This study seeks to provide an understanding of the pore system of a basaltic reservoir and contribute to the optimization of production schemes for the brownfield that shows unexpected production behavior from year to year. The metamorphosed basalt is dominated by paramagnetic minerals such as chlorite and Fe-bearing prehnite, which form a frame supporting the present pore system. The large vesicles (~mm), initially created by the degassing of the magma, appear as characteristically visible pores which are partly or completely filled with metamorphic minerals. Investigation with NMR, MICP and µXCT indicates that these impressive vesicular pores actually occupy less than 20% of total porosity. On the other hand, the signals at shorter T2 are correlated to nano pores (<micrometer) and are revealed to have higher importance in terms of volumetrics. The interconnected nano pores are responsible for the matrix permeability of around 0.1-1md.
The T2 signal from the vesicular pores indicates the shortening of bulk relaxation, probably due to the paramagnetic nature of the reservoir. Similarly, the total NMR porosity is observed to be less than helium porosity, depending on the magnetic field gradient of the samples. The correction of the NMR porosity could be achieved in relation to grain density as the magnetic susceptibility essentially has a positive relationship with the density of the minerals.
The clay bound water, chiefly belonging to chlorite, is investigated on differently heated samples with NMR. The T2 peak is at around 0.2ms at the room condition. The Gaussian mixture, fitting on the T2 distribution of saturated samples, concordantly identifies and helps to quantify the CBW separately from the nanopore region. The CBW accounts for about 10-20% of nano pores.
The permeability of the reservoir is increased by micro fractures to darcy order in maximum, as implied by comparison between µXCT observation and air permeability. The presence of the micro fractures is hard to distinguish on T2 and it remains challenging for the practical permeability estimation approach.
Major pore volume of the altered basalt is revealed to be attributed to nanometer-sized pores and the basalt is thus defined as a tight reservoir. The flow efficiently is achieved by the presence of micro fractures. The result highlights the necessity to reconsider the stimulation strategy of the field.
