A method of determining movable fluid saturation of tight oil reservoirs: A case study of tight oil reservoirs in seventh member of Yanchang Formation in Heshui area

The movable fluid saturation is one of the key factors in tight oil evaluation, and can be tested accurately by nuclear magnetic resonance (NMR) technology. The high cost and long cycle prohibited the widespread use of NMR technology to determine movable fluid saturation. The testing principles of NMR, constant-speed mercury injection and high-pressure mercury injection indicated that the relaxation time distributions, constant-speed mercury injection curves and high-pressure mercury injection curves are the reflections of pore structures and have the ingenerate consistency. The movable fluid saturations and total mercury saturations of the same samples were tested by NMR and constant-speed mercury injection respectively. Correlation between the movable fluid saturation and the total mercury saturation was closely strong. The movable fluid saturation can be calculated from total mercury saturation. Considering the similarity between total mercury saturation of constant-speed mercury injection and mercury saturation at 7.0 MPa of high-pressure mercury injection, a method to determine movable fluid saturation of tight oil reservoirs was proposed based on high-pressure mercury injection data. The calculation results indicated that the tight oil reservoirs, with high movable fluid saturation, are mainly type-Ⅲ and type-Ⅳ reservoirs, followed by type-Ⅱreservoir.