Many shale plays are being successfully developed throughout North America. These shale plays are being evaluated based on a number of criteria but primarily through typical unconventional and tight formation gas reservoir characteristics. Prospective shale plays share several interesting characteristics such as mineralogy, rock mechanics, and geomechanics. It is the intent of this paper to highlight and demonstrate the inter-relationship of these characteristics and show their importance on completion and stimulation design and more importantly to the very prospectivity of an unconventional shale play.
This paper will show through an analysis of the mineralogy that shale plays are made up of mostly silica and carbonate material and have few clay constituents. In other words, the prospective shale's are actually fine-grained clastics and not shale!
Secondly, prospective shale's tend to be brittle with the static Young's Modulus generally in excess of 3.5 x 106 psi. Of course this brittleness is related to the lack of clay constituents that make up these rocks. In addition, prospective shales tend to satisfy clastic correlations of dynamic to static Young's Modulus. They do not behave like typical shales but more like fine-grained isotropic (on a core scale) clastics!
Finally, gas can flow through induced fractures or natural fissures under effective stress conditions in these shale plays. As a result, water-frac treatments are the stimulation of choice! However, proppant is still necessary in at least the near wellbore vicinity to provide a conductive pathway to the wellbore.
This paper focuses on three key elements (mineralogy, rock mechanics, and geomechanics) of prospective shale plays and benefits the petroleum industry by:
Integrating the laboratory core work with multi-disciplinary data to develop a shale and unconventional reservoir prospectivity evaluation tool,
Illustrate how this multi-disciplinary dataset influences completion and stimulation design, execution, and well performance, and
Demonstrate how this multi-discipline dataset can be used to identify and mitigate well completion and stimulation risks in these unconventional reservoirs.
There are a number of important parameters and technical disciplines that need to be addressed to understand the viability of an unconventional gas reservoir. Unconventional gas reservoirs are somewhat unique, in that, they require "good" reservoir, completion, and fracture stimulation for success. Failure of any one of these key disciplines means a marginal or uneconomic well and success in all three may not guarantee a successful well as they are extremely price/cost sensitive.
On the reservoir side Gunter 1–2 and Newsham and Rushing 3–4 correctly tie the geology, petrophysics, and reservoir engineering to develop an integrated work flow for tight and unconventional gas reservoirs. The four stage model included:
large scale geologic architecture,
description of the rock and fluid systems,
definition of flow units through formation evaluation, and
calibration of the geologic and petrophysical models through reservoir simulation.
Geomechanics was addressed throughout their workflow. Stage 1 of their work addressed the large scale structural components of the geologic model such as faults, in-situ stresses, and fissures, Stage 2 addressed the stress dependent properties and anisotropy of the rocks, and stage 3 and 4 addresses the hydraulic fracture and natural fissure orientations and effects on well performance.
Slatt 5 et al. developed a workflow for unconventional gas shales that included (1) characterization of multi-scale sedimentology and sequence stratigraphy, (2) relate stratigraphy to log response, (3) seismic response, (4) petrophysical and geomechanical properties, and (5) organic geochemistry. In this work, the geomechanics of the prospect are brought in through step 4 and although not discussed in any great detail a relationship between mineralogy and geomechanics is suggested. Further, the authors recommend additional attention be given to the lithologic properties of the shale and the brittleness or ductility.