A Concise Guide to Geopressure: Spreadsheets Chapter 9

Downloadable spreadsheets to accompany A Concise Guide to Geopressure.

Chapter 9: Trap Integrity

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Spreadsheet 9.1: Capillary Seal

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Note: Zip archive contains one macro-enabled Excel spreadsheet (xlsm).

The spread sheet ‘Capillary Seal’ graphically calculates the column height that will be trapped by a seal with a given migration pressure according to Eq. 9.1 of Flemings, 2021:

$${ℎ}_{FWL}=\frac{u_{cmig}}{\mathrm{\Delta }\rho g}.\text{Eq. 9.1}$$

The height of the hydrocarbon column (ℎ_FWL), measured from the free-water level (the depth where the capillary pressure is zero, see discussion of free water level in Chapter 2), is equal to the migration pressure through the seal (u_cmig) divided by the difference in the static pressure gradient of each fluid phase ($\mathrm{\Delta }\rho g$).

The user inputs a migration pressure (u_cmig) from laboratory-derived mercury-air measurements (e.g., Fig. 9.4). Equation 2.22 is used to covert this mercury-air pressure to an oil-brine and a gas-brine migration pressure at in situ conditions:

$$u_{cow}=u_{cHg-air}\left(\frac{{\gamma }_{ow}cos{\theta }_{ow}}{{\gamma }_{Hg-air}cos{\theta }_{Hg-air}}\right)\text{Eq. 2.22}$$

The default values in the spreadsheet produce the results shown in Fig. 9.5.

Spreadsheet 9.2: Mechanical Seal

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Note: Zip archive contains one macro-enabled Excel spreadsheet (xlsm).

The spread sheet ‘Mechanical_Seal’ graphically calculates the column height that will be trapped by either hydraulic fracturing (Eq. 9.3 and 9.4) or by shear failure (Eq. 9.3 and Eq. 9.5):

$$u_{res}^{crit}={\sigma }_3^{seal},\text{Eq. 9.3}$$

$${ℎ}_{FWL}=\frac{u_{crit}^{res}–u_w^{res}}{\mathrm{\Delta }\rho g},\text{Eq. 9.4}$$

$$u_{crit}^{res}=\frac{{\sigma }_h^{seal}–\left(\frac{1-sin{\varphi }^{\prime }}{1+sin{\varphi }^{\prime }}\right){\sigma }_v}{\left[1–\left(\frac{1-sin{\varphi }^{\prime }}{1+sin{\varphi }^{\prime }}\right)\right]}.\text{Eq. 9.5}$$

The user inputs the least principal stress (${\sigma }_3^{seal}$) in the overlying seal, the water pressure at the crest of the reservoir ($u_w^{res}$), the fluid densities ($\mathrm{\Delta }\rho g$) and the friction angle (${\varphi }^{\prime }$) and the height to free water level is calculated. The spread sheet is populated with the parameters to produce the prediction of Figure 9.14a (Flemings, 2021).