The French Committee of Technicians has modified the "Terminology Concerning Concepts of Hydrocarbon Reserves" proposed in 1982 1, says Jean Laherrere of Total Cie. Francaise des Petroles (OGJ, Nov. 21, 1983, p. 58).
The new version makes a clearer differentiation between the two main approaches for defining recoverable hydrocarbons as a function of:
1. current geological and technical knowledge leading to the classifying of hydrocarbons in place2 that have been identified3 and that are unidentified, and
2. economic conditions leading to the classifying of hydrocarbons in place by recoverable fractions4, economic fractions5, uneconomic fractions, or fractions in the process of being assessed.
In this classification, all known or calculated quantities of hydrocarbons will be able to be accounted for at any given moment.
Each production scheme considered6 will be the subject of a quantification data sheet. For a given assessment of hydrocarbons in place, the technological progress and-or economic-political changes will move the corresponding quantities from one class to another.
This classification applies to hydrocarbons associated with individual reservoirs as well as to their sum total by geographic units and-or by degree of interest.
CLASSIFICATION
RESERVES OF CRUDE HYDROCARBON
The reserves on a given date are the quantities of identified, recoverable, and economic hydrocarbons remaining to be produced. They are subdivided into three categories: proven, probable, and possible.
Proven reserves are those that have a probability of existence greater than 85-95% (reasonable certainty) and for which the capability of being produced is demonstrated.
Probable reserves are the quantities added to proven reserves that extend the overall probability of existence to more than 50% (medium to good probability). The additional reserves obtained can be considered as probable either by attributing less restrictive values than to the petrophysical parameters to the geometry of the reservoir and to the position of unrecognized interfaces or by applying more favorable hypotheses concerning the effectiveness of the production methods retained.
Possible reserves are the quantities added to proven and probable reserves that extend the probability of overall existence to more than 5-15% (low probability). Reserves considered possible are those liable to be had by applying the most favorable hypotheses for petrophysical parameters to the geometry of the reservoir, the situation of interfaces, and the effectiveness of the production methods retained.
Mainly due to tax and legal reasons, some countries require division between developed and undeveloped reserves. For some countries the developed reserves are those liable to be recovered by existing production processes. For others, for instance, development has reached a no-return stage.
IDENTIFIED RECOVERABLE HYDROCARBON BEING ASSESSED
The decision to develop has not yet been made pending the results of technico-economic surveys.
IDENTIFIED UNECONOMIC RECOVERABLE HYDROCARBON
These are technically producible, but their production is not economic at the time of assessment.
POTENTIAL HYDROCARBON RESOURCES
These are the estimated quantities of unidentified hydrocarbons assumed recoverable and can be classed as hypothetical or speculative.
Presence of hypothetical potential resources in a basin is presumed in formations containing already identified impregnated reservoirs. Their probability of existence depends on the degree of knowledge of the formation considered.
Presence of speculative potential resources is presumed in a structural and sedimentary environment in which no impregnated reservoir has yet been identified. Their probability of existence is low, and the quantities are computed by analogies to hydrocarbon bearing basins.
TOTAL OF RESERVES
There are several ways to determine the sum total by category of reserves (proven, probable, possible):
- The deterministic method consists in calculating the reserves in the same category as being the sum of the reserves in the different reservoirs in this category.
- The probabilistic method deduces the categories of reserves by applying to the probabilistic sum total of reserves the probability thresholds corresponding to the definition of each category.
This method makes various assumptions as to the degree of linkage (in the probabilistic sense) between the uncertainties for the reserves in different reservoirs.
These assumptions, which have great influence on the final result, will have to be clearly defined.
Likewise, it is strongly recommended that these assumed reserves (hydrocarbons being assessed and uneconomic hydrocarbons) not be added to the total reserves.
- It is standard practice to figure the total reserves in a field by the mathematical expectation (R) of these reserves, or the mode, or P50 (when a distribution function is available) or by similar simplified functions such as:
R = 100% proven + 50% probable + 25% possible or
R = 100% proven + 67% probable + 33% possible
The mathematical expectation for a group of fields or potential resources is the aggregate of the mathematical expectation for each one of them.
Aggregates may include the portion of hydrocarbons calculated as being economic in the reservoirs being assessed (identified recoverable hydrocarbons being assessed).
DEFINITIONS
- Chambre Syndicate, limited edition, February 1982. Petrole et Techniques No. 287, pp. 5-9, March 1982.
- Hydrocarbons in place include all hydrocarbon components in place, liquid and-or gaseous, contained in an impregnated reservoir and expressed under reference surface conditions.
- Impregnated reservoirs are said to be identified if they are recognized by at least one borehole.
- Recoverable is the fraction of hydrocarbons in place that can be produced by the production process retained. Methods of recovery are traditional of three types: Primary recovery implements only the natural energy of the impregnated reservoir and of its aquifer, without excluding the possibility of stimulation (acidizing, fracturing) and-or activation (pumping, gas lift). Improved recovery increases the natural energy by injecting fluids (water, immiscible gas, etc.) without making any major change in the physicochemical properties of the fluids present. Enhanced recovery makes a significant change in the physicochemical state of at least part of the fluids present (injection of water to which miscible agents have been added, thermal methods, etc.). In practice, the differentiation between the types of recovery is more and more difficult, with the second one in particular often being applied very early.
- Economics applies to the recoverable quantities of hydrocarbons for which the decision in principle has been made to produce. This decision implies a sound assessment of the work to be undertaken, the market conditions, and the constraints of legislation. Likewise, economic criteria may be different for the same field depending on whether the assessment was made by the operator, the partners, the financing organization, or by the granting authority.
- Production scheme covers a single or evolutive process (natural depletion, gas flooding, etc.) as well as spacing pattern and surface installations.
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