4.1 Introduction

Can you export for me the vshale (VSH), porosity, water saturation (SW) and permeability (PERM) logs? Thanks in advance for your help!

Such request is too often the only type of collaboration between petrophysicists and geomodelers. Of course, it’s not true in every company nor every project, but it does happen more often than it should. Geologists, geophysicists and geomodelers can talk for days, if not weeks, about the structure, the stratigraphy and the facies distribution. But we tend to order logs the way we order take-away at a restaurant: we want our order fast, we want it in LAS format and we don’t really care about how the logs were cooked in the kitchen.

That’s unfortunate. All the properties that we need to model in 3D (porosity, SW…) are some expression of the rocks’ and fluids’ characteristics such as pore size, mineralogy and/or fluid composition. They are here to quantify how the rocks will behave when we attempt to produce and all these properties that we need to model are inter-connected one to the other to some degree. A good petrophysical analysis will properly capture the correlations between these different reservoir properties. As geomodelers, we must also respect these correlations when we model petrophysical logs in 3D. For example, if a rock’s characteristics are such that low porosity values are necessarily correlated to low permeability and high water saturation, we must make sure that we respect this relationship in every cell of our 3D grid. We don’t want to have cells with a mix of interpolated low porosity values, high permeability and low water saturation. It would contradict the nature of the reservoir and it might jeopardize the analysis engineers will derive from our models.

As geomodelers, we tend sometimes to consider that the challenges of the geomodeling process are building the grid and populating facies. Once this process is properly done, populating petrophysics is “easy”. The aim of this chapter is to show that an appropriate amount of time must in fact also be dedicated to distributing the petrophysical analysis in 3D correctly, and in a way that respects the nature of the rocks. This goal is easier to achieve if we keep our petrophysicists in the loop instead of trying to do it on our own.

Firstly, this chapter reviews some basic definition of VSH, porosity, SW and permeability before looking at how the logs are being upscaled (blocked) into the 3D grid. It then looks at a question sometimes asked: will it really change something if I was just to model my logs in 3D without losing time modeling facies? At last, 3D petrophysical modeling itself is covered as well as some aspects of integrating petrophysical uncertainties into the geomodel.

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