What Causes Sweet Spots in Shale Gas Reservoirs?
Australian School of Petroleum, University of Adelaide, Adelaide, SA, Australia.
Shale gas reservoirs exhibit much more production variability than conventional hydrocarbon reservoirs. Shale gas wells within a single field, completed using identical drilling and fracture stimulation programs frequently show a 2-5x variation in initial rate and/or recovery factor. These production ‘sweet spots' are very real and can change rapidly between adjacent well locations - or even between adjacent frac stages in the same horizontal well. This large variation in productivity has a significant impact on both exploration and development of shale gas fields. When exploring for a new shale gas reservoir, this statistical variation means that a number of test wells need to be drilled before a decision can be made about the commercial viability of that reservoir. In development mode, this productivity variation means that a significant portion of the development wells will be uneconomic or only marginally economic. There are thus strong economic drivers to understand this variation and evaluate new shale plays with fewer wells and to develop the economic shale fields with wells located only in the production ‘sweet spots'.
There is no single explanation for these production sweet spots and currently plenty of conjecture about the possible causes. Some of those causes include: small scale variations in clay type and source rock quality; the different porosity types in shales and how they might be controlled by depositional processes; differences in geomechanical stress which can enhance or diminish naturally occurring permeability due to micro-fractures In addition, lithology greatly influences mechanical properties and fracture stimulation and pre-existing natural fractures and faults can interact with fracture stimulation treatments to create ‘shear fractures' to greatly enhance permeability.
AAPG Search and Discovery Article #90155©2012 AAPG International Conference & Exhibition, Singapore, 16-19 September 2012