Page 62 - Fundamentals of Reservoir Engineering

CHAPTER 1

SOME BASIC CONCEPTS IN RESERVOIR ENGINEERING

1.1 INTRODUCTION

In the process of illustrating the primary functions of a reservoir engineer, namely, the
estimation of hydrocarbons in place, the calculation of a recovery factor and the
attachment of a time scale to the recovery; this chapter introduces many of the
fundamental concepts in reservoir engineering.

The description of the calculation of oil in place concentrates largely on the
determination of fluid pressure regimes and the problem of locating fluid contacts in the
reservoir. Primary recovery is described in general terms by considering the
significance of the isothermal compressibilities of the reservoir fluids; while the
determination of the recovery factor and attachment of a time scale are illustrated by
describing volumetric gas reservoir engineering. The chapter finishes with a brief
quantitative account of the phase behaviour of multi-component hydrocarbon systems.

1.2 CALCULATION OF HYDROCARBON VOLUMES

Consider a reservoir which is initially filled with liquid oil. The oil volume in the reservoir
(oil in place) is

OIP = Vφ (1 S− wc )(res.vol. ) (1.1)

where V = the net bulk volume of the reservoir rock

φ = the porosity, or volume fraction of the rock which is porous

= the connate or irreducible water saturation and is expressed as a
and S wc
fraction of the pore volume.
The product Vφ is called the pore volume (PV) and is the total volume in the reservoir
which can be occupied by fluids. Similarly, the product Vφ (1−S wc) is called the
hydrocarbon pore volume (HCPV) and is the total reservoir volume which can be filled
with hydrocarbons either oil, gas or both.

The existence of the connate water saturation, which is normally 10−25% (PV), is an
example of a natural phenomenon which is fundamental to the flow of fluids in porous
media. That is, that when one fluid displaces another in a porous medium, the
displaced fluid saturation can never be reduced to zero. This applies provided that the
fluids are immiscible (do not mix) which implies that there is a finite surface tension at
the interface between them.

Thus oil, which is generated in deep source rock, on migrating into a water filled
reservoir trap displaces some, but not all, of the water, resulting in the presence of a
connate water saturation. Since the water is immobile its only influence in reservoir