Well control is creating concern in the deepwater sand plays along continental margins around the globe. Some wells have encountered overpressure in cases where the pore pressures are close to the fracture pressure of the rocks. This problem also has been reported in the North Sea deep gas condensate and Gulf of Mexico subsalt plays.
A pressure/volume graph, displaying a typical pressure profile acquired during a pressure test.
A team from three UK universities is working on techniques that would improve pressure prediction. Where sedimentary formation is relatively young, a relationship between porosity and overpressure can be identified fairly reliably using seismic and wireline logs.
However, this is not the case in older rock sequences, where vertical and horizontal stresses have altered over the millennia. In plays where well data is sparse or non-existent, porosity values can be derived from seismic velocities, but knowledge of permeability in fine-grained sediments remains inadequate.
There is also a commercial reason to improve analysis - high pressure wells are expensive to drill. With these factors in mind, the Geoscience Project Investigating Overpressure (GeoPOP) was established in 1994. Phase 1, completed in spring 1998, drew £1 million in funding from 13 oil companies. The research program brought together geologists, geochemists, and petroleum engineers from Durham, Newcastle, and Heriot-Watt Universities, working closely with the sponsors. The main aims of Phase 1 included:
- An investigation of all potential overpressure mechanisms, particularly in Central North Sea HP reservoirs. (Most of the data supplied came from this region).
- Examining how fluctuation in overpressure over time influences fluid and rock composition.
Over-pressure findings
The major conclusion of the study was that rapid burial (disequilibrium compaction) and late petroleum generation, are the key mechanisms for generating overpressure. The researchers developed other findings:
- Longevity of seals is dictated by permeability, seal thickness, and fluid storage volume. Also, overpressure decay through a seal is geologically significant (>10Ma).
- Pressure transition zone characteristics can, in certain conditions, be identified as an overpressure mechanism.
- In two-phase systems, overpressure influences expulsion and migration of petroleum from source rocks to a trap.
The key achievement of Phase 1 was the adaptation of a medical technique, confocal laser scanning microscopy, to determine past geological pressures (palaeobarometry). It involves slicing through fluid inclusions - small time capsules of petroleum and formation water trapped in mineral cements - in 2D series to generate 3D images.
Over-pressure mechanism
According to GeoPOP Research Assistant Derek Teasdale, "As you heat the inclusion, the vapor bubbles within it expand and the liquid:vapor ratio of trapped petroleum can be determined. These ratios, in combination with computer-based modeling, enable the various physical properties of the petroleum to be calculated. The conditions that these properties represent include the pressure of the petroleum at the time that it became trapped. This allows you to calculate past palaeopressures - no one had estimated these before."
Another by-product of Phase 1 was the development of software for visualizing and interpreting pressure data, supervised mainly at Durham. A base case model was built to "back-track" progress of over-pressured sediments throughout their burial history, and also to examine the impact of rock and fluid parameters on pressure and basin stress. .
When the sponsors began requesting copies of the pressure management/interpretation tool, GeoPOP set up a separate company in Durham to commercialize it, called GeoPressure Technology. Recently an updated version of the product - PressureView 2.1 - was launched.
PressureView 2.1 allows the user to search for pressure data by geological areas or in relation to parameters such as mud weight or rock type. The multi-well plotting functions can be employed to identify pressure compartments, fracture gradients, and locations of seals and flow barriers. Oil, gas and water-bearing zones can be located using a single well plotting option.
Heriot-Watt's team modeled over-pressure generation and dissipation, their aims being to establish how this occurs, what the parameters are that influence change, and to determine how sensitive their models were to these parameters. They found that lateral transfer was a significant way of generating excess pressures at the top of a structure.
Estimation methodology
The principal aims of Phase 2 are to:
- Investigate physical and chemical behavior of low permeability rocks that retain their fluids
- Link physical properties of overpressured rocks to log response and to seismic data
- Generate a methodology for estimating overpressure in sedimentary basins.
Phase 2 research will collect similar data from contrasting basins: Australia's North West Shelf, the Gulf of Mexico (a range of young depositional systems), and the US' Delaware Basin (old, overpressured basin with a relatively straightforward burial history).
Improve evaluation
If everything comes together as planned, the result will be a new guide for examining overpressured petroleum systems, featuring a methodology for:
- Assessing the contribution of the key overpressure-generating mechanisms, as well as the magnitude of the resultant overpressure
- Pressure modeling through evolution, distribution, and dissipation over time
- Palaeopressure estimation for validating pressure history models
- Parameters for basin modeling of fluid flow and overpressure in sedimentary basins.
Studies are due to be completed in 2001.
