Fluid Identification with Multiple Nonelectrical Methods in Ultradeep Tight Gas Reservoir, A Case Study from Tarim Basin

Abstract

The complex geology of fold-and-thrust belt led to significant difficulties with the engineering aspects of drilling, logging, completion, and testing. The Bashijiqike Formation and Baxigai Formation feature a tight sandstone reservoir that exists below a salt layer with high pressure and high downhole temperature. Reservoir characterization is indispensable in the development of this Cretaceous structural fractured tight sandstone reservoir formation. Fluid identification is a key tool used to locate the sweet spot with high producibility for further development. Resistivity is the most common and straight-forward method. However, because of the mixed effect from pore structure, formation sedimentary dips, far-end fractures, and the influence of the surrounding rocks etc., gas and water cannot easily be identified based on the resistivity difference. Compressional coefficient and Poisson's ratio is without obvious cross-over effect because of the compaction effect on rock sonic waves. Under these circumstances, new methods based on other nonelectrical technologies are used. Nonelectrical means for fluid identification including T1–T2 based on 2D NMR measurement, and the spectroscopy related sigma and chlorine element method were successfully applied in this region. The 2D NMR measurements provide porosity and permeability information for T2 based analysis. When used in conjunction with T1 based measurement, the fluid identification through different T1–T2 response provides an advantage for distinguishing hydrocarbon and water, especially for gas. Since relaxation due to diffusion only applies to T2 and never to T1, given the typical magnetic field gradients of the logging tool, the oil and gas signal can easily be distinguished from the T1/T2 ratio. The continuous measurement enables the separation and quantitation of different fluids that exist in the pore system for the entire interval of the targeted reservoir. Formation water salinity is contributed by the NACL present in the fluid in this area. Advanced spectroscopy data provides chlorine measurements minus this effect in a mud system. The chlorine from the formation is calculated in this way. Formation water can be derived to further identify the main contributor of the fluid inside the pore system. Case studies are presented from this ultradeep tight gas reservoir that solved the fluid identification issue when resistivity cannot directly distinguish fluid type. The result matched the well with the test, which provided a novel solution to finalize the sweet spot interval for the targeted reservoir.