7.1 Introduction

Computing volumes of hydrocarbons in the reservoirs is an essential task of any asset team. Nowadays, many companies are using geomodels to do these evaluations. After all, considering a geomodel is meant to capture all our data, our knowledge and our assumptions about our reservoirs, it’s natural to use it for volume computations. After flow simulation, covered in the previous part of this series, running volumetrics is another important way in which engineers are using geomodels.

The first section introduces the volumetric equation.

The industry didn’t wait for geomodels to come along to develop workflows to compute volumes. As such, the arrival of geomodeling might require an asset team to adjust their current volumetric workflow. This is the topic of the second section.

Net-to-gross is an important parameter in volume computations. How should we take it into our geomodeling workflow? Should we model net-to-gross with geostatistical tools? Does it make more sense to recompute the net-to-gross in each cell by applying the cut-offs defining it in the first place? These different questions will be answered in the third section.

A key goal of geomodels is to capture our level of uncertainty about the reservoir characteristics. Several important sources of uncertainty were introduced in the previous chapters, as well as how they can be taken into account in the geomodel. These different uncertainties must be considered when running volumetics. The last section of the present chapter will summarize them.

Taking into account uncertainties will lead to generating a range of volumes instead of a single, deterministic volume. While more and more companies are now used to this probabilistic approach, many others are still not. Geomodelers might face opposition in implementing the full workflow suggested in this chapter. But we think it is at least worth it to have a discussion with your team about it. Hear what the resistance are about and adapt your workflow accordingly.

To simplify the wording, this chapter focuses on oil reservoirs but everything described applies to gas and to oil and gas reservoirs. Also, the focus is on reservoir condition oil-in-place. Lastly, no distinction is made between reserves and resources.