Log-Based Identification of Sweet Spots for Effective Fracs in Shale Reservoirs

Abstract

Abstract Hydro-fracturing and long horizontal sections are needed to generate economic production volumes in shale reservoirs. Many operators resort to multi-stage fracs in horizontal sections that are often sparsely surveyed with LWD or wireline logs. The trend is to space out different frac stages, more or less evenly, along the length of the horizontal with the expectation that reservoir characteristics will conform to those encountered in the vertical pilot hole. LWD and wireline logs examples presented here distinctly highlight sweet spots with superior reservoir characteristics that are likely to have excellent flow characteristics. Perhaps equally important is the delineation of zones that have inferior reservoir properties, where hydro fracturing will be far less effective. Sweet spots enable operators to concentrate on the most suitable intervals for stimulation to obtain a bigger bang for their bucks. In marine shales, LWD Spectral Gamma measurements highlight intervals of high Uranium corresponding to elevated Kerogen levels which are zones that are expected to be rich in hydrocarbons. An image log is generated from the cumulative GR to provide structural information. Examples show delineation of sweet spots using LWD density and Spectral Gamma combination. A LWD Acoustic measurement that employs a unique transmitter provides split fast and slow shear in horizontal sections – yielding elastic properties in the vertical and horizontal directions thereby providing information on anisotropy of these reservoirs. This once again brings out the lateral inhomogeneity and points out the sweet spots where frac dollars need to be concentrated. The cost of well surveys in a horizontal shale formation can be less than the expenditure on a single frac stage. A Quad-Combo is sufficient for a meaningful evaluation that will permit operators to concentrate on intervals with better reservoir characteristics and thus achieve improved stimulation of the formation.