Abstract

Shales that yield good production exhibit a specific set of rock properties. Ignoring the contribution of one or more of these rock properties can be fatal to profitability. Conversely, employing a rock properties-based workflow enables operators to make informed decisions about where to drill and how to frac. The economic case for exploiting a shale play is made based on the volume and maturity of the TOC (total organic content) and whether or not the shale is fracable...i.e., will it undergo fracturing and sustain a fracture network that will stimulate the flow of hydrocarbons to the borehole. Experience has shown that rock properties can be laterally variable within a rock package of interest. Consequently, to optimize production and recoverable reserves from horizontal wellbores an operator must understand variations in mineralogy, lithology, relative rock brittleness, the orientation of natural fractures, and the regional-scale differential rock stresses.

The rock properties that characterize shale sweet spots are mid-to-high kerogen content, low clay volumes, high effective porosity, low water saturation, high Young’s modulus and low Poisson’s Ratio. By systematically using these properties as a guide, a drilling program can be designed to focus on the best target rock types in the field and to optimize the recovery from each of the target wells.

In addition to simply considering rock properties, a rock-properties based workflow integrates data and information that has been gathered at the micro, meso, and macro scales. Porosity and permeability may be analyzed in the lab at the micro and meso scales. At the meso scale of the well bore, petrophysical analysis is used to quantify TOC, mineralogy, mechanical properties, and then correlates the well log data with lab and core data. Rock physics analysis can be used to remediate bad log data and sub-sequently create synthetic data by establishing the relationship between petrophysical properties and elastic properties. At the macro scale of the whole field, seismic data is used to correlate rock properties (determined with petrophysical analysis) with elastic properties (from the seismic data), which, in turn, is used to create a 3D depth-based model of the target shale package. Seismic and well log data can also be correlated to map fractures and their orientation. If azimuthally processed seismic gathers are available, maximum and minimum stress directions can be determined.

At the conclusion of this systematic workflow there will be sufficient information about the reservoir characteristics to select optimal drilling locations, as well as the orientation and placement of horizontal wells for the most effective production program.