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Reservoir modeling One step ahead of the future

You don’t know what lies beneath your feet – that is of course also true in the oil and gas industry. After all, the geoscientists and engineers can never see with their own eyes as to how it really looks down there where the oil and gas is hidden. Nevertheless, they can try to approach the reality as closely as possible – with virtual simulations of the subsurface generated on the computer.

Geologists, geophysicists, petro-physicists, reservoir engineers and production engineers: as part of their work they all gather information about oil and gas fields and develop three-dimensional models of reservoirs. These models are used to forecast the production from each field. They also provide the basis for all investment decisions. After all, before the management of an E&P company decides to develop a field, it will of course want to know how much can be produced by the field, and how much can be earned from it. The decisive factor is the economic viability. That can only be determined if the production volumes for a field are simulated across the entire lifecycle.

Static & dynamic models A two step process

The process from the initial information to forecasting the production is in principle always the same: in the beginning geophysicists evaluate data from seismic surveys to reveal information about the structure of the subsurface with the different layers of rock. This data and preliminary drilling provide the petro-physicists with information on the porosity and permeability of the rock – i.e. how many cavities are contained in the rock, how big they are and the extent of the flow paths between the individual pores.

This information provides the basis for static reservoir models, since the porosity and permeability are fixed characteristics of the subsurface that do not change, or only insignificantly, as a result of the oil and gas production.

Dynamic models, on the other hand, contain variable characteristics that change over time such as, for example, the water and oil saturation of the rock. The oil content decreases as a result of the production while the water content in the pores increases. Characteristics such as the pressure and temperature also change. Only dynamic models can visualize the changes in the reservoir and simulate the production. With their help, the experts can therefore determine how many barrels in total can be produced from a field. If the anticipated production costs, development costs as well as the oil and gas price are also taken into consideration, this produces a clear picture of the economic viability of the project.

 

The dynamic model of Wintershall’s Emlichheim oil field shows how a reservoir changes during the course of production. Since 1981 we have been injecting hot steam at a temperature of 300 degrees Celsius into the reservoir in order to improve the recovery. This has therefore gradually increased the temperature in the formation. The simulation clearly depicts this change.

Simulations Needed in every phase of a field’s lifecycle

Forecasts are produced, however, not only during the field’s initial development stage but also during every stage of the field’s lifecycle. For example, if a field is already in production, the production and plant engineers will constantly develop measures aimed at further optimizing the production. They try out all these ideas in the reservoir modeling and verify whether they are economically viable. The process from interpreting the seismic surveys to making forecasts is therefore constantly repeated.

Reservoir modeling: The process

The process - from the initial information to a concrete prediction of total output from an oil or gas field - is basically always the same, and is repeated before every important financial decision.

Seismic survey

The geophysicists begin by analyzing data from seismic surveys that reveal findings about the structure of the subsurface with its different rock strata.

Static model

With the help of seismic data as well as information from initial wells, a so-called static model with the invariable properties of the reservoir, such as the rock’s porosity and permeability, is created. It says a lot about the quality of the reservoir because cavities and flow paths in the rock are necessary for oil and gas to gather and be brought to the surface.

Dynamic model

This shows the parameters of a reservoir that change as a result of production, for example, the oil or gas content, but also properties such as pressure and temperature. If such variables are included in the 3-D images, the experts talk of dynamic models.

Forecast

Dynamic models are used to simulate the total output over a field’s entire lifecycle. The simulation needs to be done in order to create a forecast. This, in turn, helps to show if a project is economically viable.

Integrated Reservoir Modeling (IRM)

What decisively improves the quality of the models is teamwork. After all, expertise from all the disciplines is required.

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developing 3D models is a difficult task because you have to incorporate considerable data and interpret them correctly. But that is exactly what makes reservoir modeling so exciting.

Aggelos Calogirou
Reservoir Engineer

Reservoir models Approaching reality

Despite all the calculations, one thing remains clear: reservoir models only ever provide an approximation of reality. Misinterpretations and errors are possible. The quality of the model essentially depends on the quantity and quality of the data. How well known is the geological basin where the reservoir is located? Is there a high-quality 3D survey available? Things become particularly tricky when the reservoir has a very heterogeneous structure and behaves differently than expected. That makes it more difficult to draw conclusions and make predictions. These uncertainties can be reduced by drilling a considerable number of wells, since they provide a lot of important information. Generally, the more wells that are drilled, the more information is revealed about the field.

 
THE "LIFECYCLE" OF OIL AND GAS FIELDS

The service life of oil and gas fields is divided into different phases ranging from the discovery to the decommissioning.

Value Chain / Development