Fault seal

Other, less frequent fault seals are created by ... 

Figure 5.8 Fault seal as a result of clay smear and juxtaposition...

In many cases faults will only restrict fluid flow, or they may be open i.e. non-sealing. Despite considerable efforts to predict the probability of fault sealing potential, a reliable method to do so has not yet emerged. Fault seal modelling is further complicated by the fact that some faults may leak fluids or pressures at a very small rate, thus effectively acting as seal on a production time scale of only a couple of years. As a result, the simulation of reservoir behaviour in densely faulted fields is difficult and predictions should be regarded as crude approximations only. 

Fault seals are known to have been ruptured by excessive differential pressures created by production operations, e.g. if the hydrocarbons of one block are produced while the next block is kept at original pressure. Uncontrolled cross flow and inter-reservoir communication may be the result. 

Watts, N.L., 1987. Theoretical aspects of cap-rock and fault seals for single- and two-phase hydrocarbon columns. Marine and Petroleum Geology, Vol. 4, November 1987, pp. 274-307 Weber, K.J., 1982. Influence of common sedimentary structures on fluid flow in reservoir models. Journal of Petroleum Technology, March 1982, pp. 665-672 Weber, K.J., 1987. Hydrocarbon distribution patterns in Nigerian growth fault structures controlled by structural style and stratigraphy. Journal of Petroleum Science and Engineering, 1, pp. 91-104... 

Finally, there is still a trapping configuration which, although quite common, is often overlooked or misunderstood, the so-called perched water-contact (Johnson et al., 1986 Weber, 1995). This occurs when a reservoir has a U-tube shape with a fault seal or a pinch-out at the closed end. The water cannot escape from this side of the U-tube resulting in different hydrocarbon-water contacts in the same reservoir. In the North Sea, we find perched oil-water contacts in Fulmar (Fig. 16), Tern, Ulla and Logger fields, but there are probably many more cases. 

Bouvier, J.D., Kaars-Sijpesteijn, C.D., Kluesner D.F., Onyejekwe, C.C. and van der Pal, R.C. 1989. Three-dimensional seismic interpretation and fault sealing investigations. Nun River field, Nigeria. Am. Assoc. Pet. Geol. Bull., 73 1397-1414. 

Watts, N.L. 1987. Theoretical aspects of cap-rock and fault seals for single- and two-pha.se hydrocarbon columns. Mar. Pet. Geol., 4 274-307. 

Fault seal analysis successful methodologies, application and future directions... 

Fault seal prediction in hydrocarbon reservoirs requires an understanding of fault seal mechanisms, fault rock petrophysical properties, the spatial distribution of seals, and seal stability. The properties and evolution of seals within fault zones can be evaluated using the combined results of structural core logging, microstmctural and physical property characterisation, together with information on fault populations from seismic and outcrop studies and well test data. 

Successful seal analyses depends upon the amalgamation of data from the miero-scale to the macro-scale. This review demonstrates that improvements in fault seal risk evaluation are possible. The future direetions for improving fault seal risk evaluation are also discussed. The most critical of these are characterisation of the internal structure of fault zones, generation of a database for fault rock petrophysical properties and incorporation of the impact of realistic fault zone geometries into reservoir modelling programs. 

Fig. 1 outlines the important components needed for fault seal evaluation. The evaluation route shown emphasises that a range of different elements have to... 

Fig. 1. Outline of the important components in a fault seal evaluation.

The critical questions which now require evaluation in order to advance fault seal analysis are listed below ... 

What is involved in fault seal risk analysis and what future requirements can be defined ... 

Each of the questions listed above forms a subsection of this paper. A review of the recent advances in these areas, the requirements for answering the questions, and likely limitations to present solutions are discussed. The final section of the paper integrates these different aspects into a discussion of the risks associated with fault seal evaluation, and attempts to identify the future directions which will help remove the present limitations. 

The terminology and classification of fault-rocks and seal types is not yet universally agreed (Knipe, 1992a Knott, 1993). The classification presented below is based on identification of the main process responsible for the reduction in permeability associated with the faults. Mechanistic terms have been combined with textural descriptive terms to provide a more expansive nomenclature system which covers the most common fault rocks and seat types. The fault seal types and associated fault-rock types can be divided into two broad categories ... 

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... 

A large proportion of the structural features which can impact on the flow properties of a reservoir are below the resolution of seismic data. Therefore, understanding the spatial distribution and internal structure of fault zones can be fundamental to the flow behaviour of hydrocarbon reservoirs. This section reviews the important geometrical characteristics of fault zones and sub-seismic deformation features which can influence fault seal analysis. 

Is the structure of fault zones usually assumed in fault seal analysis valid and what is the impact of fault damage zones ... 

What fault zone properties need to be incorporated into a robust fault seal analysis ... 

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