Archive

No. 4, 2011

Sergey Ostroukhov ,
Senior Research Fellow of LUKOIL-Engineering,
Vitaly Bochkarev, Director of the Department of Reserve Management and Auditing Of LUKOIL

INEXHAUSTIBLE RESOURCES


The renewability of oil and gas field reserves is an aspect of nature with which oil companies must reckon

SSCIENTIFIC ARGUMENTS OVER THE RENEWABILITY of oil reserves have a long history. For many years, it was thought the process of restoring reserves of hydrocarbon deposits after their intensive exploitation would take hundreds of years. However, the experience of scientific research has shown that some fields are capable of renewing their reserves much faster. According to the concept of the two-phase generation of hydrocarbons, developed by the authors of this article, renewable fields can be divided into three categories: quickly refilling, moderately refilling, and slowly refilling. At quickly refilling fields, so-called lower gas condensate takes the place of the pumped oil in a matter of several months; at moderately refilling fields, this takes decades. At slowly refilling fields, according to preliminary estimates, the process takes 50 years and more. Practical realization of the concept could lead to major changes in present-day approaches to oil production.

The "oil phoenix"

Most of us were brought up to believe that oil is an exhaustible and non-renewable source of energy, and the significant repeated lag behind growing world production in the adequate growth of oil reserves, regardless of the latest cutting-edge exploration and survey technologies, eloquently confirms this thesis. Alternative sources of energy are actively being developed and put to use. In such seemingly hopeless circumstances, however, oil- and gasmen are increasingly daring to announce that oil and gas resources contain entirely realistic sources for their own economically advantageous renewal.

Works dedicated to the sustainability of the reserves of exploited oil and gas deposits regularly appear on the pages of scientific publications. The most important of these (by quantity of additional oil reserves) are devoted to the Romashkinskoye field. Of no less interest are the reports on the unusual development of fields on the territory of the Chechen Republic, where a huge volume of empirical statistical material on actual processes of oil reserve renewal has been accumulated over the 100 years of their operation. It has been noted in particular that after a period of enforced idleness, a number of producing wells whose initial output had declined along with the high and rising pressures of their formations and whose produce was water cut were able to restore their original parameters in three or four years. The list of such examples in different parts of the planet runs into the hundreds, indicating a systematic and regular natural process that is everywhere well-developed. This largely unstudied phenomenon in turn demands the development of theoretical grounds for the formation and reformation of deposits with renewable oil and gas reserves, and of the mechanisms for the uninterrupted accumulation of hydrocarbons in already filled traps.

At present, data on this phenomenon are still being gathered. In view of the lack of definitive theoretical, methodological, and practical developments, oil and gas companies are still producing hydrocarbons from unknown sources without consideration for their origin or how long they will be able to continue. Any rise in oil output is ascribed to imperfect assessment of the original geological reserves. A dead-end situation attains when there is already nothing further to squeeze out of the calculated parameters.

This phenomenon could be explained by involving experts in different fields. At the outset, however, a combined geological and geochemical approach would be especially effective. Using our material on the western side of the Caspian Depression and the waters of the Central Caspian, it is quite possible to establish one very important rule: all oil and gas fields are divided into two groups. The first of these is fields with renewable reserves; the second is fields with non-renewable reserves. This and all subsequent rules were established by simply analyzing the conditions of the formation and the patterns of the spatial distribution of hydrocarbon deposits.

Oil renewed by gas condensate

Since there are no super-deep wells, the question of the hydrocarbons' origin (the source of their generation) in the far depths of a sedimentary layer (as claimed by supporters of sedimentary migration genesis) or in crystalline bedrock (as claimed by supporters of the inorganic theory of origin) is most problematic. In the concept for the formation of deposits proposed below, the source of hydrocarbon generation is found in the deep-lying sediments of the Caspian Depression (west side) and the Tersko-Caspian Trench (the sea basin). In both, in proportion to the intrusion of genesis rock into the main zones of oil and gas formation, uninterrupted processes for the gradual migration of hydrocarbons occur all along the chain of the basic system of generation and accumulation.

