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Fault seal analysis for reservoir development (view source)
Revision as of 17:35, 29 September 2021
, 17:35, 29 September 2021no edit summary
Allan's Mapping Technique produces a two-dimensional model of the 3-D fault surface. It is a static model showing the lithological cut-off relationship across the fault surface for a specific temporal stage in the fault deformation history. Therefore, this model is less used in distinguishing cross-fault lithology relationships for other temporal stages in the structural evolution of faults. Knipe<ref name=Knipe_1997>Knipe, R. J., 1997, Juxtaposition and seal diagrams to help analyze fault seals in hydrocarbon reservoirs: AAPG Bulletin, v. 81, p. 187-195.</ref> introduced a technique known as the Juxtaposition Diagram ([[:file:GumelarFigure5.jpg|Figure 5]]) which can be used to determine the relative alignment of the lithology for each structural configuration. This technique uses a one-dimensional stratigraphic column of the footwall at a single spatial point along the length of the fault surface, vertically offsetting itself, to construct a diagram showing the relative alignment of the lithology across faults for a hanging-wall thrown between zero and maximum (usually equal to vertical thickness. stratigraphic column). Subject to the assumption that the footwall and hanging-wall stratigraphy are identical, the alignment relationship for the number of throwlines can be ascertained by dropping the throwline through the corresponding position on the diagram ([[:file:GumelarFigure5.jpg|Figure 5]]) using this technique, the alignment of the relationship positions at any point in time in the structural evolution of the fault can be investigated.
Allan's Mapping Technique produces a two-dimensional model of the 3-D fault surface. It is a static model showing the lithological cut-off relationship across the fault surface for a specific temporal stage in the fault deformation history. Therefore, this model is less used in distinguishing cross-fault lithology relationships for other temporal stages in the structural evolution of faults. Knipe<ref name=Knipe_1997>Knipe, R. J., 1997, Juxtaposition and seal diagrams to help analyze fault seals in hydrocarbon reservoirs: AAPG Bulletin, v. 81, p. 187-195.</ref> introduced a technique known as the Juxtaposition Diagram ([[:file:GumelarFigure5.jpg|Figure 5]]) which can be used to determine the relative alignment of the lithology for each structural configuration. This technique uses a one-dimensional stratigraphic column of the footwall at a single spatial point along the length of the fault surface, vertically offsetting itself, to construct a diagram showing the relative alignment of the lithology across faults for a hanging-wall thrown between zero and maximum (usually equal to vertical thickness. stratigraphic column). Subject to the assumption that the footwall and hanging-wall stratigraphy are identical, the alignment relationship for the number of throwlines can be ascertained by dropping the throwline through the corresponding position on the diagram ([[:file:GumelarFigure5.jpg|Figure 5]]) using this technique, the alignment of the relationship positions at any point in time in the structural evolution of the fault can be investigated.
[[file:GumelarFigure6.jpg|thumb|300px|{{figure number|6}}Displacement factor algorithm for estimating clay smears in the fault plane. (a) Potential Clay Smear; (b) Shale Gouge Ratio; (c) Shale Smear Factor (redrawn from Yielding, 1997).]]
[[file:GumelarFigure7.jpg|thumb|300px|{{figure number|7}}Plot of parameter relationship with formation ability for sealing capacity in reservoir (a) Clay Smear Potential; (b) Shale Gouge Ratio; (c) Shale Smear Factor (Yielding et al, 1997).]]
==Clay smear calculation==
Clay smear is clay that is found in the fault segment and is one of the inhibiting factors for hydrocarbon migration. However, according to Yielding, et al, (1997) there are factors that determine the formation of clay smears, namely:
* Thick coated claystone will result in a thicker clay spread.
* The thickness of the clay smear decreases the further it is from the source layer of the clay.
* Clay smear will get thinner with increasing distance of the vertical fault displacement (throw).
* The more claystone layers that are affected by faults will produce a continuous smear.
According to Yielding et al.<ref name=Yieldingetal_1997 />, calculations to approach fault sealing analysis by combining certain parameters are ([[:file:GumelarFigure6.jpg|Figure 6]] and [[:file:GumelarFigure7.jpg|Figure 7]]):
* Shale Smear Factor (SSF) or clay smear factor is how far the vertical displacement is to the thickness of the claystone that is cut by the fault, and SSF does not become a seal if it is > 7. SSF is considered effective when applied to a simple single fault plane (not multiple fault planes) and only one clay layer is enlarged<ref name=Fossen_2010>Fossen, H., 2010, Structural Geology: Cambridge University Press, Cambridge, 463 p.</ref>.
SSF = (Fault Offset)/(Thickness of clay layer (3.1)
* Shale Gouge Ratio (SGR) or the ratio of crushed clay is the percentage thickness of the clay layer that is in the fault segment to the vertical displacement (throw). The higher the SGR value, the more potential it becomes a barrier.
SGR = Σ Thickness of clay layer/fault throw (3.2)
* Clay Smear Potential (CSP) is the amount of clay present in the fault segment which is given to each layer of clay that is cut by the fault. The more the CSP value, the further the displacement is indicated.
CSP = Σ Thickness of clay layer^2/Distance to clay layer (3.3)
==Hydrocarbon column height (HCH)==
[[file:GumelarFigure8.jpg|thumb|300px|{{figure number|8}}a) & b) are examples of leaks from a major structure. HCH in a) has a lower height than b). c) has a spill point value that is above the crest depth so that no leakage occurs<rf name=Schofield+2016 />.]]
Analysis of hydrocarbon column height is a calculation to determine the resistance of a point in the fault plane to withstand hydrocarbons in the thickness domain (meters / feet). According to Schofield<ref name=Schofield_2016>Schofield, James K., 2016, Relationships between Observed Hydrocarbon Column Heights, Occurrence of Background Overpressure and Seal Capacity within North West Europe: Thesis.</ref> hydrocarbon column height is affected in 3 main things, namely:
* Limited by structure, directly affects the filling of hydrocarbons into the reservoir. The main effect is structurally limited to the elevation area ([[:file:GumelarFigure8.jpg|Figure 8]]).
* Limited by volumetric, this factor involves the lack of ingress of accumulated hydrocarbons into the trap. In this case it may be that the source rock is not producing sufficiently to fill the reservoir and the effect of migration pathways which have varying permeability becomes an obstacle for the accumulation of hydrocarbons.
* Geomechanics, related to the deformation of the cap rock that covers the hydrocarbons. In this case, it has the influence of matrix capillary control & control of the rupture of the hood rock<ref name=Schofield_2016 />. According to Schowalter<ref name=Schowalter_1979>Schowalter, T. T. 1979, Mechanics of secondary hydrocarbon migration and entrapment: AAPG Bulletin, v. 63, no. 5, p. 723-760.</ref>, it has been estimated that the minimum caprock membrane failure value of oil saturation is between 4.5 - 17% with an average of 10% based on buoyancy pressure and capillary pressure based on laboratory experiments.