The Santos Basin Pre-Salt Cluster (SBPSC), Offshore Southeast Brazil, is a unique scenario, posing great development challenges. The microbial carbonate reservoir is unusual regarding its origin and petrophysical properties; the fluids have a variable CO2 content; the few analogue reservoirs around the world do not compare in terms of volumes, water depth and distance to the coast; and there are also flow assurance issues. Considering the importance of these reserves for the Brazilian economy and the opportunity to accelerate cash flow, Petrobras and its partners have opted for a fast track development, including extended well tests (EWTs) and production pilots. The current Petrobras Business Plan (2011–15) foresees that the SBPSC areas alone will produce over 500,000 boe/d in 2015 and over 1,100,000 boe/d in 2020. These numbers refer only to Petrobras share and do not include the transfer of rights with economic compensation from the Brazilian government to Petrobras. Therefore, the initial development phase will have to cope with several uncertainties, mainly the subsurface ones. Some of the most relevant are the quality and the heterogeneity degree of each reservoir zone; the compositional grading of the fluids; the performance of different EOR methods; and the presence of fractures affecting the flow. How to specify and anticipate the acquisition of expensive equipment, such as FPSOs and subsea devices, with uncertainties to be clarified? When is it worth to invest in more data acquisition, such as EWTs, core and fluid sampling, extensive lab analysis or even more appraisal wells? The timing and the uncertainty reduction foreseen for each initiative must be taken into account. On the other hand, when is it better to pay for extra flexibilities, accepting the inevitable CAPEX increase? Some examples would be: smart completions and possibility to inject different chemical products in the wells; gas and water separated lines for each satellite injector; flexible subsea layout, allowing multiples strategies and the addition of more wells; FPSO plants designed to inject desulphated water, or to export, import or reinject the gas, and also to separate variable CO2 contents in the produced fluids. This paper aims to discuss the influence of the main subsurface uncertainties in the selection of alternatives to develop the giant fields in the SBPSC, in a fast track way.

This work presents a validation of the use of Experimental Design (ED) techniques in exploratory evaluations. The results generated by a commercial software, which uses ED techniques combined with the Monte Carlo method, have been compared with 10,000 equivalent actual flow simulations. Several oil and gas real reservoirs were studied. The static and dynamic variables were selected from real prospect studies, as well as their respective range of variation. For each case studied, the commercial software planned a small number of flow simulations to run, applying different ED techniques. Based on the simulation results, a proxy model was generated using the Response Surface Method (RSM). Making use of this proxy model, it was possible to quickly simulate 10,000 experiments and to perform statistics calculations with the Monte Carlo method. In order to verify the quality of the proxy, 10,000 simulation files were generated, with the exact same combinations of parameters used in the Monte Carlo method. A script was written to run automatically all the files and to register their results. Besides comparing the statistics generated by these two methodologies, it was possible to compare the actual results of each one of the 10,000 simulations. The results indicate that the quadratic technique is the one with the best cost-benefit ratio and suggest that the proposed methodology generates reliable results in most cases. This reliability decreases when there are complexities in the flow model, such as well and group controls, water and/or gas reinjection, etc. However, results show that the range of possible results to be emulated by the proxy model is the most influential parameter in the quality of the results generated by it. The quality of the risk analysis performed with these ED techniques is obviously influenced by the ability of the proxy model to emulate the flow simulator response. The efficiency of different ED techniques, in different reservoirs, was analyzed and the reliability and limitations of this evaluation methodology has been accessed. Introduction In the evaluation of exploratory prospects in the petroleum industry, one must deal with a large number of variables, each one of them with a certain degree of uncertainty. Therefore, probabilistic approaches have long been used in order to quantify the impact of those uncertainties in the economic evaluation of these prospects. Nowadays, exploration studies have already incorporated an uncertainty analysis in the most influential static variables, but they still tend to translate all the dynamic uncertainties into a single variable (usually the recovery factor). From this point on, simple programs or even electronic spreadsheets are used to generate production profiles, based on the estimated accumulated production. The authors hereby recommend using flow simulators together with experimental design techniques and response surface methods, to have a better understanding of the impact of dynamic uncertainties in the prediction of both the accumulated production and the production profile. The experimental design technique is used to plan a certain number of flow simulations and to try to build a response surface, i.e., a proxy model of the objective-function being studied. With this proxy model, it is possible to run thousands of simulations almost instantaneously and perform a risk analysis using the Monte Carlo method.1,2 The quality of the risk analysis will be greatly influenced by the ability of the proxy model to emulate the flow simulator's response.3 The authors have studied three different cases: a non-associated gas reservoir and two oil reservoirs, and compared the 10,000 results of the proxy model used to make the risk analysis and 10,000 actual flow simulations. By doing that, it is possible to compare the efficiency of different experimental design techniques and to access the reliability and the limitations of the evaluation methodology proposed by the authors.

This paper addresses the innovative appraisal strategy applied to the Libra project; located in ultra-deep waters offshore Brazil. It details the key role of the Extended Well Test (EWT) Program, within the field overall Risk Mitigation Plan, as well as its interfaces with additional appraisal activities. The Value of Information (VoI) for the main acquired data is described, highlighting the associated impacts for the full field development and validation of the enhanced recovery strategy. A case study approach details how the whole EWT project maximized the acquired information, mainly from a reservoir point of view. Although the EWT approach is not new to Petrobras in the offshore environment, this is the first one with simultaneous oil production and gas reinjection. Several reasons justify the use of the industry’s first dedicated offshore EWT system with this capability. Gathering data on the main dynamic parameters of the field was critical to speed-up the development, with an acceptable risk level. The incorporation of these data in the reservoir models and the impacts in the most relevant development decisions are also described. The chosen methodology brought many opportunities, as well as challenges to interpret the data and to incorporate them in the reservoir models. Furthermore, the capability to produce without continuous gas flaring makes it possible to apply such approach anywhere else in the world.

fax 01-972-952-9435. AbstractThis work presents a validation of the use of Experimental Design (ED) techniques in exploratory evaluations. The results generated by a commercial software, which uses ED techniques combined with the Monte Carlo method, have been compared with 10,000 equivalent actual flow simulations. Several oil and gas real reservoirs were studied. The static and dynamic variables were selected from real prospect studies, as well as their respective range of variation. For each case studied, the commercial software planned a small number of flow simulations to run, applying different ED techniques. Based on the simulation results, a proxy model was generated using the Response Surface Method (RSM). Making use of this proxy model, it was possible to quickly simulate 10,000 experiments and to perform statistics calculations with the Monte Carlo method.In order to verify the quality of the proxy, 10,000 simulation files were generated, with the exact same combinations of parameters used in the Monte Carlo method. A script was written to run automatically all the files and to register their results. Besides comparing the statistics generated by these two methodologies, it was possible to compare the actual results of each one of the 10,000 simulations.The results indicate that the quadratic technique is the one with the best cost-benefit ratio and suggest that the proposed methodology generates reliable results in most cases. This reliability decreases when there are complexities in the flow model, such as well and group controls, water and/or gas reinjection, etc. However, results show that the range of possible results to be emulated by the proxy model is the most influential parameter in the quality of the results generated by it.The quality of the risk analysis performed with these ED techniques is obviously influenced by the ability of the proxy model to emulate the flow simulator response. The efficiency of different ED techniques, in different reservoirs, was analyzed and the reliability and limitations of this evaluation methodology has been accessed.