A Novel Method to Predict Water Saturation from Conventional Well Logging in Tight Gas Sandstone Reservoirs

Abstract

Water saturation is an important input parameter in reserve estimation and fluid identification. Many models have been raised to predict water saturation since the Archie's equation was first established in 1942. However, water saturation estimation in tight gas sandstone reservoirs still faces great challenge due to the complicated pore structure and strong heterogeneity at present. In addition, high formation temperature also heavily affects the electrical conductivity of rocks. To accurately predict water saturation in such type of formation, the effect of pore structure and formation temperature to rock resistivity should be considered. In this study, taking the Cretaceous Qingshuihe Formation in the southern margin of Junggar Basin as an example, 20 typical core samples are recovered for the resistivity experiments under variable temperatures in laboratory (4 core samples are tested under 25°C, 75°C, 100°C and 135°C, and the rest 16 are applied under 25°C and 135°C). Meanwhile, all 20 core samples are simultaneously applied for nuclear magnetic resonance (NMR) and mercury injection capillary pressure (MICP) measurements. Analyzing the experimental results of resistivity, NMR and MICP clearly illustrate that the conductivity of tight sandstones is heavily affected by pore structure and formation temperature. This makes the conductive mechanism cannot be directly expressed based on the Archie's equation. As the pore structure of rock deteriorates, the relationships between porosity and formation factor, and water saturation versus resistivity index downward, the corresponding cementation exponent decreases, and the saturation exponent increases. Meanwhile, as the formation temperature increases, both the cementation exponent and saturation exponent decrease. Considering the in-suit formation is as high as 175 °C, we raised a new method to optimize the water saturation estimation. This method contains two steps: first, we correct the resistivity parameters to 175°C based on the experimental results of 20 core samples under 135°C. Second, we raise a theoretical model to predict the cementation exponent, and a novel method of calculating various saturation exponent in tight sandstone reservoirs. In these model and method, the effect of pore structure to rock resistivity is considered. By using our raised technique, the optimal model of predicting water saturation from porosity and resistivity in formation with high-temperature and complicated pore structure is established. We use the raised model and the typical Archie's equation to calculate water saturation in our target formation, separately. The predicted water saturations and the experimental results of core samples are compared. The comparison clearly illustrates that the calculated water saturation matches well with the core-derived results, the absolute errors between them are lower than 5.0%. However, the calculated water saturations by using the Archie's equation are generally high, and the absolute difference with core-derived results are higher than 10.0%. This research raised a novel technique to calculate water saturation in tight gas sandstone reservoirs. It can avoid the effect of complicated pore structure to rock resistivity, and will play a great important role in unconventional reservoirs estimation and fluid identification.