CRITICAL SALINITY FOR ARCHIE – NON ARCHIE MODELS IN THE JAUF SANDSTONE RESERVOIR,SAUDI ARABIA

摘要

In this case study,the effect of clay content on the resistivity of the Jauf Sandstone was investigated through multiple-salinity electrical tests on nine 1.5 inch diameter and 2.5 inch long core plug samples.Tested samples were selected from a semi-consolidated,brownish,porous and permeable quartz arenite facies from three different wells.Samples contain dominantly authigenic illite and minor chlorite clays lining the pores.Experiments were conducted under 65 °C temperature and 2,000 psi confining pressure condition.Ten different brine concentrations,starting with the highest concentration(250 kppm),were circulated sequentially through the samples while recording the electrical conductivity changes of the rock.Tests showed a 4 to 8 percent clay effect(BQv/Cw)on the electrical conductivity.The low clay effect is due to:(1)low clay percentage,(2)illite and chlorite type clays,which have a low-to-moderate cation exchange capacity,and(3)high formation brine concentration in the reservoir.The low resistivity in the reservoir is not due to clay conductance,but it is due to microporosity that is caused by the pore-lining or filling clay texture.The critical salinity corresponding to the commonly accepted 10 percent clay effect cutoff is calculated to be 100 kppm.Although the Archie model is valid for water saturation interpretation in the reservoir,low salinity brine effects,such as mud filtrate or injection water,requires the consideration of appropriate shaly sand models.