Prediction of Ultra-thin Interlayers in Shale Gas Reservoirs with Sparse Layer Inversion

Abstract

AbstractThe target shale in Block A of the Western Canada Basin is a condensate gas reservoir, with low-porosity and ultra-low permeability. The target shale is thin (the maximum seismic reflection time is 20 ms), having multiple sets of thin limestone interlayers (with thickness of 0.5–3.5 m) within the target shale. The interlayers in the shale reservoir is very important for reservoir stimulation, but it is very difficult to identify the thin limestone interlayers with seismic data. Therefore, the sparse layer inversion technology is used to improve the seismic resolution of thin carbonate interlayers in the shale reservoirs. This technology is mainly based on the tuning extrapolation of the information in the effective frequency band of the seismic data, which can be used to identify the thin layer below the tuning thickness. The main steps of sparse layer inversion are as follows: the first step is to conduct spectrum analysis and amplitude-preserving and de-noising processing on the post-stack seismic data. The second step is to optimize the processing parameters in frequency domain and to conduct inversion calculation of reflection coefficient of the sparse layer. The third step is to optimize the inversion results and analyze the application results. After the sparse layer inversion on the actual seismic data, the basic frequency and bandwidth of the seismic data have been doubled. The seismic resolution has been increased from the original 5–6 ms to 1–2 ms, and the identification accuracy of thin carbonate interlayers has reached 1–2 m. With the sparse layer inversion technology, the carbonate interlayers in shale reservoir can be identified on the seismic profile. In addition, with this technology, the distribution laws of carbonate interlayer can be made clear. This technology can effectively guide the vertical deployment of horizontal section of staged fracturing horizontal wells in the favorable layers and depths, and avoid the influence of thin carbonate interlayers as much as possible.KeywordsSparse-layer inversionThin interlayerShale gasReservoir predictionEffective bandwidth