Changes

Often porosity and water saturation (and sometimes net to gross ratio) are input as constants representing the average value over the area of interest. Otherwise the variables are entered as surface models. Creating these surface models is complex and can be done in many ways. One of the most common is to digitize a hand-drawn map and build a model. Another is to build a model from well data. During model construction and use, certain issues must be considered:

Often porosity and water saturation (and sometimes net to gross ratio) are input as constants representing the average value over the area of interest. Otherwise the variables are entered as surface models. Creating these surface models is complex and can be done in many ways. One of the most common is to digitize a hand-drawn map and build a model. Another is to build a model from well data. During model construction and use, certain issues must be considered:

* Incomplete information caused by partial penetration, eroded top, faulting, and so on is often encountered. When this happens, the value of net to gross ratio or any other parameter being modeled will only represent the portion of the rock unit present. [[:file:using-and-improving-surface-models-built-by-computer_fig24.png|Figure 24]] demonstrates this problem for a partially penetrating well. The N: G for the middle well is 0.875 (or 3.5/4), while for the left and right wells, which fully penetrate the unit, the N: G is 0.55. Clearly the partial unit value does not correctly represent the “true” unit value. If the missing portion was pay, then the N: G would be 0.95 [(3.5 + 6)/10]. If the missing portion was nonpay, then the N: G would be 0.35 (3.5/10). The true N: G lies somewhere between these two limits. Special techniques must be used to model incomplete data (<xref ref-type="bibr" rid="pt08r11">Jones et al., 1986</xref>).

* Incomplete information caused by partial penetration, eroded top, faulting, and so on is often encountered. When this happens, the value of net to gross ratio or any other parameter being modeled will only represent the portion of the rock unit present. [[:file:using-and-improving-surface-models-built-by-computer_fig24.png|Figure 24]] demonstrates this problem for a partially penetrating well. The N: G for the middle well is 0.875 (or 3.5/4), while for the left and right wells, which fully penetrate the unit, the N: G is 0.55. Clearly the partial unit value does not correctly represent the “true” unit value. If the missing portion was pay, then the N: G would be 0.95 [(3.5 + 6)/10]. If the missing portion was nonpay, then the N: G would be 0.35 (3.5/10). The true N: G lies somewhere between these two limits. Special techniques must be used to model incomplete data.<ref name=pt08r11 />

* Modeling water saturation using standard algorithms and well data generally does not produce acceptable results. This is because the amount of water in the oil or gas portion of the reservoir is dependent upon porosity, [[permeability]], height above [[Fluid contacts|fluid contact]], and other factors. Generally several engineering functions (J-curves) that relate porosity, height above fluid contact, and water saturation are used to convert structure top, structure base, fluid contact, and porosity models to a water saturation model.<ref name=pt08r9 /> The resulting model can be adjusted to honor the existing water saturation data at wells.

* Modeling water saturation using standard algorithms and well data generally does not produce acceptable results. This is because the amount of water in the oil or gas portion of the reservoir is dependent upon porosity, [[permeability]], height above [[Fluid contacts|fluid contact]], and other factors. Generally several engineering functions (J-curves) that relate porosity, height above fluid contact, and water saturation are used to convert structure top, structure base, fluid contact, and porosity models to a water saturation model.<ref name=pt08r9 /> The resulting model can be adjusted to honor the existing water saturation data at wells.

* Net to gross and sometimes average porosity can change rapidly in the area extending from the point where the base of reservoir encounters the fluid contact to the reservoir edge (wedge zone). If the vertical distribution of net rock is not homogeneous throughout the reservoir, then these variables may change significantly in the wedge zone relative to values where the reservoir is full thickness. Often these changes are ignored. Sometimes the reservoir is separated into subzones, with a full suite of volumetric models constructed for each subzone. Three-dimensional modeling of net and porosity is a more precise solution.

* Net to gross and sometimes average porosity can change rapidly in the area extending from the point where the base of reservoir encounters the fluid contact to the reservoir edge (wedge zone). If the vertical distribution of net rock is not homogeneous throughout the reservoir, then these variables may change significantly in the wedge zone relative to values where the reservoir is full thickness. Often these changes are ignored. Sometimes the reservoir is separated into subzones, with a full suite of volumetric models constructed for each subzone. Three-dimensional modeling of net and porosity is a more precise solution.