Investigating the Geologic Factors that Control Reservoir and Completion Quality within the Eagle Ford Shale
Miller, Camron; Rylander, Erik; Lewis, Rick; Sinclair, Steve; Jiang, Tianmin; Dean, Barry; Gamero, Helena
Geological factors known to impact reservoir quality (RQ) include primary depositional texture and burial diagenesis. A variety of pore types with different formative histories have been described within cores from organic mudstones. Pores hosted within organic material (OM) are created during kerogen maturation and have proven as key to gas and retrograde condensate production. Gas production mechanisms are well demonstrated for organic mudstones. Retrograde condensate production follows the same flow mechanism until the reservoir pressure falls below the dew point and creates issues with relative permeability. Oil production from organic mudstones is more difficult to understand.
Models used to identify RQ in gas and condensate producing mudstones are not applicable to those which produce oil. The flow of a liquid through nanopores may not be governed by Darcy's Law, but the industry has not reached a consensus on an alternative model. The production of oil from organic mudstones is demonstrated daily in many boreholes, and this suggests either that matrix permeability is higher or has been enhanced by natural fractures or dissolution. Production data suggests that wells producing oil from organic mudstones have a much steeper decline rate than those which produce gas.
Oil production from the Eagle Ford Shale is believed to be a function of RQ and the ability to effectively stimulate the reservoir. Matrix permeability is key to sustaining production. Initial laboratory measurements suggest that permeability to oil is a function of pore throat size, wettability and water saturation. Two main types of pore systems are present in the Eagle Ford Shale—OM pores and larger, conventional pores within the inorganic matrix. Core and log NMR measurements confirm that OM pores are hydrocarbon wet, while inter-particle (IP) pores have mixed wettability.
Pore fluids observed in core samples examined under surface conditions are not representative of in-situ conditions as the lighter portion of the hydrocarbons are expelled during core extraction. Comparison between log- and core-measured NMR allows the quantification of the expelled hydrocarbon and thus defines zones that have already produced hydrocarbons during core recovery. Understanding which portion of a mudstone reservoir contains movable oil impacts target zone selection for both vertical and horizontal well completions.
AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013