Novel response understandings and applications of azimuthal gamma-ray logging of horizontal wells in sedimentary sandstones with grain sequence features

摘要: The theory and practices of logging facies have confirmed that the grain sequence in sedimentary sandstones is closely related to the amplitude and morphological characteristics of vertical well logs. However, the corresponding studies of longing responses of horizontal wells in sedimentary sandstones with grain sequences have not been publicly reported, especially lacking the corresponding response characteristics of fundamental gamma-ray logging. This situation leads to difficulties in geosteering and reservoir evaluation of horizontal wells. To address it, the grain sequence characteristics of traditional sedimentary sandstones are sorted out. Combining geological features of sandstones and the geometric characteristics of grain size logs, the core features of different grain sequences are extracted. In sandstone reservoirs, a semi-quantitative quantitative ideal grain sequence models, i.e., cylindrical-shaped, bell-shaped, funnel-shaped and eggshaped grain sequences, is proposed, and the corresponding formation models with horizontal wells are constructed. Then, the Monte Carlo method is employed to establish the forward model of azimuthal gamma-ray logging (AGR) of horizontal wells in sandstones with different grain sequences. The AGR responses of these formation models are calculated. The results show that the AGR response of the formation model with cylindrical-shaped grain sequence is consistent with that of traditional three-formation models. However, the AGR responses of the formation models with bell-shaped, funnel-shaped and egg-shaped grain sequences are characterized by "double left-handled spoons", "double right-handled spoons" and "double inverted isosceles triangles", respectively, which are significantly different from that of traditional three-formation models. Based on the geological and engineering characteristics of horizontal well drillings, eight different basic spatial relationships between horizontal wells and formations are sorted out. Combined the forward models, the AGR response characteristics and modes of formation models with different grain sequences under the eight spatial relationships are established and abstracted through geometric morphology. After clarifying the application scheme, the research understandings are applied in two typical horizontal wells with different grain 2 sequences to verify the reliability and convenience of the above response characteristics and modes. In addition, the adaptability, advantages, disadvantages and limitations of this study are discussed in detail. The study can provide fundamental understandings and supports for geosteering and reservoir evaluation of horizontal wells in different grain sequence sandstones.