Steep Decline and Low Hydrocarbon Recovery in Fractured Shale: What and Why?

Abstract:

Since 2000, the technological advances of horizontal drilling and hydraulic fracturing in the United States have led to a dramatic increase in hydrocarbon (gas and oil) production from shale formations, changing the energy landscape in the US and worldwide. Since 2005, the surge in tight oil production from shale formations has provided tremendous optimism regarding future United States hydrocarbon production, unexpectedly becoming the fastest-growing frontier of unconventional resources. According to the Energy Information Administration’s newly released report in 2014, US oil output from tight oil prospects will almost double from the 2012 level of 2.5 million barrels per day, to 4.8 by 2019. However, total gas recovery from the Barnett play was reported to be only12–30%, and the tight-oil recovery rate from shale formations is even lower at 5–10%. The main barrier to sustainable development of US shale, the pore structure of the nanopores storing and transporting hydrocarbons, has been quietly ignored.

We have studied pore structure, edge-accessible porosity, and how wettability is associated with mineral and organic kerogen phases, from four complementary tests: vacuum saturation with vacuum-pulling on dry shale followed with tracer introduction, tracer diffusion into fluid-saturated shale, fluid and tracer imbibition into partially-saturated shale, and Wood's metal intrusion followed with SEM imaging and elemental mapping. The first three tests use tracer-bearing fluids (API brine or n-decane), with tracer distribution on shale mapped with micro-scale laser ablation-ICP-MS analyses. These innovative approaches indicate the limited accessibility (several millimeters from shale sample edge) and connectivity of nanopores in shales under atmospheric condition, which is linked to the steep initial (e.g., 1^st year) decline and low overall recovery because of the limited connection of hydrocarbon molecules in the shale matrix to the stimulated fracture network.