Abstract

Economic and risk analysis are important tasks in a petroleum engineering study. This work considers the analysis of the results obtained from a real Brazilian offshore field simulation study in which the decision making process for field development is discussed considering geomechanical aspects. The focus is on the evaluation of uncertainties of geomechanical parameters that may influence the response of a petroleum reservoir submitted to seawater injection for secondary recovery. A decision tree has been built and solved using utility theory and the expected monetary value (EMV) concept. A sensibility analysis for different discount rates and oil prices has been also performed.

First, this work identifies if the reservoir is a candidate for a deeper analysis using a matrix approach consisting of important geomechanical parameters. Second, lab stress information is used and the results obtained compared with the traditional reservoir simulation, which considers compressibility invariant with time. Third, a decision tree is constructed and solved. Based on the assumptions chosen for these analyses, it is concluded that coupled reservoirgeomechanics simulations should be applied in order to more precisely forecast reservoir performance.

Introduction

The application of geomechanics to reservoir engineering problems is relative new and is receiving more focus as the oil and gas industry begins to exploit unconventional hyrdrocarbon assets. The relative importance of geomechanics in this context, however, remains a question.

In this work, a new methodology is developed which enables, through a comprehensive listing of some major geomechanical parameters, the establishment of the important mechanisms associated with these parameters and a method for assessing their relative importance. This methodology is demonstrated by the use of traditional simulation and geomechanical approach in an economics analysis using a decision tree approach to obtain the critical parameters.

The focus for this study is an offshore unconsolidated sandstone reservoir field located in Campos Basin, Brazil. The field has undergone water injection since the beginning of production and was chosen for this study due to strong uncertainties related to project's development. As is typical, the amount of information is incomplete in the initial phase of a project and a sensitivity analysis must be done to justify the acquisition of additional information and to determine whether a simple black-oil model will be sufficient to assist with development decisions.

As mentioned by Onaisi et al.1, flood breakthrough leads to local effective stress changes and compressibility or volume changes. The addition of these stress-dependent effects into traditional reservoir simulation parameters (e.g.: porosity, rock compressibility and permeability) show that they can impact reservoir productivity and fault transmissibility. For the Brazilian offshore field, an important issue is the possibility of fluid leaking from the reservoir to the seabed due to flow along existing fault structures. The determination of rock mechanics parameters in unconsolidated sandstones is also an important step to better characterize the rock, providing important inputs for better forecasts.

Soares and Ferreira2 noted that in some reservoirs large deformations could occur during oil production. These deformations result from increasing effective stresses as the reservoir fluid pressure declines. As the rock structure changes, it may cause permanent damage to the reservoir lowering the permeability and the final oil recovery.

In general, reservoir engineering studies do not take into account the effect of in situ stresses on production. In most cases, the permeability is assumed to remain constant along the oil production. However, for limestone and unconsolidated sandstone, as in the deepwater reservoirs of Campos Basin, Brazil, this assumption is likely not valid and a better understanding of reservoir-geomechanical processes will help optimize the ultimate recovery from these classes of reservoirs.

The Field " A"

The Field "A" is located southeast of Brazil in a water depth of 1,500m (Figure 1). The real oilfield name and exact position is not revealed due to confidentiality aspects. The reservoir model built to depict the field is shown in Figure 2.

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