Uncertainty Analysis in the Seismic to Reservoir Simulation Workflow for a New Offshore Heavy Oil Development
Authors: Sahni A. , Swain J. , Merrell J. , Abriel W. , Bluhm C. , Unser S. , Lin T. , Wei M. 
Published: March 2010

This paper presents the work of a multi-disciplinary team of earth scientists and engineers to create a development plan for an offshore heavy oil field while accounting for uncertainties in the geological and reservoir simulation models. The field is located in shallow waters and has oil ranging in viscosities from 20 cp to 200 cp. The depositional environment is a fluvial meandering river system where the sands are relatively thin and disconnected. The field is penetrated by a good spread of appraisal wells, is covered by 3D seismic data, and has extensive measurements of rock-fluid properties and geochemistry analysis. Some of the appraisal wells also have good quality DST and MDT data which was used to constrain the geological model.

The workflow is comprised of constructing a geological model using deterministic geobodies identified from interpretive analysis of seismic relative acoustic impedance coupled with stochastically defined sands in areas of poor seismic quality or below the threshold of seismic resolution. The distribution and density of these stochastic sands were tied to well control and guided by stratigraphic layering statistics using a truncated Gaussian simulation technique. Results of both techniques were merged into one final model, embedded into a shale background facies.

Various field development scenarios were considered, including inverted five-spot and nine-spot pattern waterfloods, along with the application of horizontal wells. Development scenarios and predictions incorporated uncertainties in wateroil contacts, net-to-gross ratios, oil viscosities, relative permeability, residual oil saturations, injectivity, and skin factors. The use of experimental design techniques and parallel simulation enabled the team to capture the range of uncertainty in subsurface and engineering parameters while keeping computational times manageable. The paper discusses some of the key uncertainties and challenges in bringing this and similar reservoirs to development.

Introduction

The ability to make quick yet intelligent and value-added decisions to develop new fields has always been of great significance. In situations where the capital expenses and subsurface risk are high, carefully analyzing the inherent uncertainties in the reservoir and how they impact the predicted production becomes a daunting task. The problem is compounded in offshore environments, especially in the presence of heavy oils and disconnected sands, where the margin for error is small. In many cases, realizing the value of a prospect is very time-dependent and resolving uncertainties of complex reservoir systems needs a systematic and efficient approach. This paper presents the case study of the uncertainty analysis workflow for a new offshore heavy oil reservoir for which a development decision had to be made in a short timeframe (a few months).

The field is located in shallow waters and is penetrated by 11 exploration/appraisal wells. The depositional environment is predominantly a fluvial meandering river system, where the sands are relatively thin (5 – 15 m in thickness) and disconnected. All the appraisal wells have good quality log data and several attempts were made to tie reservoir zones to 3D seismic attributes.