Weyburn seal capacity

Exploring for Oil and Gas Traps
Series	Treatise in Petroleum Geology
Part	Predicting the occurrence of oil and gas traps
Chapter	Predicting reservoir system quality and performance
Author	Dan J. Hartmann, Edward A. Beaumont
Link	Web page
Store	AAPG Store

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.

Total oil column height

Weyburn and nearby Steelman fields appear to produce from a single, pressure-communicated oil column. 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:

${\mbox{h}}={\frac {0.670\times \gamma \cos \theta }{{\mbox{R}}_{\rm {bt}}(\rho _{\rm {w}}-\rho _{\rm {h}})}}$

where:

• γ = interfacial tension (dynes/cm)
• θ = contact angle
• R_bt = breakthrough pore throat size (μ)
• ρ_w = formation water density (g/cc)
• ρ_h = hydrocarbon density (g/cc)

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).

Other parameters:

• Reservoir temp. = 150°F (66°C) (possibly a low estimate)
• GOR = 100 CFG/BO (18 m³ gas/m³ oil) (probably low estimate)
• Reservoir press. = 3,000 psi (20.7 × 10³ kPa)
• γ = 35 dynes/cm at STP
• θ = 0° (seal is assumed to be very strongly water wet)

Estimates of in situ values:

• ρ_w = 1.01 g/cc (from his Figure 2)
• ρ_h = 0.85 g/cc (from his Figure 3)

Therefore:

• (ρ_w – ρ_h) = 0.16 (0.12 from approximations in preceding section)
• γ = 27 dynes/cm (from his Figure 11)
• cos θ = 1 (very strongly water wet)

Therefore:

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

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

A generally accepted concept is that oil migrates after filling only the minimum possible percentage of the largest pore throats that is required to establish a continuous, thread-or rope-shaped channel through the rock This is the breakthrough or critical nonwetting phase saturation. Clearly, estimating R_bt or the size of the largest connected pore throats that control hydrocarbon breakthrough is critical to this analysis.

There are at least three ways to estimate R_bt.

• Measure R_bt directly on core samples, then correct it to reservoir conditions using methods of the preceding section (e.g., ).
• Estimate the breakthrough pressure from the shape of a capillary pressure curve.
• Use the Winland method to derive a measure of pore throat size in the seal.

Winland approach to estimate r_bt

The Winland approach is perhaps the simplest method for obtaining R_bt because it uses readily available core analyses. The method relates a core sample's porosity and permeability to the pore throat size indicated at a given nonwetting-phase saturation. Once breakthrough saturation is estimated, the Winland method yields pore throat sizes representative of that mode of pore throats, or R_bt.

Choosing a breakthrough saturation

The difficulty is knowing the breakthrough saturation for a formation without lab data from samples from that formation. There are conflicting opinions about how to estimate breakthrough saturation:

• Thomas et al., Schowalter, and general industry opinions suggest oil or gas migration through plug-size samples occurs at nonwetting phase saturations of about 10% (4-17%), i.e., that the largest 10th percentile of pore throats controls breakthrough.
• Catalan et al. observed breakthrough saturations of 4-20% in pack experiments. Relative permeability analysis of core plugs shows the first nonwetting phase flow occurs at approximately the same saturations for most rocks.
• Other workers (Alan Byrnes, personal communication, 1995) have observed breakthrough in plug samples at highly variable saturations—sometimes more than 50%.

It would seem best to calculate R_bt for different reasonable breakthrough saturations to test the sensitivity of the solution.

Winland's r₁₀

Breakthrough saturation of 10% is reasonable to use for most rocks. Using a statistical analysis similar to that of Winland, Franklin developed the following formula for R_bt at 10% nonwetting phase saturation (also called r₁₀):

$\log {\mbox{R}}_{\rm {tt}}=0.353+0.427\log {\mbox{K}}_{\rm {a}}-0.184\log \phi$

where:

• Ka = air permeability, md
• φ = porosity, % (not decimals)

Most of the cores in the porous mudstone facies found updip from the productive area have porosities of about 10% and permeabilities of about 0.1 md (or less), based on routine core analyses from the 1960s. Unfortunately, the core permeabilities are too high, given (1) the tendency to “high-grade” core plugs in better rock and (2) the fact that the parameters used on these samples could not measure values bt = 0.4μ for these rocks, consistent with petrographic data. If r₃₅ is a better approximation of R_bt, then Winland's equation yields R_bt = 0.1μ.

Weyburn oil column height

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

Hannon 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

• Evaluation of trap type
• Weyburn field location and trap problem
• Midale lithofacies and distribution
• Midale porosity, pore geometries, and petrophysics
• Effect of pore geometry on Sw in midale rocks
• Midale seal capacity and trap type

External links

find literature about Weyburn seal capacity

• Original content in Datapages
• Find the book in the AAPG Store

drill stem test

American Petroleum Institute

gas-oil ratio