Pore Connectivity and Thermal Maturation of Various American and China Shales

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. However, the total tight-oil recovery rate from shale formations is as low as 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. With estimated shale gas reserves greater than the US's and Canada's combined, China has an ambitious shale development program. China has several types of shale (by area, 26% are marine, 56% marine-terrestrial transitional, and 18% terrestrial), whereas nearly all US producing shales are marine. Sinopec recently reported that its 1st marine shale well (Jiao-Ye #1HF, drilled Feb. 14, 2012 and completed Nov. 24, 2014) initially produced 2.0×105 m³ gas/day, and maintained stable daily production of 6.6×104 m³ over the next7 months. This production behavior, though of limited duration, is consistent with the 60% 1st year decline observed in US wells. Shale geology could be a bottleneck to its sustainable development. We have collected a variety of leading hydrocarbon-producing shale formations in both U.S. and China. These formations have different ages and geologic characteristics (e.g., porosity, permeability, mineralogy, organic matter content, thermal maturation). We 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 imaging and elemental mapping. The first three tests use tracer-bearing fluids (API brine or n-decane) to examine the association of tracers with mineral or kerogen phases, using a combination of elemental laser ablation-ICP-MS mapping and high-resolution SEM approaches. These innovative approaches indicate the limited accessibility (several millimeters from shale sample edge) and connectivity of nanopores in shales, which could lead to the steep first-year decline and low overall recovery because of the limited connection and migration of hydrocarbon molecules in the shale matrix to the stimulated fracture network.

AAPG Datapages/Search and Discovery Article #90216 ©2015 AAPG Annual Convention and Exhibition, Denver, CO., May 31 - June 3, 2015