Seal capacity

Fig. 3.12 A sill intruded between carbonatic rock beds forming a local aquielude. Weathering into clay minerals may improve the sealing capacity of a sill.

Hydrodynamic conditions influence the system of hydrocarbon migration in a basin, i.e. the volumes of hydrocarbons available for entrapment in a certain part of the basin, and the trapping energy conditions in the basin, i.e. the location of potential trapping positions and the sealing capacity of rocks and faults (Sections 5.2 and 5.3). 

The sealing capacity of a rock under hydrostatic conditions is determined by the minimum hydrocarbon-water displacement pressure of the rock, which depends on the radius of the largest connected pore throats in the rock and the oil-water and gas-water interfacial tensions, and in addition on the densities of groundwater and hydrocarbons accumulating in the adjacent reservoir rock. The maximum height of an oil or gas column that can accumulate below a seal is given by Equation 4.17 (Section 4.1.3)... 

The sealing capacity of a rock changes with depth. This is because the characteristics of the rock change with depth (e.g. the porosities and permeabilities decrease with depth), the interfacial tensions of oil and gas change (Section 4.3.1) and the densities of groundwater, oil and gas change (Section 4.3.1). At shallower depths (< 2 km) the gas-water interfacial tension of gas is greater than the oil-water interfacial tension, while at depths of more than > 2 km the gas-water interfacial tension is similar to the oil-water interfacial tension (Section 4.3.1). 

The maximum height of a hydrocarbon column that can be contained in a hydrostatic trap is determined by the sealing capacity and geometry of the rocks, or rocks and faults that form the trap. When the vertical distance from crest to spill plane of the trap (Figure 5.1) is less than the maximum height of the hydrocarbon colunm Zj (Equation 4.17), the accumulating hydrocarbons may fill the trap to its spillpoint. As hydrocarbons continue to migrate into the... 

Hydrodynamic conditions affect the sealing capacity of a rock or fault, and consequently influence the holding capacity for hydrocarbons of hydrostatic structural, stratigraphic and combination traps. In addition, hydrodynamic conditions may create additional regions of minimum potential energy for separate phase hydrocarbons, i.e. purely hydrodsmamic trapping positions. 

Under the assumption that the capillary pressure gradient across the carrier rock-barrier rock interface is the only significant resistant force affecting hydrocarbon accumulation in a conventional hydrostatic trap, i.e. the influence of the hydrodynamic condition in the barrier rock on its sealing capacity can be considered to be negligible, the maximum height of the hydrocarbon column below the barrier rock can be given by Equation 4.22... 

Those that occur in conventionally closed lithological structures (i.e. in hydrostatic traps). In these traps the hydrocarbon-water contact may have any degree of tilt from the horizontal to the maximum dip of the barrier boundary at the downstream side of the closure. Although hydrocarbons may become trapped in the conventional hydrostatic traps of sufficient sealing capacity, the hydrocarbon accumulation is not necessarily present in the same position within the trap, as its actual position depends on the hydrodynamic condition in the carrier-reservoir rock (Figure 5.10). 

Tectonic forces may affect the hydrocarbon accumulation and entrapment indirectly by changing the hydrodynamic condition of the basin (Chapter 4) and thus the sealing capacity of rocks and the holding capacity of hydrostatic traps. 

The physical principles of the trapping of oil and gas are well established but quantification remains problematic in many cases. Consequently, the trapping capacity of structures and the retention of hydrocarbons with time is difficult to predict in exploration ventures. In a production setting, with a number of similar fields, the situation is much more favourable for the quantification of sealing capacity. 

Without a detailed understanding of the fundamental processes which control the evolution of fault rocks and their properties, the prediction of sealing capacity and the evaluation of the behaviour of a faulted reservoir will never be anything more than speculative. 

The basic requirement for mapping fault seals is the generation of a realistic, maximum probability map of sealing capacities along individual fault zones. This involves evaluation of the possible juxtaposition patterns within the zone as well as an assessment of the variance of fault rock properties. 

The prediction of fault seal capacity requires the evaluation of each fault seal mechanism and its possible effect on fault sealing. 

Fig. 1. Classification of fault seal processes. Fault seals are divided into two types juxtaposition fault seals and fault gouge seals (e.g., cataclasis and clay smear). One fault may have a combination of different sealing processes affecting its sealing capacity.

Fig. 2. Strategy for the analysis of fault seals over geological time scales. Based on a fault juxtaposition diagram, the effects of the different fault seal processes are assessed in a systematic manner for their seal capacity due to clay smear, brittle fault sealing and juxtaposition fault seal.

Fig. 6. Brittle fault seal analysis strategy. The strategy aims to quantify the sealing capacity of brittle faults by first predicting the deformation mechanism. Particulate flow faults are treated as non-sealing. Cataclastic faults have variable sealing properties according to fault throw and matrix properties. The chart in the lower left of this figure is reproduced at a larger scale in Fig. 8.

Faults in sandstones deformed at depths greater than 1 km tend to deform by cataclasis. Permeability and entry pressure of such faults can be predicted from estimates of matrix properties. Static seal capacities of cataclastic faults depend on the minimum sealing properties, which are related to the fault displacements. 

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