Seal description

General about seal mechanisms

A HC accumulation creates a pressure differential because the in-situ density of oil and gas will usually be lower than that of the formation water. The buoyancy pressure can be so strong that the seal may not be able to hold the HC column. To be fairly complete for most circumstances three seals are considered. Foremost is the top-seal. For fault traps or reefs a lateral seal may also be relevant. For stratigraphic traps even a seat-seal will be required to form a valid trap. A seal calibration can best be made by considering only the top seal and this approach has been followed by Nederlof & Mohler (1981). Their results suggest that for a practical retention modeling the idea of a "membrane seal" based purely on capillary pressure theory and measurements is not sufficient. A wider approach is required in which the membrane seal aspects are of course taken along, but the fracturing, faulting, discontinuities of the top seal are also taken into account.

Watts (1987) has made a distinction between "membrane" and "hydraulic seals" in an excellent overview of top and lateral seals. The first type is dependent on capillary pressure conditions, the second type on the wedging open by faults and fractures. In GAEAPAS the retention calibration covers both types, not because we do not believe in this distinction, but simply because the data available for prospect appraisal will not normally tell the difference.

Seal description distiguishes three seal types:

  1. Top seal
  2. Seat Seal
  3. Lateral Seal

Top seal

The top seal thickness appears normally to be a significant factor for the retention potential, especially for the hydraulic type seals. The input is in the form of a UDD control. Other factors that are important are the lithology, the depth and the degree of fracturing and faulting. Although the correlations are not very strong, they are important enough to model a retention potential for the top seal and seat seal. Note that valid codes for the lithology possibilies are limited. Valid lithology codes are listed in the list boxes for this purpose. Click on a code from the list in the listbox that appears when the arrow is clicked at the right side of the list box. The Lithology and Faulting/Fracturing codes represent constants that are listed in the Gaeacalib file. Lithology is clearly a proxy variable for capillary pressure information. In view of this, one could imagine that the top seal effect would be exclusively one of lithology. However, the thickness of the seal appears also to have an effect. This is explained in the following way: the thicker the seal, the more continuous over the structure it is bound to be. So if no leakage takes place through an unfaulted, unfractured lithology, it may do so in the case of fractures. The risk that an otherwise good seal is failing by fracturing/faulting is reduced for a thick seal. The risks to a top seal are not only the presence of undetected small faults and fractures, but also occasionally sand injectites or sand channels creating a leak and diffusion of gas through the top seal, a slow but sometimes significant process. The risk that a top seal fails completely is taken care of in the Risk/POS table.

The input for the top seal consists of a UDD input table and two listboxes for the lithology and the faulting/fractiring.

Seat seal

Seat seal description is required in combination with a stratigraphic trap statement. The Lithology is then also required, but it is assumed that the faulting/fracturing of the seat seal is the same as for the top seal.

A stratigraphic trap might leak HC through the top seal, but also through the seat seal. In a wedge-shaped reservoir on a monocline, HC may travel through the seat seal, if not strong enough, into an underlying reservoir that is not stratigraphically closed. Still the HC would move laterally and upwards. The retention modeling is taking the "weakest link", i.e. the minimum retention potential is taken from the top seal and seat seal potentials. Care should be taken not to exaggerate the seat-seal potential, for instance by assigning a great thickness to it that exceeds the vertical closure.

The seat seal input is exactly the same as for the top seal, with a UDD and listboxes.

Lateral seal

The seal calibration studies mentioned above have not addressed the problem of the lateral seal, such as against a fault. The variety in geological situations is such that a global calibration becomes very complex. However, most geologists will agree that a good juxtaposition of reservoir in the trap against shale on the other side of the fault will often be sufficient to hold a large column of HC. There are a number of systems to estimate and quantify fault smear that would even allow juxtaposition of reservoir against reservoir to hold HC. Juxtaposition maps (Allan diagrams) etc. attempt to look at the three-dimensional situation. Sometimes data are insufficient to do this and a one-dimensional approach based on the stratigraphy or lithology column can be made. A few good examples are given by Linji Y, (2009). There are several commercial fault seal analysis programs on the market. As a shortcut, it is always useful to take a wirelinelog showing the reservoir intervals and shift a copy of it against the first over the depth interval of the estimated throw as a rough indication of juxtaposition problems. With growth faults, the downthrown section may be expanded and this should be taken into account.

The lateral seal aspect is closely related to the trap description. As the contoures for the trap are given on another form (form 6) the contour depths are automatically repeated in a label above the spillpoint UDD statement for easy reference. The spill/leakpoint statements should not contradict the trap geometry input. However spillpoint depths below the last contour given will not increase the trap capacity, but will be simply ignored.

It should be noted that the spill/leak point statement combines any uncertainty about what the closing contour is (geophysical uncertainty) and about the lateral seal (geological uncertainty, such as juxtaposition). This dual purpose is a good reason to make the input in the form of a depths table with possible leak points, and a doubt on position of last closed contour.