# A Concise Guide to Geopressure: Spreadsheets Chapter 9

## Chapter 9: Trap Integrity

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:

$$ℎ _{FWL} = {u _{cmig} \over \Delta \rho g}. \hspace{20pt} \textnormal{Eq. 9.1}$$

The height of the hydrocarbon column (ℎFWL), 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 (ucmig) divided by the difference in the static pressure gradient of each fluid phase ($$\Delta \rho g$$).

The user inputs a migration pressure (ucmig) 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:

$$u _{cow} = u_{cHg-air} \left( { \gamma _{ow} \hspace{3pt} cos \theta _{ow} \over \gamma _{Hg-air} \hspace{3pt} cos \theta _{Hg-air}} \right) \hspace{20pt} \textnormal{Eq. 2.22}$$

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

$$u _{res} ^{crit} = \sigma ^{seal} _3 , \hspace{20pt} \textnormal{Eq. 9.3}$$
$$ℎ _{FWL} = {u ^{res}_{crit} \hspace{3pt} – \hspace{3pt} u ^{res}_{w} \over \Delta \rho g} , \hspace{20pt} \textnormal{Eq. 9.4}$$
$$u ^{res} _{crit} = { \sigma ^{seal} _h – \left( {1-sin \phi ^\prime \over 1+sin \phi ^\prime} \right) \sigma _v \over \left[ 1 – \left( {1-sin \phi ^\prime \over 1+sin \phi ^\prime} \right) \right] }. \hspace{20pt} \textnormal{Eq. 9.5}$$
The user inputs the least principal stress ($$\sigma ^{seal} _3$$) in the overlying seal, the water pressure at the crest of the reservoir ($$u^{res}_w$$), the fluid densities ($$\Delta \rho g$$) and the friction angle ($$\phi ^\prime$$) 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).