DETERMINATION OF PORE SPACE COMPRESSIBILITY USING NUCLEAR MAGNETIC RESONANCE (NMR) MEASUREMENTS

Pore space compressibility is an important parameter for reservoir characterization and simulation of reservoir behavior. Compressibility of the pores can provide estimation of energy required to drive fluids from the reservoirs during production methodologies. The production of hydrocarbon from a reservoir increases the effective stress with reduction of porosity, permeability, and changes in pore space compressibility. This is linked with reduction in hydrocarbon production.

The main objective of this work is to calculate the change in pore space compressibility for sandstone rocks across wide ranges of porosity and permeability with low-field, 2 MHz, nuclear magnetic resonance (NMR). Analysis of changes in porosity and permeability under varying confining pressure was also performed. Measurements for this work were conducted on 12 Hibernia oil reservoir cores and 2 virgin Berea samples. Porosity values range from 12% to 34% and permeability range from 4 md to 6500 md. This work investigates NMR response under a maximum confining pressure of 1500 psi and electric conductivity/resistivity for confining pressure of 2500 psi. Calculation of pore space compressibility from acoustic measurements and porosity determination using helium porositimeter up to pressure of 2500 psi were used to verify these obtained values.

The NMR results indicate pore space compressibility at T2 relaxation times between 40-800 ms for experiments conducted on three Hibernia and one virgin Berea samples. Decrease in NMR signal amplitude and relaxation time for larger pore sizes is seen with increasing confining pressure. This corresponds to an increase in NMR signal amplitude for smaller pores. Compressibility and compliance of larger pore sizes that is not observed in smaller sizes shows permeability modification on larger pore sizes, restricting flow with increasing confining core pressure.