Porosity. Theory on porosity. Determination of porosity. pores in rocks, pore types, porosity. Determination of porosity in laboratory

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Erica Sharp
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1 1 Theory on porosity pores in rocks, pore types, porosity Determination of porosity Determination of porosity in laboratory

2 2 Theory on porosity

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4 4 Theory on porosity Reservoir rock has interconnected pores filled with fluid (oil, gas, water) that can flow through the rock. There are also pores that are not interconnected (isolated pores). Generally; Porosity is the fraction of void space in a rock to its bulk volume. V V V V p b p b 100 volume of bulk volume of porespace rock The unit for porosity in the Eq 1 is in percent. The unit can also be in fraction. 1

5 5 Theory on porosity Porosity can be classified into; 1. Original porosity 2. Induced porosity The original porosity is developed during the deposition of the rock material. intergrannular porosity in sandstone intercrystalline and oolitic porosity in limestones Induced porosity is developed by geologic process subsequent to the deposition of the rock. fractures vug or solution cavities commonly found in limestones

6 6 Theory on porosity Sand grain Cementing material Effective porosity/interconnecte (25%) Isolated porosity (5%) Total porosity (30%)

7 7 Theory on porosity 3 types of porosity 1. Effective porosity 2. Isolated porosity 3. Total porosity Effective porosity only consider interconnected pores. Isolated porosity refers to the isolated pores. Total porosity includes both interconnected and isolated pores. Which type of porosity is important in reservoir engineering?

8 8 Theory on porosity Porosity is influenced especially by: 1. Arrangement and shape of the grains 2. Mixture of various shapes and sizes of the grains 3. Amount of cementing material Large sphere Porosity 36% Porosity 20% Small sphere Cubic packing 48% Rhombohedral packing 26% Varied sizes of spheres affect the type and volume of the pore. Effect of cementing material

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10 10 Porosity measurement V V V V p b p b 100 volume of pore bulk volume Measurement made in lab: V p, volume of pore can be directly or indirectly measured V b, bulk volume can be measured directly or indirectly

11 11 Porosity measurement V b, bulk volume can be measured directly or indirectly Direct measurement, V b Regular shape samples (cylindrical, cube etc): Measure the dimensions and calculate the bulk volume A L

12 12 Porosity measurement V b, bulk volume can be measured directly or indirectly Indirect measurement,, V b Regular and irregular shape samples A Two methods will be discussed: L - volume of displaced mercury - gravity method

13 13 Porosity measurement V b, bulk volume can be measured directly or indirectly Indirect measurement,, V b volume of displaced mercury Hg Additional volume of mercury after the sample is pushed into the mercury is equal to bulk volume of the sample.

14 14 Porosity measurement V b, bulk volume can be measured directly or indirectly Indirect measurement,, V b Gravity A Wt d = weight of dry sample Wt hg = weight of mercury B Wt b = balance reading due to the sample being immersed into the mercury Weight of displaced mercury = (Wt b - (Wt k + Wt hg )) Hg Volume of displaced mercury = Weight of displaced mercury Mercury density Balance Balance Bulk volume of sample = Volume of displaced mercury A B

15 15 Porosity measurement V p, pore volume can be measured directly or indirectly Direct measurement, Vp Saturating sample with liquid Wt d = weight of dry sample Wt t = weight of saturated sample Wt f = weight of liquid in the sample = Wt t - Wt k H 2 O V p = (Wt t - Wt k )/r f

16 16 Porosity measurement V p, pore volume can be measured directly or indirectly Direct measurement, Vp Saturating sample with liquid Vb Wtd Wtt Wf rhof Vp 35 cc 20 g 25 g 5 g 1.25 g/cc 4 cc H 2 O Porosity

17 17 Porosity measurement V p, pore volume can be measured directly or indirectly Indirect measurement, Vp Grains volume by using Boyle s Law P 2 =? line Sample cylinder P 2 V 2 T 1 Helium tank Stainless steel discs P 1 V 1 T 1 P 1 V 1 = P 2 V 2 V 2 = V 1 + V l + V s -V c

18 18 Porosity measurement V p, pore volume can be measured directly or indirectly Indirect measurement, Vp Grains volume by using Boyle s Law P 3 =? line Sample cylinder P 3 V 3 T 1 sample Helium tank Stainless steel discs P 1 V 1 T 1 P 1 V 1 = P 3 V 3 V 3 = V 1 + V l + V s -V c - V g

19 19 Porosity measurement V p, pore volume can be measured directly or indirectly Indirect measurement, Vp Grains volume by using Boyle s Law P 1 V 1 = P 2 V 2 V 2 = V 1 + V l + V s - V c V 1 + V l + V s - V c = V 2 = (P 1 V 1 )/ P 2 V 1 = reference volume of helium tank P 1 V 1 = P 3 V 3 V 3 = V 1 + V l + V s - V c - V g V g = V 1 + V l + V s - V c - V 3 V l = volume of tubing connecting helium tank to the sample cylinder V s = volume of empty sample cylinder V g = (P 1 V 1 )/ P 2 - (P 1 V 1 )/ P 3 V c = volume of stanless steel discs V p = V b - V g V g = grains volume of the core sample = V p / V b = (V b - V g )/ V b

20 20 P 2 =? V g = (P 1 V 1 )/ P 2 - (P 1 V 1 )/ P 3 Helium tank P 1 V 1 T 1 Vb P1 V1 P2 P3 35 cc 100 psia 100 cc 68 psia 85 psia P 3 =? Vg cc Vp 5.59 porosity %

21 21 Average porosity

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Chapter 2 Porosity (2.15)

Chapter 2 Porosity (2.15) 2.3 Measurement Porosity can be estimated through volumetric measurements of core samples, or from geophysical logs, which measure a property of the rock and infer porosity, or from Petrographic Image