Abstract

The process of estimating oil and gas recoveries for producing field continues throughout the life of the field. There is always uncertainty in making such estimates, the level of uncertainty is affected mainly by the Quantity and quality of the geological, engineering, and geophysical data, the magnitude of uncertainty however, decreases with time until the economic limit is reached and the ultimate recovery is realized.

Experimental (ED) design techniques are increasingly been adopted as a standard, to consistently combine the uncertain parameters to construct a range of reservoir models to evaluate recoverable volumes. However, ED technology is a time and resource consuming workflow, requiring a suite of static and dynamic models to be generated and a full history matching process to be applied, however, when a project team is time constrained full ED approach, should be limited to only those reservoirs which have a maturity less than 70% and capture 90% of the impact to total remaining recoverable volumes.

Traditional deterministic simulation methods, calculate ultimate recovery based on the assumption that all input parameters are exactly known, however they sometimes ignore the variability and uncertainty in the input data which creates inherent problems resulting in over pessimistic or optimistic numbers.

This paper details Modelling strategy aimed at achieving a risked based and scalable solution to delivering realistic recoverable volumes for the candidate reservoirs in the planned time frame.

Introduction

Oil and gas volumes evaluation is achievable through any of the following methods: volumetric based on analogues, Performance based Decline curve Analysis, Material balance and Simulation (deterministic or probabilistic).

In the early stages of development of a reservoir, recoverable volumes estimates are restricted to and analogy and volumetric calculations.

The volumetric method, involves determining the areal extent of the reservoir, the rock pore volume, and the fluid content within the pore volume. This provides an estimate of the amount of hydrocarbons-initially-in-place. A recovery factor estimated using analogues is then applied to obtain the recoverable volumes. The analogue technique involves identifying a mature reservoir with similar structural and geological setting as the subject reservoir. The drainage radius and recovery factor from this mature analogue reservoir is then used to calculate recoverable volumes for the relatively greener subject reservoir.

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