Reservoir overpressured

For apexes below 3500 m, i.e., the deepest subcompartments of the north Hank, there is an increased possibility of actual breaching (vertical fracturing) of the Spekk Formation cap rock. This could cause direct coupling between the Jurassic reservoirs (overpressured to the extent controlled by the Spekk Formation prior to breaching) and the overlying less overpressured Lower Cretaceous semi-permeable silty (occasionally sandy) claystones. Once attained, this situation might be expected to cause relatively dramatic pulses of vertical leakage (e.g., Mandl and Harkness, 1987). 

As illustrated in Fig. 14b, pressure-inhibited charge is less likely in cases where hydrocarbons are able to migrate across the boundary of different pressure cells. In these cases the concept of pressure equilibrium does not apply as reservoir pore pressure is unrelated to the migration of hydrocarbons from the overlying seal. Excess reservoir overpressure relative to the seal may therefore induce hydraulic fractures which breach both the seal and the overburden. It follows that closures located near to frequently reactivated boundary faults have a relatively greater... 

Fig. 12. Effective stress evolution in a pseudo-well on the top of the Snorre structure. Notice the rapid drop in effective stress in the reservoir (overpressure build up) during the rapid Pliocene burial.

An intermediate casing is usually set above the reservoir in order to protect the water bearing, hydrostatically pressured zones from influx of possibly overpressured hydrocarbons and to guarantee the integrity of the well bore above the objective zone. In mature fields where production has been ongoing for many years, the reservoir may show depletion pressures considerably lower than the hydrostatically pressured zones above. Casing and cementing operations are covered in section 3.6. 

Growth faulted deltaic areas are highly prospective since they comprise of thick sections of good quality reservoir sands. Deltas usually overlay organic rich marine clays which can source the structures on maturation. Examples are the Niger, Baram or Mississippi Deltas. Clays, deposited within deltaic sequences may restrict the water expulsion during the rapid sedimentation / compaction. This can lead to the generation of overpressures. 

In abnormally pressured reservoirs, the continuous pressure-depth relationship is interrupted by a sealing layer, below which the pressure changes. If the pressure below the seal is higher than the normal (or hydrostatic) pressure the reservoir is termed overpressured. Extrapolation of the fluid gradient in the overpressured reservoir back to the surface datum would show a pressure greater than one atmosphere. The actual value by which the extrapolated pressure exceeds one atmosphere defines the level of overpressure in the reservoir. Similarly, an underpressured reservoir shows an pressure less than one atmosphere when extrapolated back to the surface datum. 

Moist diethyl sulphate was stored in an iron reservoir, which detonated after a little while. This accident was explained by the hydrolysis of the sulphate present that gives rise to the formation of sulphuric acid. By reacting with iron, sulphuric acid formed hydrogen that caused the overpressure responsible for the detonation. 

The washing of capillaries with dilute alkaline solution is advisable before analysis. The alkaline solution can be followed by deionized water and buffer. Capillaries can be washed between runs too. Samples can be introduced into the capillary by hydrodynamic and electro-kinetic methods. The hydrodynamic method applies a pressure difference (5-10 sec) between the two ends of the capillary. The pressure difference can be achieved by overpressure, vacuum or by creating a height difference between the levels of the buffer and sample reservoirs. In the case of electrokinetic injection, the injection end of the capillary is dipped into the sample for a few seconds and a voltage of some thousand volts is applied. 

The 8 UGS candidates were chosen among other oil and gas fields because these sites (with some variation) are well documented (e.g. 2D seismic surveys and a number of wells) and the reservoirs has a proven seal, which has been able to withhold a gas overpressure for million of years. This is important both for safety reasons and for operating the gas storage without risks of gas leaking away in the substrata. 

Pore pressure gradients are very difficult to estimate with the same accuracy in shales outside the reservoir zones, where RFT or DST measurements are impossible. We have, however, estimated pressure gradients in three wells on the border between the Melke and Gam Formations, based on the drilling data in Fig. 5. We have attempted to calculate the flow of water from the overpressured Upper Jurassic and Lower Cretaceous shales, into the underlying Middle Jurassic sandstones. The main uncertainty in... 

Fig. 8. Hypothetic pressure-depth plot showing the thickness of a hydrocarbon column in a normally (hydrostatic) versus overpressured reservoir. As the pore pressure in the water phase increases, a smaller hydrocarbon column can be trapped before the cap-rock reach hac-ture pressure. Note that the oil-water contact is presumed but realistic.

Within single reservoir units, formation pressure data indicate lateral stepwise increasing overpressures from approximately 70 bar above hydrostatic in the south-east to approximately 120 bar in the north-west, controlled by major northeast trending sealing faults, which subdivide the Njord structure into a series of hydraulic compartments. There is also a stepwise formation pressure increase with depth, corresponding to Triassic and Jurassic stratigraphic boundaries. 

The capillary pressure equation (Berg, 1975) predicts that capillary leakage will occur when the upward pressure of the hydrocarbon column is greater than the capillary resistance of the seal. The magnitude of upward reservoir pressure is determined by hydrocarbon buoyancy pressure plus any overpressure in the reservoir relative to the seal. The capillary resistance of the seal is determined by the size of interconnected pore throats and the interfacial tension between hydrocarbon (gas or oil) and formation water. The relationship between these factors can be expressed as follows if the angle of contact of the hydrocabon-water interface is assumed to be 0° (Clayton and Hay, 1994) ... 

Fig. 12. GEA pressure/depth plot showing the relationship between aquifer overpressure, fracture pressures, crestal reservoir pressures and closure style. Aquifer pressures are grouped into a terrace domain and a deep graben domain.

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