Changes

  | part    = Predicting the occurrence of oil and gas traps

  | part    = Predicting the occurrence of oil and gas traps

  | chapter = Predicting reservoir system quality and performance

  | chapter = Predicting reservoir system quality and performance

  | frompg  = 9-1

  | frompg  = 9-145

  | topg    = 9-156

  | topg    = 9-147

  | author  = Dan J. Hartmann, Edward A. Beaumont

  | author  = Dan J. Hartmann, Edward A. Beaumont

  | link    = http://archives.datapages.com/data/specpubs/beaumont/ch09/ch09.htm

  | link    = http://archives.datapages.com/data/specpubs/beaumont/ch09/ch09.htm

==Weyburn trap model==

==Weyburn trap model==

A working trap model for Weyburn field is that of a macroporous vuggy packstone reservoir lying downdip from a microporous intercrystalline mudstone seal. How much hydrocarbon column could a trap like this retain, especially since superficially the seal doesn't appear to be a seal at all. Instead, it consists of rocks with appreciable [[porosity]], local oil staining, local log-calculated water saturations less than 100%, and the capability of producing significant amounts of water on DST.

A working trap model for Weyburn field is that of a macroporous vuggy packstone reservoir lying downdip from a microporous intercrystalline [[mudstone]] seal. How much [[hydrocarbon column]] could a trap like this retain, especially since superficially the seal doesn't appear to be a seal at all. Instead, it consists of rocks with appreciable [[porosity]], local oil staining, local log-calculated water saturations less than 100%, and the capability of producing significant amounts of water on DST.

==Total oil column height==

==Total oil column height==

Weyburn and nearby Steelman fields appear to produce from a single, pressure-communicated oil column.<ref name=ch09r24>Hannon, N., 1987, Subsurface water flow patterns in the Canadian sector of the Williston Basin: RMAG 1987 Symposium Guidebook, p. 313–321.</ref> If so, then the total height of this combined column is about [[length::600 ft]] [[depth::(180 m]]). Could rocks of the porous mudstone facies act as a lateral seal for this much oil column? To calculate oil column height, we use the following equation:

Weyburn and nearby Steelman fields appear to produce from a single, pressure-communicated oil column.<ref name=ch09r24>Hannon, N., 1987, Subsurface water flow patterns in the Canadian sector of the Williston Basin: RMAG 1987 Symposium Guidebook, p. 313–321.</ref> If so, then the total height of this combined column is about 600 ft (180 m). Could rocks of the porous mudstone facies act as a lateral seal for this much oil column? To calculate oil column height, we use the following equation:

:<math>\mbox{h} = \frac{0.670 \times \gamma \cos \theta}{\mbox{R}_{\rm bt}(\rho_{\rm w} - \rho_{\rm h})}</math>

:<math>\mbox{h} = \frac{0.670 \times \gamma \cos \theta}{\mbox{R}_{\rm bt}(\rho_{\rm w} - \rho_{\rm h})}</math>

==Calculating oil column height at weyburn==

==Calculating oil column height at weyburn==

We can calculate the potential oil column height that could be sealed by the porous mud-stone facies using maximum reasonable estimates for the above parameters. Weyburn field oil densities grade from 35°API in the updip portion of the field to 27°API near the base. A representative gravity of 30°API is used for the column as a whole. The formation water is brackish NaCl brine (35,000 ppm).

We can calculate the potential oil column height that could be sealed by the porous mud-stone facies using maximum reasonable estimates for the above parameters. Weyburn field oil densities grade from 35°API in the updip portion of the field to 27°API near the base. A representative [[gravity]] of 30°API is used for the column as a whole. The formation water is brackish NaCl brine (35,000 ppm).

Other parameters:

Other parameters:

* Reservoir temp. = [[temperature::150&deg;F]] ([[temperature::66&deg;C]]) (possibly a low estimate)

* Reservoir temp. = 150&deg;F (66&deg;C) (possibly a low estimate)

* GOR = 100 CFG/BO (18 m³ gas/m³ oil) (probably low estimate)

* GOR = 100 CFG/BO (18 m³ gas/m³ oil) (probably low estimate)

* Reservoir press. = [[pressure::3,000 psi]] (20.7 × 10³ kPa)

* Reservoir press. = 3,000 psi (20.7 × 10³ kPa)

* γ = 35 dynes/cm at STP

* γ = 35 dynes/cm at STP

* θ = 0° (seal is assumed to be very strongly water wet)

* θ = 0° (seal is assumed to be very strongly water wet)

* γ cos θ = 27 dynes/cm (26 dynes/cm from approximations in preceding section)

* γ cos θ = 27 dynes/cm (26 dynes/cm from approximations in preceding section)

Substituting these values into the above equation results in h [[length::( ft]]) = 176/R_bt (μ). All that is left is to estimate R_bt.

Substituting these values into the above equation results in h = 176/R_bt (μ). All that is left is to estimate R_bt.

==Estimating r_bt==

==Estimating r_bt==

==Weyburn oil column height==

==Weyburn oil column height==

If R_bt = r₁₀ = 0.4μ and h = 113 ft/R_bt, then the estimated oil column is [[length::283 ft]] [[depth::(86 m]]). If R_bt = r₃₅ = 0.1μ, then h = [[length::892 ft]] [[depth::(272 m]]).

If R_bt = r₁₀ = 0.4μ and h = 113 ft/R_bt, then the estimated oil column is 283 ft (86 m). If R_bt = r₃₅ = 0.1μ, then h = 892 ft (272 m).

==Using estimated oil or gas column heights==

==Using estimated oil or gas column heights==

Hannon<ref name=ch09r24 /> calculated only [[length::100 ft]] [[depth::(30 m]]) of [[seal capacity]] for this field. His calculations assumed a breakthrough pressure of 10-15 psi (69-103 kPa), based on “a multitude of capillary pressure curves” that he did not document. Yet we can estimate several reasonable breakthrough pressures from any given capillary pressure curve, depending on the assumed nonwetting phase saturation.

Hannon<ref name=ch09r24 /> calculated only 100 ft (30 m) of [[seal capacity]] for this field. His calculations assumed a breakthrough pressure of 10-15 psi (69-103 kPa), based on “a multitude of capillary pressure curves” that he did not document. Yet we can estimate several reasonable breakthrough pressures from any given capillary pressure curve, depending on the assumed nonwetting phase saturation.

==See also==

==See also==

[[Category:Predicting the occurrence of oil and gas traps]]  

[[Category:Predicting the occurrence of oil and gas traps]]  

[[Category:Predicting reservoir system quality and performance]]

[[Category:Predicting reservoir system quality and performance]]