow rates. Another point of concern is the proper disposal of the produced water: the recommended solution would be to re-inject the water.

Production problems may also arise: strong water-oil emulsions, flow assurance problems due to the formation of gas hydrates in pipelines and poor artificial lift efficiency. Gas lift method is not effective for high water cuts; on the other hand the use of electrical submersible pumps (ESP) in satellite wells lacks in reliability. Moreover, most offshore developments are based on FPSO production systems and satellite wells (equipped with the so called wet christmas trees) where one has limited (and highly expensive) well access. This development concept hinders well workovers due to high costs and limited availability of workover rigs with dynamic positioning facilities able to operate in deep waters. This results in fewer possibilities for an active management of the producing reservoirs. A flexible and timely reservoir management is particularly important for heavy oil reservoirs, as they present early water production problems, with lower recovery rates and associated flow assurance problems. In this context production units allowing direct access to the wells, like tension leg platforms (TLP) and Spars, become more attractive for heavy oil fields in deep waters (2).

2. General Aspects of OHO Reservoirs in Campos Basin

Heavy oil reservoirs are usually shallow (up to 1000m below mud line) and therefore present low reservoir temperatures (between 40 and 60oC). Low sedimentary overburden tends to ease the biodegradation of the oil mainly when associated to bottom aquifers. It also contributes for creating geomechanically fragile structures were faults can occur creating geological compartments and heterogeneities. Such kind of reservoir may also have low seal pressure which may cause the dissolved gas to leave the oil, increasing its viscosity. The reservoir lithology is usually sandstones deposited as turbidites; high permeability (in the order of Darcys) and porosity are quite common. High permeability may compensate the elevated oil viscosity resulting in high well productivities. The group K*h/m (ratio of the product of permeability per net pay over oil viscosity) is an important parameter when dealing OHO projects, depending on the specific value the development plan is directed towards the use of horizontal or multilateral wells.

Well sampling of heavy oil is a very important step in the appraisal of a new reservoir since the correct viscosity and other crude assay defines the economical feasibility of the development plan. The best practice is to use the so called single phase samplers once the oil and gas recombination in a PVT laboratory is not effective for extra heavy oils. OHO usually present high total acid numers (TAN) sometimes far above the usual 0.5 mg/g KOH. This is one of the most important parameters which negatively impacts oil valuation. Interestingly, sulfur content has not shown higher levels in Campos Basin. Compositional grading along the sedimentary column and in different places of the field is common. Asphaltene deposition at water oil contacts may form tarmats which can hydraulically isolate the oil leg from the bottom water, this must be carefully studied in order to avoid bad placement of injection wells.

For proper fluids sampling some exploratory or appraisal wells must be cased and gravel packed in order to be adequately evaluated. This fact imposes extra costs to the appraisal phase. Well tests require careful planning aiming at the estimation of horizontal well productivity indexes (translating vertical productivities to horizontal ones may be tricky in heavy oil accumulations), vertical permeability, coring and for collecting reliable oil and water samples (water composition may be very important since sea compatibility studies should be performed for modeling scale and souring tendency). Another concern is what to do with the gas: these reservoirs have a very low GOR, insufficient to support an exportation pipeline. On the other hand, gas will have to be imported in the future to generate power.

For the OHO shallow reservoirs, geo mechanical data are receiving increasingly importance for the correct planning of the injection and production practices and as an input for reservoir modeling (geomechanics were already employed for drilling and gravel packing projects). As a consequence, mini fracs, image logs, oriented cores are becoming more usual. Correct definition of maximum working pressures (drawdown and water injection) in certain cases must rely on a flow and geomechanical coupled model.

3. Two Case Histories of OHO Appraisal in Campos Basin

Petrobras experience in testing heavy oil fields is briefly described for two particular fields: Jubarte and the extra heavy X reservoir. In the first one a 1070m horizontal section well located in a 1330m water depth was successfully brought into production (17 API oil) using an electrical submersible pump installed above the mud line. An extended well test followed by a pilot production provided the dynamic information needed for the refinement of the geological and simulation modeling of the reservoir. Figure 1 shows the excellent match obtained for water cut adjusting net to gross ratio and vertical permeability in the geological model (2).

The second case history depicts an extra heavy (13 API) reservoir. This is a shallow carbonate reservoir at 100 m water depth. Value of Information analysis and sensitivity studies based on reservoir simulation models justified the drilling and completion of a slanted pilot well followed by a 2000m horizontal section well for data acquisition. Figure 2 shows some results of the sensitivities simulations performed. It is planned a pilot production for in this reservoir in 2008 (3).

Further details on these two topics are provided in the PowerPoint presentation.

4. Final remarks

Although difficult, the exploration and production of heavy oil in offshore fields is pushing the limits of offshore technology. Petrobras is facing this challenge through a well organized internal research and development program - PROPES. The program is multi-disciplinary covering most of the upstream disciplines and an interface with the downstream area. It comprises 8 areas: reservoir engineering; well technology; artificial lift; flow assurance; oil processing (separation and treatment); extended well tests; subsea and production facilities; pre-treatment / refining. The technology development is being made on a step-by-step basis, with equipment suppliers, service companies, consulting companies and universities ascertaining the growth of its Exploration and Production businesses.

References

(1) Pinto, A.C.C., Branco, C.C.M., Matos, J.S., Vieira, P.M., Guedes, S. S., Pedroso Jr., C., Coelho, A.C.D., Ceciliano, M.M.: “Offshore Heavy Oil in Campos Basin: The Petrobras Experience”. Paper OTC-15283 presented at the Offshore Technology Conference, Houston, May, 5-8, 2003.

(2) Branco, C.C.M., Santos, R.L.A., Peres, A.M.M., Prais, F., Pinto, I.A.: “Reservoir Study Considering a TLP and Dry-Completion Development for a Deep Offshore Heavy Oil Field”. SPE-94863 presented at the SPE Latin American and Caribbean Petroleum Engineering Conference held in Rio de Janeiro, Brazil, 20 – 23 June 2005.

(3) Branco, C.C.M., Pinto, A.C.C., Tinoco, P.M.B., Vieira, P.M.F., Sayd, A.M..S., Santos, R.L.A., Prais, F.: “The Role of the Value of Information and Long Horizontal Wells in the Appraisal and Development Studies of a Brazilian Offshore Heavy Oil Reservoir”. SPE-97846 presented at the 2005 SPE International Thermal Operations and Heavy Oil Symposium held in Calgary, Alberta, Canada, 1–3 November 2005.

Figure 1. Water cut history match.