For the profitable development of shale reservoirs, it is critical to understand the natural fractures in the rock and how they may or may not interact with hydraulic fracturing. When performing core analysis, it is also important to assess whether the measured properties have been altered by fractures induced during coring. NMR is an important tool in understanding the shale pore networks and ultimately how to best produce the reservoirs.
A key indication of the quantity of the fractures can be obtained by measuring the amount of porosity contained in the fracture network. There is usually at least two distinct pore networks in shale formations – one associated with the inter granular pore network and one associated with the pores within the organic rich kerogen. The fracture network, either natural or induced, could be considered a third pore network which at ambient pressure would be the largest in size. NMR is still able to distinguish the different pore networks because the organic pores are much smaller than their non-organic counterparts. In addition, the T2 distributions of the three pore networks can change differently as a function of confining pressure allowing further distinction of the three pore networks to be made.
Modern NMR instruments can be used with high temperature and pressure cells, such as the P5 Overburden Cell, to mimic reservoir conditions. NMR can then be used to obtain not only the total porosity of the shale samples but also quantify the amount of porosity arising from the fractures. NMR T2 relaxation times can be used to identify the pore networks as well as their dependence on stress.
More information can be found about quantifying the changes to the pore network with added pressure can be found in this Application Note entitled Understanding changes in porosity as a function of overburden pressure in shales.
Our sales team can help you find the right NMR instrument and software to help with your understanding of shale pore networks. If you are interested in working with us to conduct this testing on your shale core samples, please visit our in-house laboratory website.