Page 43: of Offshore Engineer Magazine (Nov/Dec 2014)

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of Black Sea due to fewer data. to discharge areas. In these ? ow systems

Drilling

Pressure-elevation (p(z)) pro? les show the main ? ow direction is horizontal in signi? cantly overpressured zones and the midrange areas. It can be observed on super-hydrostatic gradients from both Fig. 1 where near vertical potential lines study areas. Overpressure is usually indicate horizontal ? ow in the upper part

Fig. 3: 1000 m formed by rapid sedimentation when of the section. If there is an underlying equipotential pore water cannot leave the sediments overpressured zone then it will be super- surface and it has to carry the burden of imper- imposed and limit the downward exten- (~10 MPa meable overlying layers, which act as a sion of the gravitational system resulting overpressure) in the southern pressure seal. Since this seal is thought in upward directed ? ow.

part of the to be responsible for maintaining the In an offshore environment the pres-

Pannonian overpressure, its permeability, porosity, sure exerted by the water column above basin.

thickness and geometry are of crucial the sea? oor has to be considered and importance. In the Pannonian basin these this would result in constant head cells components that show toward deep of excess pressure, which are delta slope sediments, whereas in the for the layer just on the sea? oor. In our grabens. No signi? cant discrepancy can would correspond to different severity at

Black Sea these are Miocene rocks below case study from the Black Sea, the water be observed between the results of the different depths.

the Intra-Pontian Unconformity (IPU). depth is approximately 70m, which two calculation methods, however, the On the other hand, ? gure 4 is a graphi-

Some calculations (Almási, I.: is about 7bar constant pressure on the congestion of equipotential lines is more cal illustration of relative values for the

Petroleum Hydrogeology of the Great sea? oor. The modeled section is approxi- expressed by the ? nite element method, Black Sea: showing pressure values as

Hungarian Plane, PhD thesis, 2001) sug- mately perpendicular to the dip direction which is in more accordance with the a percentage in excess of hydrostatic gest that overpressures formed during of the slope, therefore water depth was abrupt pressure change observed on p(z) pressure. This can provide very useful sedimentation should have been at least assumed constant along the section. pro? les. information for choosing appropriate partly dissipated in geologic times, even Once boundary conditions are set the Figure 2 shows the hydraulic cross sec- mud weight when drilling.

through very low permeability pres- pressure distribution will be de? ned by tion from the western part of Black Sea

Conclusion sure seals. However, this is not the case, the porosity, permeability and the geom- as a result of the hydrodynamic model- which means there must be an additional etry of the model layers. The parameters ing. It can be noticed from the section Hydrodynamic modeling can be used for phenomenon maintaining overpressure. of the pressure seal will mainly in? uence that highly overpressured Oligocene overpressure prediction in on- and off-

Some geoscientists believe it could be how super-hydrostatic pressure dissi- layers are sealed by Miocene rocks and shore environments, however, a slightly – among others – tectonic compression. pates with distance. the excess pressure is dissipated over different approach has to be taken mainly

These are assumptions that have to be Figure 1 depicts hydraulic cross sec- these sediments and it slowly disappears with regards to boundary conditions.

made and transformed into boundary tions from the Pannonian basin, where above the IPU. As ? ow is perpendicular In general the model results have sev- conditions in order to model overpres- the ? uid potential ? eld is represented by to the equipotential lines it is mainly eral practical applications: • sured areas regardless of whether it is equipotential lines. There are two almost directed upwards. However, it also has a Predicting the spatial distribution of on- or offshore. identical sections on the ? gure because horizontal component showing up-dip of pressure and especially overpressure is of

Since the natural ? ow system is the simulation was run with two differ- IPU (red arrows). This suggests that IPU global importance from interrelated eco- modeled, steady-state conditions are ent methods: ? nite difference and ? nite might play in important role in hydrocar- nomic, technical and HSE (Health Safety assumed, which means the model element method. Both methods start from bon migration. and Environment) aspects.

• converges to equilibrium where ini- the Laplace- and continuity equations Since the hydrodynamic model pro- The determination of migration path- tially overpressured cells would become that describe permanent ? ow, but the vides a hydraulic head (pressure) value ways is of crucial signi? cance from the hydrostatic. To avoid this, constant head mathematical method for solving these to each single model cell it can be a exploration point of view.

• cells have to be applied at the bottom of equations and the geometry of the model useful tool for predicting overpressures. The presented method can be power- the model space to maintain the over- grids are different. There are different ways of visualizing ful in both well-explored mature areas pressure (i.e. to simulate the effect of Figure 1 shows that ? ow is mainly the model prediction. and less-known territories regardless of tectonic compression). directed upwards, since it is perpen- Figure 3 presents an equipoten- whether it is on- or offshore.

The main difference between on- and dicular to the equipotential lines. tial surface of the 3D model from the Nonetheless, it can never be overem- offshore modeling is between the bound- Potentiometric mounds occur above the southern Pannonian basin. The 1000m phasized that all available information ary conditions applied to near surface basement highs, which suggests that equipotential surface (approximately 10 has to be carefully studied and evaluated layers. Onshore, the undulations of the extreme overpressures are related to the megapascal overpressure) indicates the during the modelling process because terrain create gravitational ? ow systems, horsts of the basement. Above basement depth at which this overpressure would misconceptions can lead to unrealistic highs ? ow directions have horizontal be encountered. It gives an absolute value where water ? ows from recharge areas highs ? ow directions have horizontal be encountered. It gives an absolute value results and misleading predictions.