Third EAGE Workshop on Pore Pressure Prediction

Non-hydrostatic fluid pressure is a geologically transient phenomenon. It is found in basins that are not in hydraulic equilibrium at present day, such as in young deltas and rift basins, in fold and thrust belts, or in salt basins. There are various geological processes that can generate overpressure in the subsurface such as disequilibrium compaction, tectonic stresses or thermal transformations such as HC generation or clay dewatering. Overpressures usually imply tight formations that prevent fluid flow and therefore generate fluid pressures higher than hydrostatic. Only disequilibrium compaction can be identified from sonic data. As the principles of fluid flow in porous media are well known, most of these transient phenomena can be modeled numerically. Workflows have been developed to cope for these various pressure generating processes. Coupled poromechanical simulators become increasingly available to consider the effect of geological deformations and horizontal stresses on pressure, but challenges remain in terms of parametrization, simplification of physics, resolution and gridding in modeling approaches.

In this work we have carried out a full field trail, starting with a pre-drill study using a basin scale pore pressure modelling with a Monte-Carlo uncertainty approach. We have tested the workflow, with the new data coming in while drilling in a digital twin. The updated pore pressure and mud-weights are fed back to the drilling operator in a real-time operation. The automated workflow allows researchers and operators to follow the drilling operations at office or home office.

The Waxman-Smits equation can be used in combination with careful petrophysical and geological investigation of geophysical well logs to estimate subsurface shale pressures from electrical resistivity logs. The main benefit of this approach is the transferability between different wells, even across an entire sedimentary basin, once the normal compaction trend (NCT) is constrained. The main disadvantage is given by the dependency on a large number of factors, which, in the North Alpine Foreland Basin, might result in overestimation of pore pressures in some areas. Nevertheless, this further highlights the necessity of involving the local and regional geological boundary conditions in pore pressure studies. In the context of deep geothermal exploration in the overpressured North Alpine Foreland Basin in SE Germany, the results of this study are of particular relevance, since finding a reasonable trade-off between data acquisition and cost minimization is key to ensure economic deep geothermal projects. Utilizing data sources, which are already available and measured while drilling, to estimate subsurface pressures in overpressured target areas is therefore an important step forward to safe and economic deep geothermal exploration.

Pre-drill pore pressure prediction includes significant amount of uncertainties, especially for vast underexplored offshore regions. Level of ambiguity increases due to lack of any operational data. Hence, pore pressure cubes generated from interval velocities is a widely used alternative method to identify the profile. This study favours point data extraction of interval velocities to construct a robust 3D pore pressure model around recently discovered shallow gas anomalies in the offshore Evros Basin, NE Aegean Sea. As being the SW extent of gas prone Thrace Basin, study area is represented by Eocene to recent sedimentary succession that can be observed along the seismic sections. Seismic facies analysis has revealed the presence of 2 mud volcanoes and 5 gas chimneys. Petrophysical models and pore pressure cube have yielded satisfactory results by illustrating meaningful anomalies such as a rapid velocity decrease along gas chimneys and sharp density transitions between seismic units. Abrupt pressure increase around gas chimneys indicates a possible lateral charge into the coarse grained lithologies. Findings have shown the importance of pore pressure cubes for the early phases of offshore exploration projects to detect potential reservoirs and reduce drilling risks.

Pore pressure prediction is a critical aspect of safe well delivery in basins around the world. Understanding the spatial and temporal distribution of pore pressure is also beneficial to the economic evaluation of exploration targets.

In basins where overpressure develops due to sediment loading, the redistribution of pressure or “focussing” of fluid flow is often referred to as the centroid or lateral transfer effect. Centroid theory describes the redistribution of pressure and pore fluids in laterally continuous and permeable bodies such as a sandstones that are encased in a less permeable sediments, such as a mudstones. The subsequent pressure relationships between the sands and mudstones impacts both the column height potential and drilling risk. The impact on fluid flow can have implications for migration and petroleum systems assessment. Many uncertainties exist in the exploration domain, this series of 2D models seeks to take common geological uncertainties such as reservoir geometry or hydrocarbon fill and demonstrate the effect of these on the pressure-depth relationship at a prospective structure/anticline.

We predict pore pressure and stresses from seismic velocity over a volume around a salt dome in Green Canyon 955, Gulf of Mexico, using the new, Full-Effective-Stress (FES) method. The salt dome significantly perturbs the magnitude and orientation of stresses in surrounding sediments. The FES method uses a geomechanical model to account for these perturbations and their impact on pressure and stresses predicted from velocity. In this study, we use the FES method along with a 3D geomechanical model and show that predicted pore pressure, stresses, and drilling windows significantly differ from those predicted by a 2D model. Thus, the FES method should be used with a 3D geomechanical model for accurate pore pressure and stress prediction near salt.

The presence of overpressure is a well-known phenomenon in the Pannonian Basin, but the main generation mechanisms have not been clarified yet. Previous studies mainly focused on the current subsurface pressure regime, but the paleo pressure conditions have not been evaluated. The suggested 2D basin modeling approach can deliver adequate information about both the paleo and current abnormal pressure conditions and the main drivers of the development.

Two neighboring subbasins have been selected and simulated for presenting the advantage of 2D basin modeling in regional studies. Comparing the Derecske Trough with the Békés Basin, the measured static pore pressure data prove that the magnitude of overpressure is different, despite of the similar geological history. The 2D basin model suggests that mainly three processes control the subsurface pressure regime development, namely the undercompaction, the centroid effect and the timing of burial or sedimentation.

The main parameters from the master model has been linked with a Monte Carlo simulation and recalculated. The uncertainties in the depth converted seismic maps have resulted only minor effect in the calculated pore pressure model, but uncertainties in porosity and permeability can strongly affect the pressure simulation.

In this study, the Mechanical Earth Model MEM was built for a vertical exploration well within Wielkie Oczy Graben in order to investigate the origins of encountered overpressure zone in the Miocene age Mb1 strata within generally normally pressured homogenous profile. It was interpreted that at the depths of approx. 3100–3250 m, there is a layer of sand surrounded by relatively impermeable shale which could be responsible for the centroid effect. Due to the fact that a centroid is a three-dimensional form, it is impossible to describe it on the basis of the wellbore dataset. The top and the bottom could be merely assumed. Using the anomalous pressure recorded during well tests as a calibration, applying gradient from overlying strata and assuming the top of centroid approximately on the basis of geological cross-section, the resulting bottom of the centroid must be bounded by some deep zone (of approx. depth 4550 m) connected via Krakowiec fault zone.

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