Engineering Applications of Phase Behavior of Crude Oil and Condensate Systems (includes associated papers 16046, 16177, 16390, 16440, 19214 and 19893 )
Authors: Moses Phillip L. 
Published: December 2007

Distinguished Author Series articles are general, descriptiverepresentations that summarize the state of the art in an area of technology bydescribing recent developments for readers who are not specialists in thetopics discussed. Written by individuals recognized as experts in the area, these articles provide key references to more definitive work and presentspecific details only to illustrate the technology. Purpose: to informthe general readership of recent advances in various areas of petroleumengineering.

Fluid samples must be taken early in the life of a reservoir toobtain samples truly representative of the reservoir fluid. They should betaken only after a carefully planned well conditioning and testing program. When the PVT data obtained from these samples are used, care should be taken toadjust FVF's and gas/oil ratios (GOR's) for surface separator conditions.

Introduction

The proper development, engineering, and production of an oil or gasreservoir requires a considerable amount of planning. At the same time thatplans are formulated to develop the field, plans should also be made for adata-gathering program to facilitate reservoir engineering months and evenyears into the future. This data-gathering plan should include a sufficientnumber and variety of electric logs and cores on key wells to describe thereservoir adequately. Electric logs and core analyses evaluate the reservoirrock. If reservoir engineering calculations are to be made to optimizeproduction from a reservoir, including EOR, then the properties of thereservoir fluids must also be known. The properties of the reservoir water fallwithin narrow ranges and are seldom studied at reservoir pressures andtemperatures. The properties of the reservoir water determined are normallyconfined to chemical analysis and possibly compatibility tests in cases ofinjection projects. This paper is concerned primarily with the study of thehydrocarbon fluids contained in a reservoir. Coring and logging programs shouldcontinue throughout the development of a reservoir. The data obtained from thelast well drilled are as valuable as the data obtained from the first well. This is usually not the case for reservoir fluids. Samples representative ofthe original reservoir can be obtained only when the reservoir pressure isequal to or higher than the original bubblepoint or dewpoint. Methods areavailable for extrapolating fluid data obtained after some pressure decline. Remember, however, that extrapolation is an educated man's word for guessing. Plans for obtaining reservoir fluid samples and analyses should be made earlyin the life of a reservoir. Reservoir fluid samples should be taken beforesignificant reservoir pressure decline has been experienced.

Oil Reservoirs

Oil reservoirs can be divided into two categories: ordinary oil reservoirsand near-critical reservoirs. The ordinary oil reservoirs are sometimes calledblack-oil reservoirs. This misnomer does not reflect the color of the reservoirfluids. It is meant only to distinguish them from near-critical oil reservoirfluids. The near-critical fluids will be discussed in subsequent paragraphs. Ordinary oils are characterized by GOR's up to approximately 2,000 ft3/bbl [360m3/m3], oil gravities up to 45 degrees API [0.8 g/cm3], and FVF's of less than2 bbl/bbl [2 m3/m3]. Remember that there is no sharp dividing line between anordinary and a near-critical oil. Such factors as composition and reservoirtemperature greatly influence the behavior of the reservoir fluid. It is oftenimpossible to determine whether a fluid should be studied as a near-criticaloil or as an ordinary oil until it is actually in the laboratory beingobserved.

JPT

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