Porosity determination reservoir rocks

Porosity determination reservoir rocks

The presence of pores and voids in rock is called porosity. The porosity of oil-bearing rocks is characterized by the porosity coefficient.

The porosity coefficient (m) is the ratio of the pore volume of a sample (Vn) to the apparent volume of the same sample (V0) (formula 1).

[pic 1].                                        (1)

The porosity coefficient is expressed in fractions of a unit or as a percentage of the sample volume.

Rock porosity is a very important parameter necessary for estimating oil reserves and elucidating filtration processes in a porous medium.

The following types of rock porosity are distinguished:

1) total (absolute, physical or total) porosity, which includes the volume of all pores in the sample, i.e. connected and unconnected pores. Total porosity is determined by the difference between the volume of the sample and the volume of its constituent grains;

2) open porosity or saturation porosity, which includes all interconnected pores into which a given liquid (gas) penetrates at a given pressure (vacuum). Those pores into which liquid does not penetrate at the considered saturation pressure are not taken into account. Typically, kerosene is used as a saturating liquid (it penetrates well into the pores and does not cause swelling of clay particles) and saturation occurs under vacuum;

3) dynamic (effective) porosity, including only that part of the pore channels that is occupied by moving fluid during the filtration process when the rock is completely saturated with fluid. This does not take into account the volume of subcapillary pores (with a diameter of less than 0.0002 mm) and pores where the liquid is held by molecular-surface forces. Dynamic porosity in the same sample does not have a constant value, but varies depending on the pressure drop, filtration rate and fluid properties.

The presence of pores and voids in the rock is called porosity. The porosity of oil-bearing rocks is characterized by the coefficient of porosity.

The porosity coefficient (m) is the ratio of the pore volume of the sample (Vn) to the bulk volume of the same sample (V0) (formula 1).

. (1)

The porosity coefficient is expressed in fractions of a unit or as a percentage of the sample volume.

Rock porosity is a very important parameter necessary for assessing oil reserves and elucidating filtration processes in a porous medium.

There are the following types of rock porosity:

1) total (absolute, physical or total) porosity, which includes the volume of all pores in the sample, ie connected and unconnected pores. The total porosity is determined by the difference between the sample volume and the volume of its constituent grains;

2) open porosity or saturation porosity, including all interconnected pores into which a given liquid (gas) penetrates at a given pressure (vacuum). The pores into which the liquid does not penetrate at the considered saturation pressure are not taken into account. Usually, kerosene is used as a saturating liquid (it penetrates well into the pores and does not cause swelling of clay particles) and saturation takes place under vacuum;

3) dynamic (effective) porosity, which includes only that part of the pore channels that is occupied by a mobile liquid during filtration when the rock is completely saturated with liquid. This does not take into account the volume of subcapillary pores (less than 0.0002 mm in diameter) and pores where the liquid is held by molecular surface forces. Dynamic porosity in the same sample does not have a constant value, but varies depending on the pressure drop, filtration rate and fluid properties.

1.1. Typical task

Determine the coefficient of open porosity of the rock sample according to the data given in Table 1 (measurement data of open porosity were obtained by the gravimetric method).

Given:

Table 1.1

                Find:mo

Solution:

1) Determine the volume of open interconnected pores:

[pic 2].                                        (2)

2) Determine the volume of the sample of the rock being studied:

[pic 3]                                        (3)

3) Determine the open porosity coefficient:

[pic 4]                                                (4)

1.2.Tasks for independent work

Determine the coefficient of open porosity of the rock sample according to the data given in Table 1.2 (measurement data of open porosity were obtained by the gravimetric method).

Legend:

Rs– weight of the dry sample in air, g;

RK– weight in air of a sample saturated with kerosene, g;

Rk.k– weight in kerosene of a sample saturated with kerosene, g;

ρTo– kerosene density, kg/m3.

1, ..., 120– option number.

Table 1.2