Porosity determination Introduction Porosity is the ratio of void space within a rock(or sediment) relative to bulk volume, reported either as a decimal fraction or as a percentage of total bulk volume. Sedimentary rocks that form petroleum reservoirs(e.g, Figure 1)typically have an average porosity value that will be between 5 and 40% Permeability is the ability of porous media to transmit fluid. At the most basic level, porosity determines the amount of fluid present within a reservoir, permeability governs what is recoverable. Because porosity and permeability represent important characteristics of a reservoir rock, the detection and evaluation of porosity and the inferred presence or absence of permeability are two important duties of the geologist. 159mD There are many ways of determining the porosity of a reservoir rock. Porosity can b Figure 1. Drill cuttings containing 14% intergranular visually estimated from drill cuttings or core fragments. porosity, click on the image to zoom in/out (image ourtesy of Calgary Rock and Materials Services In Determined for a given interval from petrophysical logs. and Hayden Geological Consultants). Measured by analyzing the injection behavior of a fluid or gas into rock cuttings or core fragments However, when values from different types of analysis are compared to one another, for the same sample or sample interval, discrepancies may occur. One potential discrepancy relates to the physical size of the sample and sample interval. Visual estimates of porosity and laboratory measured porosity values are typically conducted on small samples, derived from drill cuttings(Figure 2A)and/or selected core plug samples removed from drill core(Figure 2B) Drill cuttings represent a sampled of a drilled interval, the size of which is prescribed in the drilling prognosis or plan Typically sample intervals are 3 to 5 m(approx. 10 to 30 feet)in the upper part of an offshore exploration well (known as"topB hole ), decreasing to perhaps a I m(3 feet) sample interval as the drilling rate slows towards total depth(TD); for the sample in Figure 2A the interval is 3 m(approx. 10 feet). Because of the relative high cost of drill core, cored intervals are selected over zones of specific interest (i.e, potential reservoir) and therefore vary in length, but are typically 9m to 30m(27 to 90 feet). The drill core in Figure 2B is 5.7m(19 feet) Figure 2. Geological samples, showing drill uttings obtained over a sample interval of 3 m(approx. 10 feet), the yellow scale bar is 0.5 cm (A); drill core in which the measured length is 5.7 m (19 feet), the scale bar is subdivided into cm. The removal of a core plug leaves cylindrical holes in the drill core. he scale bar is in cm (drill core image courtesy of KM Bergman)1 Porosity determination Introduction Porosity is the ratio of void space within a rock (or sediment) relative to bulk volume, reported either as a decimal fraction or as a percentage of total bulk volume. Sedimentary rocks that form petroleum reservoirs (e.g., Figure 1) typically have an average porosity value that will be between 5 and 40%. Permeability is the ability of porous media to transmit fluid. At the most basic level, porosity determines the amount of fluid present within a reservoir, permeability governs what is recoverable. Because porosity and permeability represent important characteristics of a reservoir rock, the detection and evaluation of porosity and the inferred presence or absence of permeability are two important duties of the geologist. There are many ways of determining the porosity of a reservoir rock. Porosity can be: • Visually estimated from drill cuttings or core fragments. • Determined for a given interval from petrophysical logs. • Measured by analyzing the injection behavior of a fluid or gas into rock cuttings or core fragments. However, when values from different types of analysis are compared to one another, for the same sample or sample interval, discrepancies may occur. One potential discrepancy relates to the physical size of the sample and sample interval. Visual estimates of porosity and laboratory measured porosity values are typically conducted on small samples, derived from drill cuttings (Figure 2A) and/or selected core plug samples removed from drill core (Figure 2B). Drill cuttings represent a sampled of a drilled interval, the size of which is prescribed in the drilling prognosis or plan. Typically sample intervals are 3 to 5 m (approx. 10 to 30 feet) in the upper part of an offshore exploration well (known as ‘top hole’), decreasing to perhaps a 1 m (3 feet) sample interval as the drilling rate slows towards total depth (TD); for the sample in Figure 2A the interval is 3 m (approx. 10 feet). Because of the relative high cost of drill core, cored intervals are selected over zones of specific interest (i.e., potential reservoir) and therefore vary in length, but are typically 9m to 30m (27 to 90 feet). The drill core in Figure 2B is 5.7 m (19 feet). B Figure 2. Geological samples, showing drill cuttings obtained over a sample interval of 3 m (approx. 10 feet), the yellow scale bar is 0.5 cm (A); drill core in which the measured length is 5.7 m (19 feet), the scale bar is subdivided into cm. The removal of a core plug leaves cylindrical holes in the drill core, the scale bar is in cm (drill core image courtesy of K.M Bergman). A Figure 1. Drill cuttings containing 14% intergranular porosity, click on the image to zoom in/out (image courtesy of Calgary Rock and Materials Services Inc. and Hayden Geological Consultants)