Computer simulation is becoming more widely used for predicting the effects of industrial objects on the environment.
I.А. Gishkeluk, D.V. Еvlanov, V.I. Kovalenko
Transportation, processing and production of hydrocarbons and natural gas is associated with the risks of pollution by oil products and chemicals.
In order to assess these risks, as well as monitoring and control of factors affecting them, computer simulation methods are more widely starting to get occurrence in oil and gas industry, which also can play an important role in increasing the extraction of oil and gas and efficiency of their processing.
Leading foreign companies providing services in oil and gas industry, – Schlumberger, Paradigm, Geophysical and directly oil and gas companies themselves – Total, BP, Royal Dutch Shell, Chevron, ConocoPhillips, Statoil Hydro, China National Petroleum and many others in their work for decades widely use methods of computer simulation, which, unfortunately, are less spread in Russian oil and gas companies. At the same time, just to solve the environmental problems, occurring in oil and gas industry, computer simulation allows:
- perform environmental study of the possibilities for location and functioning of objects of increased environmental threat: filling stations, oil storage objects, transport service stations, sites for industrial waste storage;
- explore different scenarios of emergency situations related to the transportation or storage of petrochemicals and natural gas, and to assess the risk of such situations;
- evaluate the contamination of soil, groundwater and surface water (rivers, lakes, seas) with accidental spills of oil and oil products;
- assess the negative impact on the environment of thermal pollution from the burning of petrochemicals and natural gas.
The main environmental problem when petrochemicals ingress on the surface of the earth is connected with the fact that they seep to the surface of groundwater and begin to form floating on the water lenses that can migrate with the groundwater, causing water intakes and surface water pollution. Let assess the possibility of petrochemicals’ ingress into the reservoir produced during the exploitation of the industrial object by an example of filling station (PFS) using computer simulation methods (Fig. 1).
FIG.1. MAP OF THE INDUSTRIAL OBJECT LOCATION
1–area for industrial object location;
To solve this problem, it is possible to use open source software MODFLOW and MT3DMS.
Software MODFLOW, using finite difference method allows to simulate three-dimensional movement of groundwater. Model implemented in the software can take into account the influence of water wells, areas of recharge and discharge of rivers and other surface water objects on the movement of groundwater. Software MT3DMS, using the results of groundwater flow computation obtained with MODFLOW, simulates a three-dimensional groundwater transfer of dissolved contaminants in them on the basis of the solution of the convective diffusion equation.
The process of computer simulation of petrochemicals distribution using software data is the following. On the area that is adjusted to the filling station the boundaries of the simulated area are selected. In this case, in order to formulate correctly boundary conditions for the equation of ground water flow it is necessary to select the boundaries of this area by the height of lands (as the flow of water through them equals to zero) or along the lines of rivers and lakes (because the level of groundwater is known there ). Next in MODFLOW hydrogeological model of the territory is created. And each layer is assigned to its groundwater hydrological properties (Fig. 2).
FIG. 2. hYDROGEOLOGICAL MODEL OF THE RESEARCH AREA:
1 – industrial object
2 – free-flow layer formed by sandy loam and loam;
3 – pressure layer formed by sand;
4 – waterproof layer formed by sandy loam
As a result of computer simulation, distribution of groundwater hydraulic heads is obtained, which determine the magnitude and direction of the water table rate of motion (Fig. 3).
FIG.3. SIMULATION RESULTS
|Distribution of groundwater pressures
|Vector field of groundwater speed
For transfer simulation of dissolved in groundwater petrochemicals in software MT3DMS, like in many others, as boundary conditions it is necessary to set the value of petrochemicals flow at the boundary of groundwater with aeration zone (the upper zone of the earth’s crust between its surface and the water table).
However, when solving most of practical problems (objectives) only the concentration of petrochemicals on the surface is known. It should be noted that when using commercial software occurs a situation when the applicable software does not take into account all relevant factors and requires the specification of parameters that the user doesn’t possess. Therefore, in most cases it is necessary to complete existing or develop new mathematical models and user interface directly according to the user’s tasks and available source data .In this example, the problem of practical software application MODFLOW and MT3DMS is solved with the help of special software developed by the authors, which computes the intensity of oil ingress into the ground water, if assume that the surface layer of soil due to the work of filling station is contaminated in some concentration of C0. C0. Further, software MT3DMS simulates distribution of petrochemicals in the groundwater. From the simulation results (Fig. 4) it is seen that during the operation life of the station (30 years) there is no possibility of getting petrochemicals into the reservoir.
FIG.4. SIMULATION RESULTS OF CONTAMINANTS DISTRIBUTION IN GROUND WATER:
|After 10 years of filling station operation
|After 30 years of filling station operation
Thus, computer simulation plays an important role in selecting the location of environmentally hazardous facilities, taking into account their influence on the environment.
Let us note that this prediction is relied on the use of mathematical model, based on Darcy equation (describes the groundwater movement) and convective diffusion. However, some of the model parameters, such as filtration rate, variance and coefficient of distribution cannot be determined exactly and therefore have some uncertainty. In addition, geological environment in which the groundwater movement and transport of pollutants are predicted consists of interbedded rocks with different physical and chemical properties, and thus it is not possible to create a completely accurate geological model with all the irregularities. All this increases the level of uncertainty of predictions. One of the methods to solve this problem is to use a stochastic approach in computer simulation.
In contrast to the deterministic approach, where for prediction one model that provides the best approximation to the real system is used, stochastic approach creates a number of models that with some probability describe the simulated system and which are used for prediction, in accordance with a given simulation scenario. Further the received results are applied to estimate the probability of the prediction.
Consider the implementation of the stochastic approach for the previous problem. In this case, when predicting petrochemicals transfer in groundwater the equation of convection-diffusion with the adsorption is used, where for coefficients of distribution and dispersion empirical function of probability density on the basis of data on the values of these coefficients for different geological environments is built. Each probability density function obtained for analyzed parameters is divided into five segments, and for each combination of parameters the distribution of petrochemicals using convective diffusion equations is computed. This allows obtaining multiple predictions of petrochemicals distribution from the filling station and the probability of their implementation.
On the basis of the obtained simulation results a probability map of pollutants distribution is created (Fig. 5).
FIG.5. DISTRIBUTION OF PROBABILITIES OF PETROCHEMICALS EXCESS AFTER 30 YEARS OF THE OPERATION OF FILLING STATION:
|More than 5% of the maximum allowable concentration
|More than 10% of the maximum allowable concentration
Thus, computer simulation is an effective tool for the risk assessment of environmental pollution, control and management of factors affecting them, and also it facilitates decision-making for the wider challenges facing oil and gas industry.