Shales form a particular petroleum system wherein the same rock formation is simultaneously source, reservoir rock, sealing rock and the trap, while gas migration occurs solely in a micro-scale or is absent. Quite a challenge for the petrophysicists who have to use more advanced techniques of research, especially in micro- and nano-scale.
Shale properties can be described in different scales of observation:
- mineral grains (nano- and micro-scale),
- packages of lithologically-uniform laminae (millimetre or centimetre scale),
- higher order sedimentary complexes displaying internal lithological variability and higher order patterns of the sedimentary structure (scale of metres).
In order to characterize the petrophysical properties of the rocks, i.e. rock capability to accumulate and transport reservoir fluids, it is necessary to determine the values of two key parameters:
- porosity, and
- permeability.
Moreover, the characterization of the reservoir fluid (oil, gas, water) present in the rock is an important aspect. An illustrative petrophysical model of an organic matter-rich shale rock is presented below.
Porosity of shales
Porosity means the volume of void space in the rock (expressed as percentage) which can be filled with oil, gas or water. Therefore, porosity determines the volume of reservoir fluids accumulated by the rock.
Two types of porosity are distinguished:
- total porosity – calculated as the total pore volume divided by bulk volume of the rock, and
- effective porosity – calculated as the volume of interconnected (permeable) pores divided by bulk volume of the rock.
Shale rock is built of micro- and nano-sized space pores with varying degrees of water saturation and partly of residual organic matter. Void spaces also occur between rock grains (inorganic pores and micro-pores), but their volume is minimal. Effective porosity appears as a result of fracturing.
So far, in the case of conventional oil and gas reservoirs (sandstones, carbonate rocks) porosity was defined as the void space between rock grains (inorganic pores and micro-pores). In that space and within laminae enriched in silica, as well as in the system of natural fractures and micro-fractures, the gas is accumulated in the form of free gas.
In shale rocks, natural gas occurs as:
- free gas within rock particles,
- free gas within the dispersed organic matter,
- gas adsorbed by the dispersed organic matter,
- gas adsorbed by certain clay minerals.
In addition to the aforementioned various accumulation spaces, free gas is present in clay-mud shale complexes also within laminae that are enriched in organic matter. However, a significant amount of natural gas is present in organic pores located within insoluble organic matter which is called kerogen.
Permeability and fracturability of shales
Permeability is associated with the presence of natural cracks/fractures in the rock which enable the flow of reservoir fluids between pore spaces. Permeability enables the flow of natural gas or oil into the borehole and their production. Permeability coefficient is dependent on:
- the size of pores,
- relative configuration of the rock-building grains,
- grain grading and cementation, and
- rock fracturing patterns.
In the case of shale rocks, both permeability and porosity are highly dependent on:
- mineral composition,
- organic matter distribution,
- quantitative (%) content of organic matter, and
- thermal maturity of organic matter.
Shale rocks characterized by low permeability it basically prevents any unrestrained flow of hydrocarbons.
Accordingly, stimulation jobs (such as fracturing operations) must be performed in order to connect the pores to the borehole and allow for an unrestrained flow of gas and reservoir fluids.
author: Ireneusz Dyrka
