Capillary entry pressure

Relationship between capillary entry pressure, limiting capillary pressure, and permeability of the medium. (Reproduced with permission from Ref. 41. Copyright 1986 SPE-AIME.)... 

In laboratory experiments and field applications the gas is delivered to the rock face either as continuous gas or as a course gas-liquid dispersion. In both cases, for the gas to move into the porous medium, the gas pressure at the rock face must be higher than the capillary entry pressure. For a gas finger or a bubble train to advance through the porous medium, the face pressure must be maintained at a level above the maximum capillary pressure that the gas finger or bubble train will experience along its path through the medium. 

Several saturation regions can be identified on this curve. S<0,3 represents a pendular saturation state or bridging range. For S>0,8 no liquid bridges exist in the capillary state and ends when the first liquid bridges form between the particles. The relation between maximum tensile strength and the saturation of the cake with the special case of the capillary entry pressure can be written as follows (9,10) ... 

Fundamental to a successful fault seal analysis is quantification of the petrophysical properties of the different fault rocks present in the hydrocarbon field under investigation. The critical properties which require quantification are permeabilities, capillary entry pressures, transmissibility, fault-rock thickness and the strength of the fault rocks. One of the reasons why fault seal analysis and reservoir modelling has proved difficult has been the absence of data on these properties. Analysis of the petrophysical properties of... 

Accurate permeability measurements of fault rocks as well as characterisation of capillary entry pressure data of fault rocks requires different equipment and procedures than those applicable to... 

If a fault is hydrocarbon-wet or the pressure difference across it exceeds the capillary entry pressure, it will leak and become a permeability barrier to flow. The probability that a hydro carbon column will be retained by a permeability barrier over geological time is a function of the rate of flow through the fault. In the case of Darcy flow, the rate of flow (Q) per unit area is proportional to the pressure gradient (AF/Ax) across the fault, the fault permeability (/c), and inversely proportional to the fluid viscosity (a) ... 

A pre-requisite for fault seal analysis is a consistent structural model, with sufficient detail and proper fault linkage relationships. The first step of static fault seal analysis (Fig. 2) involves the construction of a juxtaposition diagram (Allan, 1989), in which areas where reservoir is juxtaposed against a sealing lithology are identified. The retention capacity is calculated from the minimum capillary entry pressure of the juxtaposed lithology, which can be measured or... 

The second stage of static fault seal analysis is the evaluation of the properties of the fault gouge (Fig. 2). In the presence of clay layers, the introduction of clay into a fault is one way of strongly increasing the capillary entry pressure. A common process is the... 

Fig. 9. Permeability versus capillary entry pressure. Entry pressure increases with decreasing permeability. The regression line drawn through the data was derived for a range of lithologies by Ibrahim et al. (1970) (Watts, 1987 Antonellini and Aydin, 1995). The relationship appears to hold for the cataclastic faults as well. Some of the deviations from the regression are due to uncertainties in the thickness of the deformation bands and slip planes, which lead to overestimated fracture permeabilities of up to one order of magnitude.

Juxtaposition seal of reservoir against non-reservoir can be assessed by fault-plane diagrams. Additional seal may be developed (at reservoir juxtapositions) if fault-plane processes increase the capillary entry pressure. In Oseberg Syd, clay smearing is considered to be dominant because of the relatively shaly nature of the Brent Group and the shallow burial depths during faulting (<500 m). 

Most seals in clastic sequences are membrane seals (Watts, 1987). The dominant control on seal failure is the capillary entry pressure of the seal-rock, that is, the pressure required for hydrocarbons to enter the largest interconnected pore throat of the seal. A number of mechanisms have been recognised whereby fault planes can act as a membrane seal (e.g.. Watts, 1987 Knipe, 1992) ... 

Direct observations of sub-surface pressure allow a calibration to be made between the SGR and seal capacity. Ideally, an in situ measurement of the pore-pressure in the reservoir and that inside the fault zone would allow the capillary entry pressure of the fault to be calculated. However, fault-zone pressures are rarely available. Instead, the pressure difference between the two walls of the fault is a more general parameter that can be derived from pressure measurements in pairs of wells across the fault. Fig. 7a shows one such calibration, based on the Nun River dataset of Bouvier et al. (1989). From their strike projections of Fault K , values of SGR have been calculated on a dense grid across the fault surface. On the same grid, minimum across-fault pressure differences have also been derived, using the proven distribution of hydrocarbons in the footwall sands to calculate buoyancy pressures. Fig. 7a shows a cross-plot of these two parameters for the areas of sand-sand contact at the fault surface. The dashed line indicates the inferred relationship between SGR and seal capacity. At SGR < 20%, no fault-sealed hydrocarbons are observed the shale content of the slipped interval... 

Fig. 1. Simplified evaluation strategy for top seal assessment. The flow chart begins by determining if faults throws are greater than the top seal thickness. If so, then a fault seal analysis is an additional requirement. Top seals are simplified into three main types (1) massive shale, (2) layered shale/sand/silt, and (3) massive strata of other coarser grained lithologies. Key top seal risks and the data required to carry out their assessments are shown in the flow chart. The rectangles represent leakage scenarios and the ellipses indicate data which will contribute to analysis of the scenarios (abbreviations Fluid P, formation fluid pressure <5 hor, minimum horizontal stress Entry P, capillary entry pressure HC prop s, hydrocarbon physical properties, including wetting characteristics).

Laboratory measurements of capillary entry pressures are commonly performed on Hg-air systems. To calculate maximum hydrocarbon column heights, mercury-air capillary pressure data must first be converted to hydrocarbon-water pressures, using the following equation (Watts, 1987) ... 

Our database indicates mercury-air capillary entry pressures for siltstones between 20 and 30 MPa (equivalent to a 400-7(X) m column of 30° API oil sealed at 2.5 km depth), and 45-55 MPa for mudstones (900-1200 m oil columns). For shales, these... 

This happens because, under these conditions, once the capillary seal has been breached, the flux into the trap is always greater than the flux out of the trap. The accumulating column of hydrocarbons will therefore always have a buoyancy force greater than the capillary entry pressure, which renders the capillary seal ineffective. The result is that leakage is controlled by the permeability of the seal rock alone. 

These results suggest that dynamically stable, underfilled, traps can be expected to be rare in real subsurface cases and that most traps will either be full to spillpoint, or will have leaked their charge completely. It has been demonstrated above that capillary entry pressures are not the sole control on maximum column heights in top seals. The relative importance of permeability and entry pressure, in controlling maximum columns, changes in response to changes in petrophysical parameters, which are. 

The relationships between the pore-pressures of the Jurassic reservoirs, the estimated overburden pore pressures and the formation integrity trends of the structure are taken to suggest that capillary entry pressures (membrane seal failure), possibly in combination with cap rock microfracturing, are the main controlling mechanisms for vertical leakage. 

Phc-water consists of a fluid flux (Darcy) component and a non-wetting phase capillary entry-pressure... 

Capillary pore throat blockage isolates bodies of oil as the capillary entry pressure cannot be overcome under static flow conditions. 

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