Abstract

The Fault Block Model is an alternative simulation tool, developed with the purpose of forecasting well requirement and reservoir performance in strongly faulted gas and gas condensate reservoirs. This model is particularly designed to handle reservoirs where the uncertainty related to gas communication between the various parts or compartment blocks of the field, is pronounced and thought to be the dominant source of reservoir uncertainty. The model can predict reservoir production behavior with reference to a reservoir data basis, as to display the reservoir uncertainty with respect to business opportunities and risks (upside and downside forecast).

Based on geological characterization and fault seal analysis, reservoir compartmentalization is implemented. The model forms drainage volumes based on a statistical evaluation of the communication probability between neighboring compartment blocks. Uncertainty analysis is carried out using error propagation techniques, forming an optimistic and a pessimistic view of the reservoir. Production is simulated by material balance calculation technique, producing the reservoir reserves through wells allocated to different drainage volumes.

The Fault Block Model has proven to be an advantageous tool in the early stage field development of a North Sea gas-condensate reservoir where the effect of regional well location, number of wells and the optimum well production sequence have been studied. Production profiles from different well location strategies are investigated and various tests involving uncertainty in inter-block communication and in block volume are presented. Uncertainty analysis also includes suggestions on how to reduce reservoir uncertainty and recommendation for an optimum well location strategy.

Introduction

A classic dilemma in starting up many petroleum reservoir development projects is the restricted access of good and reliable reservoir information, and concurrently, the demand for strategic, well-based and balanced decisions regarding future optimization and reservoir development. At such an early phase in the development process of a new field, lack of reliable reservoir data makes it difficult to forecast future production scenarios, revealing potential financial risks and/or opportunities for enhanced production from the field.

In strongly faulted reservoirs, this dilemma appears to be even more pronounced since communication between reservoir segments in the field are strongly related to the faults effectiveness as barrier for fluid flow. Faults generally reduce the communication in the field and consequently increase compartmentalization of reserves. Reduced communication between reservoir segments and related uncertainty makes it more difficult to assess an optimal drainage strategy, defining well locations and sequence of well production.

Alternative Reservoir Models

The accumulation of reliable reservoir information is considered to monotonically increase with time, as data from field seismic is processed, new appraisal wells drilled and test well production data is being analyzed. Reservoir information, as depicted in Fig. 1, is further increased as more wells are drilled and set into production. By far the most information is obtained by long time production data analysis. A comfortable level of reservoir knowledge is unfortunately achieved late in the lifetime of field production, when the need of such information is not urgent since practically all reserves have nearly been produced.

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