Goals for Formation Evaluation

A goal is something presently out of reach; it is something to strive for, to move toward, or to become. It is an aim or purpose so stated that it excites the imagination and gives people something they want to work for, something they don't yet know how to do, something they can be proud of when they achieve it. Some Basic Thoughts and Definitions For the purposes of this article, formation evaluation is defined as "the determination of the economic worth of a natural resource occurring in the subsurface". In this sense, the early-day mining assayer can be thought of as a specialist in formation evaluation, inasmuch as his responsibility was to determine the worth of an ore body. This responsibility was usually executed by making direct measurements of the desired rock and mineral properties on a sample assumed to be representative of the ore body. The in-place value of a petroleum reservoir is defined by its primary properties: its areal extent and thickness, its pore size distribution (fractional porosity and permeability), its fluid distribution (the fraction of porosity saturated with hydrocarbons and the fraction porosity saturated with hydrocarbons and the fraction of that hydrocarbon saturation that is recoverable), and the composition of the hydrocarbon fluids. Its economic worth is determined by relating this asset value to factors such as the capital investments, operating difficulties and expense, and time patterns of income production required for exploitation of the asset. Unfortunately for the assayer's latter-day counterpart in the petroleum industry, the petroleum reservoir is generally not so accessible as a mineable ore body. In fact, the formation evaluation specialist must often rely on measurements of petroleum reservoir properties from samples buried in the subsurface as properties from samples buried in the subsurface as much as 4 miles beneath his feet. To further complicate matters, the primary properties of a reservoir can seldom be reliable or economically measured. The nature of a petroleum reservoir rock and its contained fluids dictates that secondary properties of the reservoir must usually be those measured, and the desired primary properties must be inferred from such primary properties must be inferred from such measurements. An intelligent system of formation evaluation requires a complete understanding of the primary reservoir properties and the relationships among them, then an understanding of the secondary reservoir properties and the relationships both among them and with the primary properties. Next, the methods used to measure primary properties. Next, the methods used to measure the properties - the data-gathering phase, the core and fluid analysis and well logging aspects of formation evaluation - must be thoroughly understood. Finally, the interpretation methods used in formation evaluation must be mastered. We propose to briefly review here the current level of understanding in most of these aspects of formation evaluation and to comment on how future investment opportunities will face the technologist with future challenges to expand and apply this understanding. Primary Reservoir Properties Primary Reservoir Properties Twenty to thirty years from now, petroleum engineers will be able to provide better estimates of the spatial distribution of the primary reservoir properties. P. 55