Influence of Prospecting and Crude Oil Production on Geoecological Parameters of

Sergey N. Buldovich1 and Elena V. Stanis2

1. Department of Geocryology, Moscow State University, Moscow 119633, Russia

2. Department of Geoecology, Peoples Friendship University of Russia, Moscow 113093, Russia

Received: March 21, 2011 / Accepted: April 27, 2011 / Published: February 20, 2012.

Abstract: The large reserves of oil are disposed in zones of permafrost in shelf and continental fields. In Europe the subarctic and arctic tundra are abundant only in Nenetsky autonomous area and in the north-east of the republic of Komi. Oil and gas production activity has threatened richest biological resources and health of the population of the region. A singularity of petroliferous fields of the north of Russia is the existence of the thick strata of permafrost. On top of them different heat-insulating overlying layers (snow, biogenic covers) reside. State and dynamics of permafrost cause progressing dangerous exogenic geoecological processes, which are exhibited completely. State of permafrost is instituted by exchange of heat in near-surface strata of a geological section. During the development of oil fields there is a disturbance of a temperature schedule of permafrost. For estimation of technical attack the analytical computational method of influence of the different factors on value of annual heat exchange and prediction of geocryological conditions is proposed. It is shown, that such dangerous phenomena as degradation of permafrost and thermokarst would develop in the first place, which would cause the destruction of natural ecosystems.

Key words: Heat exchange, permafrost, thermal parameters of rocks, seasonal thawing layer, thermokarst.

1. Introduction??

The large reserves of oil are disposed in zones of permafrost in shelf and continental fields. The region of oil-fields of the Bolshezemelskaya tundra of Russia is considered in the paper. In Europe the subarctic and arctic tundra are abundant only in the Nenetsky autonomous area and in the north-east of Komi. They are considered to be the standard of flat tundra of Europe. The Bolshezemelskaya tundra is the major region of reproduction of the white partridge. The indigenous population long since uses these territories for cultivation of deer, hunting and fishery. Oil and gas production activity has threatened richest biological resources and health of the population in the North European.

The main difference of oil- and gas-bearing fields of a north of Russia from the majority of regions is availability of permafrost, somewhere up to 350 m thick. The stratum of permafrost formations affects the state of all the parameters of ecosystems in the region so it is the major geoecological parameter. Thus the prognosis of state and dynamics of permafrost is indispensable in budgeting and conducting of all complex of operations, bound with discovery and development of oil fields in the Bolshezemelskaya tundra.

2. Experiment

First of all, state of permafrost formations depends on temperature schedule and value of annual heat exchange in top strata of the rocks. The patterns of seasonal thawing of formations depend on these factors, that define a developmental character of dangerous geoecological (or modern exogenic) processes in the oil field. Those to mention are the degradation of a permafrost, solifluction, thermokarst, bogging, swelling, etc. concern to these phenomena [1].

The considerable influence on thermal parameters of formations of a zone of permafrost renders the following technical processes: a repeated and single fare of a full-track and high-gravity wheel carrier, road-building and exploitation, building construction, development of opencasts, well boring, construction of drill floors, warehousing and dumping of waste metals of boring, production activity and primary petroleum refining, leakage at transportation of oil, fill of earth for the different purposes, etc..

The simple and precise method of analytical calculation facilitating factor-by-factor analysis and straight prediction of cryopedological conditions for an estimation of influence of technical attack of crude oil production on the upper field of zones of permafrost is designed [2].

The proposed method is based on a postulate: the values of annual heat exchange in near-surface strata of a geological section are of the first magnitude in forming of mean annual temperatures of sediments in conditions of periodic annual temperature fluctuations on a surface.

The amount of heat annually absorbed by rocks within the half cycle of heating up period, and which is lost by them during the cold season, is called annual heat exchange in formations. This amount of heat characterizes the level of heat exchange between grounds and the atmosphere. The heat turn value depends on a lot of factors: thermal properties of rocks(and proportion of these parameters in a thawed and frozen state), availability and properties of different heat-insulating overlying strata on a surface of formations, nature of seasonal fluctuations of temperature on a daylight area (surface of overlying strata) and average annual temperature of formations. The forming of this average annual temperature is instituted by the value of annual heat turn in formations, as the temperature contribution of all the temperature generative factors of an environment depends on it.

The proposed method is based on the analysis of the above mentioned interdependence between the temperature schedule of the formations and the level of annual heat change in them. In the calculated schema the massive of permafrost formations is considered, on a surface of which are different heat-insulating overlying strata (snow overlying strata, ground vegetative overlying strata, artificial coatings). They are allowed in the schema as thermal resistances, which value is considered as a constant during summer or winter seasons [3].

The basic parameters of cryopedological situation for numerical definition at prediction are: mean annual ground temperature ?t of permafrost on the base surface of a seasonal thawing layer, depth of seasonal thawing of rocks ? and value of annual heat turn in formations B .