 This process is provisionally divided into two stages: a first (oil) stage, in which mainly oil reservoirs are formed, and a second (gas condensate) stage, in which the oil reservoirs are gradually transformed into gas condensate reservoirs. The oil reservoirs formed in the first stage are replenished and then fully replaced by the products of the next stages of generation: gas condensate and gases. This process leads to the oil reservoirs gradually being replaced by gas condensate concentrations throughout the formation as the depth increases, and the gas condensate concentrations are themselves replaced with deposits of dry methane gas. This is expressed in a specific redistribution of hydrocarbons within their main classes. The effect of the gas flow thus leads to the formation of light oils of gas condensate genesis, secondary condensates, intermediate oils of gas condensate genesis, and residual oils. The continuity (smoothness) of this process results in a wide range of oil and gas condensate solutions of various proportions in the zones of accumulation.

Fields' reserves become renewable in a continuous process of hydrocarbon entrapment at the first and second stages. The deposits are replenished with hydrocarbons both as a result of their recovery and before the recovery of oil, due to the slow process of changing the composition and properties of the oil itself through large quantities of arriving condensate being dissolved in it.

The following characteristic of the division of fields and reservoirs with renewable and non-renewable reserves is associated with the second stage. If the fields (or reservoirs) are in a section of the continuous movement of hydrocarbons along the path of migration (starting from the zone of generation), they are provided with an inflow of more and more portions of hydrocarbons (i.e., they are renewable with the acquisition of desirable commercial characteristics of oils). So long as they have an active connection to the present zone of hydrocarbon generation, such fields will be fed by new, lighter hydrocarbons in the course of their exhaustion until their reservoirs of oil are completely replaced with reservoirs of gas condensate.

If the fields (deposits) at the second stage are for some reason cut off from earlier regular paths of oil migration, their reserves are not renewable. One obvious example of this is the oil reservoirs in the Tithonian, Kimmeridgian, Oxfordian, and underlying Jurassic deposits at the 170 km and Khvalynskoye field in the Central Caspian. In the oil reservoirs of these fields that were cut off from hydrocarbon sources (they contain no traces of gas condensate) the process of their degeneration continues to this day (the aging of remnant aggregations of heavy oil in the stagnant zone).

Rates of replenishment

The classification of fields into renewable and non-renewable according to the composition of their fluids allows us on the basis of geochemical studies to assess the degree of a field's (deposit's) replenishment with additional volumes of hydrocarbons. Another characteristic of the division of fields with renewable reserves is the relation between the rate of replenishment with new amounts of trapped hydrocarbons and the field's distance from the zone of generation.

According to this relation, the nearer the field to the zone of generation, the faster the process of restoration. At the Alekseyevskoye field (on the western side of the Caspian Depression), the zone of generation lies under the field; according to a published scheme (a deposit - a zone of fractured rock in the fault plane of a dislocation, i.e., a source of hydrocarbon gas generation), hydrocarbons are supplied to producing deposits in a matter of months, and the oil reservoirs become gas condensate reservoirs with renewable reserves.

In the waters of the Caspian Sea, the zone of generation is a considerable distance away. There are thus several paths of oil migration to the Rakushechno-Shirotny zone of oil and gas collection: vertical migration (along the fracture zones of faults and lateral faults) totals from 780 to 3020 meters (in the Khvalynskoye and Promyslovskoye fields, respectively), and lateral migration totals from 60 to 190 kilometers (in the Khvalynskoye and Promyslovskoye fields, respectively). The processes of deposit renewal here are in direct dependence on the intensity of the fields' development. The relatively thin bank of oil (20 meters in the Upper Jurassic and Neocomian) being developed at present at the Yu. Korchagin field could disappear in the first years of development, and the classification of the field could change from oil and gas condensate to the category of gas condensate.

Our analysis of the gradual formation of reservoirs and the consistently changing ratios of oil and liquid-phase hydrocarbon condensate in renewable fields allows us to establish the patterns of the spatial distribution of hydrocarbon deposits in the region and to conduct directed searches for fields of certain phase states and specific physiochemical properties of oils (condensates).

Many fields have the potential for renewing their reserves, and the production of oil and gas condensate from renewable reserves entails relatively low additional costs. The renewability of fields is thus one factor that must be considered and carefully weighed in the course of their development.




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Oil of Russia, No. 4, 2011
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