18. preservation, degradation & destruction of accumulation

18.

Preservation, Degradation &

Destruction of Accumulation

by: Awang Harun Satyana

INDONESIAN PETROLEUM ASSOCIATION (IPA)

REGULAR COURSE, SHERATON LAMPUNG, 26-30 AUGUST 2013

Preservation of Petroleum

Further deformation may breach petroleum traps. In this case,

petroleum will re-migrate to surface becoming HC seeps or to other

traps.

Changes may take place in the physical and chemical properties of

petroleum while it is in the trap. These changes may have an

important impact on the recoverable fraction and commercial value of

HC accumulation.

Processes causing in-reservoir alteration include :

biodegradation

thermal degradation

water washing

CO2 pollution

etc.

Bissada et al. (1992)

Blanc and Connan (1994)

In-Reservoir Alteration

Biodegradation: bacterial alteration of crude oils; attack firstly light n-

alkanes, branched (iso) alkanes, cyclo-alkanes, finally aromatics; occurs

at low reservoir temperatures < 80ºC; need supply of fresh, nutrient-rich,

oxygenated water.

Thermal degradation: thermal cracking of oil into gas. Heavy compounds

are replaced by progressively lighter ones, until only dry gas methane is

present. At high temperatures (> 160 ºC), oil cracking reaction proceed so

rapidly that an oil accumulation may be destroyed within a geologically

short period of time.

Water washing: to occur in association with biodegradation if the

reservoir temperature is too high ( > 70 ºC) or if other condition for

microbial attack are not met. Light alkanes and low boiling point aromatics

(benzene, toluene) are the most soluble and preferentially removed. The

net result is anomalously heavy oil though not biodegraded.


Gas souring: production of H2S in deep, hot carbonate/ evaporite

reservoirs via reaction of methane with gypsum. With increasing

temperature (> 150 ºC), proportion of H2S increases.

CO2 pollution: during diagenesis, fermentation reactions as well as

oxidation by bacteria liberate CO2; during thermal maturation of

organic matter CO2 is liberated via decarboxylation of e.g. fatty acids

or esters in kerogen. Other possibilities, only viable at high

temperatures (> 150 ºC), are thermal decomposition of inorganic

carbonates and out-gassing of the earth’s mantle. Production of CO2

from organic matter is thought to be the most significant mechanism

for contributing CO2 to reservoirs.


Gas deasphalting: a process whereby the precipitation of the

heavy asphalthene compounds in a crude oil takes places as

a result of the injection of light C1-C6 hydrocarbons. This may

occur when an oil accumulation experiences a later gas

charge as its source kitchen becomes highly mature.

Gravity segregation: in high petroleum columns of reservoir,

various hydrocarbons vary systematically with depth due to

gravity. Denser high molecular weight components tend to be

more concentrated at the bottom of the reservoir.

Jobson et al. (1972)

Hunt (1996)

Waples (1985)

Hunt (1996)

Clayton and Fleet (1991)

Biodegradation changes

regular 25-hopane to 25-

norhopane

Palmer (1994)

Phase Changes in Oil and Associated Gas

Retrograde condensation: single-phase dense-gas

system. When the pressures reduced, it converts to a two-

phase system of free gas over oil. Such reservoir have

GORs 3000-150,000 ft3/bbl. The gravities of the separated

liquids usually range from 40-60 ºAPI. Retrograde

behavior is generally observed at pressures above 2500

psi.

Phase Changes in Oil and Associated Gas

Evaporative fractionation: as oil matures, it generates

increasing amounts of gas. In addition, gas from deeper

maturing oils and source rock can pass through a carrier bed or

reservoir, picking up light HC (gas stripping). As reservoir

pressure build up, the trapped gas escapes, carrying with it

some dissolved oil. The oil condenses out at successively

shallower levels up the section as the pressures and

temperatures decrease. This results in the non-biodegraded,

shallower oils having higher API gravities than deeper oils. This

is in contrast to thermal condensates that usually show the

general trend of increasing API gravities at greater depths.

Hunt (1996)

Degradation-Destruction of Accumulation:

Conclusions

Among the factors influencing petroleum composition, secondary

alteration processes that occur after oil entrapment are most important,

as they can lead to considerable changes in both the composition and

quality of the oil.

Knowledge of conditions and mechanisms of degradation processes

are needed for petroleum exploration, not only for economic reasons

but also because multidisciplinary approaches must be developed to

improve already established techniques and to create new techniques.

Presently, molecular chemistry is a useful tool that is able to recognize

incipient biodegradation, to detect oil gravity segregation, and likely to

discriminate a pyrobitumen from a precipitated asphaltene, whereas

bulk analyses are currently inaccurate.

18. preservation, degradation & destruction of accumulation

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