Parameter ?t institutes a calorific state of permafrost formations and allows to determine their power and stability to different technical influence. The depth of penetration of a zero isotherm in a frozen massive in a warm season ? institutes a developmental character of the majority of cryogenic geological processes: different kinds of swelling of earths, bogging and possibility of thermokarst processes. The value of heat turn B allows to determine quantitatively the temperature contribution of any factor of an environment, taken into account by the calculated schema.

The proposed method allows to determine the values of each of the three cryopedological parameters irrespective of two other. However in practice it is more efficient to determine one of them at first, and to compute remaining values from the coupling equations.

The calculation schema describes a massive of frozen formations, therefore in the beginning it is expedient to receive a solution for temperature ?t . If temperature is below zero, i.e. in these conditions permafrost formations are advanced, the authors further compute values ? and B . If retrieved temperature of formations is positive, frozen formations in considered natural environments could not exist. They were either not present initially, or they will inevitably begin to degrade as a result of variation of conditions of heat exchange.

The solution for average annual temperatures of frozen formations is

s??

where: ?t—mean annual temperature of permafrost formations on the base surface of a seasonal thawing layer; fth??,—heat conductivity factors of thawed and frozen formations; fthCC,—volumetric heat capacity of thawed and frozen formations;

Q—volumetric heat of phase changes in formations;

csR

R , —total thermal resistance of all strata of

insulation (vegetation, artificial coatings) on a surface of earth in summer and winter time (the thermal resistance of each stratum is equal to the ratio of its thickness to heat conductivity factor: i

R—mean thermal resistance of the snow overlying strata for winter time (close to a value of resistance of snow in the middle of winter); s

?,—summer and winter sums of degree-hours on a surface (in winter—on a surface of snow) taken with plus or minus(winter sum is below zero); s?—duration of summer season (with positive temperatures of air); T—duration of year.

Then it is possible to determine the value of heat exchange B

(3)

The temperature influence of summer and winter ground overlying strata and snow on average annual temperature of formations will be

(4)

Mean annual surface temperature of rocks under overlying strata

(5)

The authors can see from Eqs. (4) and (5) that this temperature is largely determined by the value of annual heat turn in rocks.

Mean annual temperature of rocks at the sole of a seasonal thawing layer ?t differs from that at its surface by the value of temperature offset ?t?, related to difference in thermal properties of formations in a thawed and frozen state.

Value of this temperature offset

Knowing the value of heat turn it is easy to find temperature influence on average annual temperature of formations of any of surface overlying strata using Eq. (4) and value of temperature of shifting in a seasonal thawing layer from Eq. (6).

The prediction of reorganization of the temperature order of formations under influence of variation of any factor is possible only based on the final result of new calculation using Eqs. (1)-(3), as such variation inevitably forms the other value of heat turn B and variation of the temperature contributions of all remaining factors. For example, from Eq. (4) follows that if the thermal resistance of the ground vegetative overlying cover does not change during the seasons of the year (because of variation of humidity, compaction under thick snows, etc.), the summer and winter influences of this overlying cover countervail one another, and its combined temperature influence turns to zero. The elimination of this cover will cause growth of the values of heat turns B, increase defrosting influence of snow and raise average annual temperature of formations.

This feedback can have composite, sometimes unobvious nature. So, the intensification of defrosting influence of any purely summer natural factor will lead to increased heat turns in formations, that results in augmentation of defrosting influence of snow overlying strata in winter time, i.e. this interplay is carried out time lagged during the year. It is important, that additional defrosting influence of snow can significantly exceed the influence of the summer factor itself. With the help of the proposed method the eliciting of such singularities of process of heat exchange between formations and atmosphere is carried out in a simple way.

Let the authors explain the capabilities of the method on a concrete example of calculation. The lease of the Severo-Chosejdajskoe oil-field is considered. The top of a geological section is presented with clay sand and argillaceous sand formations with mossy and lichen cover on a surface. In winter time the thickness of snow is on the average close to 0.5 m. The thermal performances of rocks and overlying covers following: th? = 1.26, f? = 1.74 W/m·K; thС = 630, fС = R= 1.25 m2·K/W. The temperature schedule of a daylight area under the data of a meteorological station Choseda-Hard is characterized by the following values:? = -68,000 degree·hour; endurance of summer phase s? = 4 months = 2,920 hours. From Eq. (1) there is mean annual temperature of rocks ?t = -0.60 oC, from Eq. (2) value of annual heat exchange in rocks B = 34,143 W·hour/m2 and from Eq. (3) depth of seasonal thawing of rocrs ? = 1.34 m. From Eq. (4), there is mean warming influence of snow snt?= + 4.87 oC and from Eq. (6), value of temperature offset in the seasonal thawing layer ?t? = - 0.55 oC.

The computed values ?t and ? are in close correspondence with the observed parameters of frozen formations in undisturbed areas.

