Ultrasonic wave attenuation dependence on saturation in tight oil siltstones

Ultrasonic P-wave attenuation was measured in tight oil siltstones, carbonates and a tight sandstone with two independent estimation methods. The dependence on saturation in gas-water partially-saturated siltstones at in-situ conditions shows a different behavior compared to the other rocks. The siltstones in our experiments exhibit a behavior characterized by a gradual decrease of attenuation with increasing water saturation in the presence of gas, and full-gas saturation shows more attenuation than full-oil and full-water saturations. However, previous theoretical and experimental studies show that gas-water saturated carbonates and sandstones have the highest attenuation at high water saturations, and generally, a liquid-saturated rock shows more attenuation than a gas-saturated one. Poroelasticity theory shows that the two dominant loss mechanisms (due to fabric heterogeneity and patchy saturation) have peaks at different frequencies for siltstones, resulting in a gradual decrease of attenuation with water saturation, while these mechanisms overlap at ultrasonic frequencies for carbonates and sandstones, leading to an attenuation peak at high water saturations. The predicted attenuation dependence on fluid type agrees with the measurement for most samples. Regarding the tight oil siltstones, although the model fails to explain the experimental results for oil-water saturation, it can be concluded that for gas-water saturation the squirt flow caused by fabric heterogeneity dominates the attenuation, which differs from carbonates and sandstones. Experimental studies show that the attenuation dependence on saturation in tight oil reservoirs can be associated with fabric texture. The theory describes these behaviors, which can potentially improve the practices of detecting and monitoring multi-phase fluids in the reservoirs.