Pratiksha Tathed is a graduate research assistant working with Dr. Siddharth Misra. Her research work recently got published in the Fuel Journal (https://www.sciencedirect.com/journal/fuel). The publication is titled: “Hydrocarbon saturation in upper Wolfcamp shale formation”. Yifu Han, a recent graduate from MPGE, has contributed to the data inversion code implemented in this publication. Fuel has been the leading source of primary research work in various broad aspects of fuel sciences. This journal has an impact factor of 4.6
Fuel 219 (2018) 375–388, Volume 219, 1 May 2018, Pages 375-388
Hydrocarbon saturation in upper Wolfcamp shale formation
Formation evaluation in conventional reservoirs generally involves the estimation of water saturation from the deep-sensing or high-resolution electromagnetic (EM) logs, such as laterolog, induction log, and dielectric dispersion logs. In unconventional reservoirs, water saturation estimation is difficult due to complex mineralogy, higher clay content, low porosity, and high salinity. Conventional EM-log-interpretation models tend to break down for organic-rich shale formations because they neglect the interfacial polarization effects and the dispersive behavior of EM properties of such geomaterials. Interpretation with only 1-GHz dielectric permittivity log with dielectric tool or with only laterolog or induction resistivity log or with only 8 dielectric dispersion logs in the frequency range of 10 MHz to 1 GHz is sensitivity to model assumptions, noise in data, noise in model inputs, and has low sensitivity to certain petrophysical properties.
These challenges can be addressed by jointly processing the resistivity and dielectric dispersion logs using an integrated mechanistic model. We implement a novel log interpretation technique for the improved estimation of water saturation (), brine conductivity (), textural index/cementation exponent (), and saturation exponent () in the upper Wolfcamp shale and Bakken Petroleum System. Log processing was performed with an integrated mechanistic model, which combines Complex Refractive Index (CRI) model to analyze the conductivity and permittivity logs acquired at 1 GHz, Stroud-Milton-De (SMD) model to analyze the 3-conductivity dispersion and 3-permittivity dispersion logs in the frequency range of 20 MHz to 0.3 GHz, and Waxman-Smits (WS) model to analyze the deep galvanic resistivity log (RLA5) measured by the EM laterolog tool at 1 kHz.
In the upper Wolfcamp shale and Bakken Petroleum System, estimates derived from the joint inversion were robust in the presence of pyrite, low water saturation, and low porosity as compared to estimates from the inversion of only four-frequency dielectric dispersion logs. Formation brine conductivity and saturation-exponent estimates are more reliable compared to water saturation and cementation exponent estimates. Estimates of cementation exponent obtained using the proposed inversion exhibit higher variation with the increase in depth indicating an increase in heterogeneity/layering with depth. Hydrocarbon saturation in the interval XX334-XX346 ft is close to 30%. There are thin-layers in the interval XX412-XX444 ft having hydrocarbon saturation close to 20%. Average relative errors in fitting the 1 laterolog resistivity and 8 dielectric dispersion logs using the estimates obtained from the proposed method are 10% and 20% in the 520-ft depth interval of the upper Wolfcamp shale formation. Water saturation and brine conductivity estimates are most certain in the depth interval XX720-XX750 ft of Middle Bakken, where the formation exhibits high porosity, low clay content, and low anisotropy. Hydrocarbon saturation in this interval is close to 35%. Hydrocarbon saturation close to 60% exists in the 45-ft depth interval of Middle Bakken formation. Inversion-derived brine conductivity and saturation exponent estimates are most uncertain in Lodgepole and Three Forks 2 formation probably due to wide range of pore size distribution.