Potential Global Implications of Gas Production from Shales and Coal for Geological CO2 Storage

Abstract

Production of natural gas from shale formations and coal deposits is increasing, and new potential productive horizons are being identified. Producers are using horizontal drilling and hydraulic fracturing technologies to economically produce oil and gas from shales and coal seams. These technologies open up the possibility of using shales and coals as actual storage media for carbon dioxide (CO2) by increasing permeability and injectivity; though some worry that the same technology may compromise the integrity of shale cap rocks in some basins.This paper builds upon previous work to assess the global potential for geological storage of CO2 in shale and coal formations. This includes assessment and characterization of: (1) the global status of hydrocarbon production from shales and coal seams; (2) the potential theoretical capacities for CO2 storage in shales and coals; and (3) containment issues arising from shale fracturing, both for shales as a storage medium, and in terms of cap rock integrity for underlying storage units, particularly deep saline formations.Technical recovery potential for methane from the world's coal seams is estimated to be 79 trillion cubic meters (Tcm) globally, which could facilitate the potential storage of nearly 488 billion metric tons, or Gigatonnes (Gt), of CO2 in unmineable coal seams. In gas shales, an estimated 188Tcm of shale gas resources are potentially technically recoverable globally, and could facilitate the potential storage of 740 Gt of CO2 in gas shales.Concerns about the potential adverse impacts of the use of saline aquifers underlying gas shale formations for CO2 storage overlook the critical third dimension – depth. Sedimentary basins consist of thousands of meters of multiple layers of shale, sandstones, limestones, etc. (that may also be “tight” or largely impermeable). If one shale formation layer directly above a storage zone is fractured; additional layers of impermeable rock overlying the fractured area could block upward migration of the CO2. Moreover, in many cases, even the targeted shale will not be fractured throughout its entire thickness, which could be hundreds of meters. With sensible safeguards, CO2 storage reservoirs can, in most areas, coexist with conventional and unconventional oil and gas operations, including shale gas production and hydraulic fracturing.