A Case Study in Using Compositional Grading to Improve Reservoir Characterization
Authors: James B. , Patience R. 
Published: March 2010

This paper presents a case study of how compositional grading was used as a tool to greatly affect the subsurface interpretation of a deepwater field. Originally, the main reservoir at the field was viewed as being compartmentalized. There were also two viscous low API gravity oil reservoirs that were of little economic interest.

A new interpretation based on compositional grading suggests that the main reservoir is, in fact, much better connected. There is also a risk of a viscous asphaltene-enriched zone at the oil-water contact that was not previously considered. Fortunately, if it exists, it would likely only delay aquifer drive. The two low API gravity oil reservoirs of little economic interest likely have much better quality oil updip making them good appraisal candidates. The different interpretation improves the overall view of the field and has implications for optimal development well locations, initial production rates and facility gas capacity.

Compositional grading is particularly useful in aiding subsurface characterization in areas where grading is strong, and in deepwater offshore developments where uncertainty is great and development and appraisal costs are high. Fundamental geology and geochemistry are critical to being able to recognize a priori areas where compositional grading is strong.

Introduction

Compositional grading of hydrocarbons within a reservoir refers to the gradual and continuous change in fluid chemistry and physical properties such as density, gas-oil ratio and viscosity within the reservoir as a function of spatial location; in particular changes in elevation. Experience has shown that compositional grading is a common phenomenon(1–3).

This paper demonstrates how compositional grading was used at a deepwater field as a valuable tool to aid in subsurface characterization. Such a tool is particularly useful in deepwater where development costs can be on the order of a billion dollars, uncertainty is great and appraisal costs are high.

Yet, despite its common occurrence and potential use in subsurface characterization, compositional grading is poorly recognized in general and little used in practice to aid in characterization. For example, in the recent past, often only a single fluid sample would be taken in a reservoir. The fluid properties from this single sample would then be applied uniformly throughout a reservoir. Even when grading is recognized, there is often a belief among many practicing reservoir engineers that grading can be easily predicted using commercially available thermodynamic software. We discuss below how this can be misleading.

Whilst grading occurs in both gas condensate and oil fields, in this paper we focus on grading in black oil columns; in particular in gas-oil ratio and in situ density.

We first present background information on the theory of compositional grading and its practical manifestation including one of its by-products; namely asphaltene-enriched zones. We then discuss the case history. We feel that one of the keys to understanding compositional grading and using it in subsurface characterization is in the integration of a wide range of information, particularly from reservoir engineering, geochemistry and geology.