The typical technical influence on leases of construction of fields is the full or partial destruction of a ground vegetative overlying stratum by driving engineering in summer time. The dependence of cryopedological parameters upon thermal resistance of a surface heat-insulation layers at conservation invariable of all the remaining parameters mentioned above can be characterized by the following data: at values of thermal resistance of a overlying stratum 0, 0.1, 0.2, 0.3 and 0.4 m2·K/W mean annual ground temperature ?t will be correspondingly 0.0, -0.3, -0.6,-0.85 and -1.09 oC, and the depth of seasonal thawing of formations ? = 1.59, 1.46, 1.34, 1.22 and 1.12 m. It is evident that at decrease of a heat-insulating role of surface covers, the temperature of formations and the depth of their seasonal thawing are simultaneously increasing. This results in drop in stability of frozen strata and increases probability of progressing of thermokarst depressions of a surface. In case of full destruction of a ground vegetative overlying stratum the degradation of permafrost formations starts.

The relevant factor influential in a temperature schedule of frozen strata is the humidity of rocks. Their thermal properties (thermal conductivity, heat capacity and heat of phase changes) defining conditions of heat exchange between formations and atmosphere depend on humidity. On oil fields, the variation of humidity conditions can be connected with customary reasons(variation of conditions of drainage, etc.), as well as with leakage of oil products and their invading. In this case oil partially extrudes and substitutes water in a pore space and changes thermal properties of formations as at decrease of humidity (the thermal conductivity of formations is slightly moderated and proportionally to impurity the heat of phase changes decreases). Let’s estimate influence of humidity of rocks on its temperature mode. At volumetric water content in formation 10%, 20% and 30% (phQ= 9,280, 18,560 and 27,840 W·hour/m3) mean annual ground temperature ?t will be -1.75, -0.88 and -0.21 oC, and thickness of a seasonal thawing layer ? = 1.89, 1.45 and 1.21 m. At reduction of humidity (iciness) of frozen rocks their temperature also decreases, their stability is augmented, and the depth of seasonal thawing will increase, creating hazard of progressing of thermokarst processes.

Compaction or destruction of snow overlying strata results in simultaneous decrease of values of temperature of frozen formations and depth of their seasonal thawing. It is the most favorable situation for conservation of permafrost formations and avoidance of dangerous cryogenic processes. However, expertise demonstrates that the breaking down of a snow overlying strata is typical only for the beginning stage of development of oil-fields, at opening-up of platforms, raising of buildings and equipment installation. At the stage of exploitation the buildings start to execute a function of holding of snow and promote accumulation of a more thick snow cover, than in vivo.

The carried out quantitative estimation of influencing of different variations on conditions of existence of permafrost formations became the basis of constructing of prediction maps.

Thus during the development of the Severo-Chosejdajskoe, oil field should be expected progressing of the following processes.

Partial destruction of surface vegetative cover causes the situation when the temperature of rocks and depth of their seasonal thawing are simultaneously increasing. This results in drop in stability of frozen strata and increases probability of progressing of thermokarst depressions of a surface. In case of full destruction of a vegetative cover the degradation of permafrost starts.

The relevant factor influential in a temperature schedule of permafrost is the humidity (iciness) of rocks. In the case oil partial extruding and substituting of water in a pore space thermal properties of formations change. Thermal conductivity of formations is slightly moderated and the heat of phase changes decreases proportionally to impurity. Thus average annual temperature of frozen formations is depressed, and the depth of seasonal thawing will increase. The hazard of progressing of thermokarst processes appears.

Compaction or destruction of snow overlying strata results in simultaneous decrease of values of temperature of frozen formations and depth of their seasonal thawing. It is the most favorable situation for conservation of permafrost formations and avoidance of dangerous cryogenic processes. Such situation is typical only for the beginning stage of development of oil fields. At the stage of exploitation the buildings start to execute a function of holding of snow and promote accumulation of thick snow cover. This can reduce to irregular thawing and surface subsidence under them.

4. Conclusions

The proposed method allows to determine the temperature schedule of permafrost using cryopedological parameters: average annual temperature of permafrost formations on the sole of a seasonal thawing layer, depth of seasonal thawing of earths and value of annual heat turn in formations.

The analytical calculations of variation of heat parameters of permafrost of the Severo-Chosejdajskoe oil field demonstrate that the most dangerous geoecological phenomena for the region are degradation of permafrost formations, progressing of thermokarst.

References

[1] E.D. Ershov, General Geocryology, Moscow State University Press, Moscow, 2002, pp. 343-348.

[2] S.N. Buldovich, The express-method of the estimation and forecastings of the warm-up mode of permafrost rocks, in: Materials of Second Conference Geocryologists of Russia, Vol. 2, Moscow State University Press, Moscow, 2001, pp. 61-70.

[3] S.N. Buldovch, The methods of the analytical estimation and forecastings geocryologycal parameters in two-layer construction layer seasonal thaw sorts, in: Materials of Third Conference Geocryologists of Russia, Vol. 2, Part 3, Moscow State University Press, Moscow, 2005, pp. 27-35.