Changes

* 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 (<xref ref-type="bibr" rid="pt08r11">Jones et al., 1986</xref>).

* 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 (<xref ref-type="bibr" rid="pt08r9">Hamilton and Jones, 1992</xref>). 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.

* If more than one of the four models input to the HPT equation contain negative values, then additional incorrect volumes could result. This is because these models are multiplied together, and if two have negative values at the same location, the resulting value will be positive, creating a volume where no volume should exist. A commonly used safety measure is to clip porosity, water saturation, and net-to-gross models to a minimum value of zero, eliminating the problem. Zeros in the model often produce a very jagged zero line contour. However, that is preferred rather than significant volume errors. There are techniques for correcting these jagged zero contour lines.

* If more than one of the four models input to the HPT equation contain negative values, then additional incorrect volumes could result. This is because these models are multiplied together, and if two have negative values at the same location, the resulting value will be positive, creating a volume where no volume should exist. A commonly used safety measure is to clip porosity, water saturation, and net-to-gross models to a minimum value of zero, eliminating the problem. Zeros in the model often produce a very jagged zero line contour. However, that is preferred rather than significant volume errors. There are techniques for correcting these jagged zero contour lines.