With a slightly different approach, hydrodynamic modeling can be used for overpressure prediction in on and offshore environments. Marcell Lux, Ahmed Amran, and Marianna Vincze explain.
Fig. 1: W-E hydraulic cross section from the southern part of the Pannonian basin. |
Overpressure is a serious problem and a challenge for drillers in many petroleum provinces of the world, including in the Pannonian basin in Central Eastern Europe and in the Black Sea.
MOL looked into how hydrodynamic modeling can be used for (over)pressure prediction onshore and offshore by presenting case studies from the southern part of the Pannonian basin and from the western part of Black Sea, respectively.
Pressure prediction is generally done by detailed analysis of offset well data. A hydrodynamic model can incorporate well data, seismic data and any (hydro)geological information that could improve the model.
By solving the Laplace- and continuity equations it simulates present-day, steady-state conditions, giving a possible realization of the fluid-potential (pressure) field of the study area (Fig. 1 and 2). This enables us to predict overpressure and flow directions, which may also serve as hydrocarbon migration routes. Naturally, the reliability of the model prediction depends on the quantity and quality of the available data and on how appropriate the boundary conditions and the modeler’s geological assumptions are.
Fig. 2: SW-NE hydraulic cross section from the western part of Black Sea. |
Since MOL’s core exploration area has been the onshore Pannonian basin for over 75 years there was more data available compared to offshore. This data allowed for more detailed 3D hydrodynamic modeling in the Pannonian basin. Conceptual 2D modeling was done in the western part of Black Sea due to fewer data.
Pressure-elevation (p(z)) profiles show significantly overpressured zones and super-hydrostatic gradients from both study areas. Overpressure is usually formed by rapid sedimentation when pore water cannot leave the sediments and it has to carry the burden of impermeable overlying layers, which act as a pressure seal. Since this seal is thought to be responsible for maintaining the overpressure, its permeability, porosity, thickness and geometry are of crucial importance. In the Pannonian basin these are delta slope sediments, whereas in the Black Sea these are Miocene rocks below the Intra-Pontian Unconformity (IPU).
Fig. 3: 1000 m equipotential surface (~10 MPa overpressure) in the southern part of the |
Some calculations (Almási, I.: Petroleum Hydrogeology of the Great Hungarian Plane, PhD thesis, 2001) suggest that overpressures formed during sedimentation should have been at least partly dissipated in geologic times, even through very low permeability pressure seals. However, this is not the case, which means there must be an additional phenomenon maintaining overpressure. Some geoscientists believe it could be – among others – tectonic compression. These are assumptions that have to be made and transformed into boundary conditions in order to model overpressured areas regardless of whether it is on- or offshore.
Since the natural flow system is modeled, steady-state conditions are assumed, which means the model converges to equilibrium where initially overpressured cells would become hydrostatic. To avoid this, constant head cells have to be applied at the bottom of the model space to maintain the overpressure (i.e. to simulate the effect of tectonic compression).
The main difference between on- and offshore modeling is between the boundary conditions applied to near surface layers. Onshore, the undulations of the terrain create gravitational flow systems, where water flows from recharge areas to discharge areas. In these flow systems the main flow direction is horizontal in the midrange areas. It can be observed on Fig. 1 where near vertical potential lines indicate horizontal flow in the upper part of the section. If there is an underlying overpressured zone then it will be superimposed and limit the downward extension of the gravitational system resulting in upward directed flow.
In an offshore environment the pressure exerted by the water column above the seafloor has to be considered and this would result in constant head cells for the layer just on the seafloor. In our case study from the Black Sea, the water depth is approximately 70m, which is about 7bar constant pressure on the seafloor. The modeled section is approximately perpendicular to the dip direction of the slope, therefore water depth was assumed constant along the section.
Once boundary conditions are set the pressure distribution will be defined by the porosity, permeability and the geometry of the model layers. The parameters of the pressure seal will mainly influence how super-hydrostatic pressure dissipates with distance.
Figure 1 depicts hydraulic cross sections from the Pannonian basin, where the fluid potential field is represented by equipotential lines. There are two almost identical sections on the figure because the simulation was run with two different methods: finite difference and finite element method. Both methods start from the Laplace- and continuity equations that describe permanent flow, but the mathematical method for solving these equations and the geometry of the model grids are different.
Figure 1 shows that flow is mainly directed upwards, since it is perpendicular to the equipotential lines. Potentiometric mounds occur above the basement highs, which suggests that extreme overpressures are related to the horsts of the basement. Above basement highs flow directions have horizontal components that show toward deep grabens. No significant discrepancy can be observed between the results of the two calculation methods, however, the congestion of equipotential lines is more expressed by the finite element method, which is in more accordance with the abrupt pressure change observed on p(z) profiles.
Figure 2 shows the hydraulic cross section from the western part of Black Sea as a result of the hydrodynamic modeling. It can be noticed from the section that highly overpressured Oligocene layers are sealed by Miocene rocks and the excess pressure is dissipated over these sediments and it slowly disappears above the IPU. As flow is perpendicular to the equipotential lines it is mainly directed upwards. However, it also has a horizontal component showing up-dip of IPU (red arrows). This suggests that IPU might play in important role in hydrocarbon migration.
Since the hydrodynamic model provides a hydraulic head (pressure) value to each single model cell it can be a useful tool for predicting overpressures. There are different ways of visualizing the model prediction.
Figure 3 presents an equipotential surface of the 3D model from the southern Pannonian basin. The 1000m equipotential surface (approximately 10 megapascal overpressure) indicates the depth at which this overpressure would be encountered. It gives an absolute value of excess pressure, which would correspond to different severity at different depths.
On the other hand, figure 4 is a graphical illustration of relative values for the Black Sea: showing pressure values as a percentage in excess of hydrostatic pressure. This can provide very useful information for choosing appropriate mud weight when drilling.
Fig. 4: Overpressure percentages above hydrostatic in the western part of Black Sea. |
Conclusion
Hydrodynamic modeling can be used for overpressure prediction in on- and offshore environments, however, a slightly different approach has to be taken mainly with regards to boundary conditions.
In general the model results have several practical applications:
Predicting the spatial distribution of pressure and especially overpressure is of global importance from interrelated economic, technical and HSE (Health Safety and Environment) aspects.
The determination of migration pathways is of crucial significance from the exploration point of view.
The presented method can be powerful in both well-explored mature areas and less-known territories regardless of whether it is on- or offshore.
Nonetheless, it can never be overemphasized that all available information has to be carefully studied and evaluated during the modelling process because misconceptions can lead to unrealistic results and misleading predictions.
Marcell Lux has been with MOL Plc. as an exploration geologist since 2012. He holds BSc in earth science engineering and a MSc in hydrogeology. Currently, he is doing his PhD studies in geology at the University of Szeged specializing in subsurface hydrodynamic modeling.