--> Applications of Diamondoids to Unconventionals

AAPG ACE 2018

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Applications of Diamondoids to Unconventionals

Abstract

Diamondoids are nanometer-sized, hydrogen-terminated diamonds. They form during petroleum generation from hydrocarbon rearrangements and are present in all crude oil and rock extracts. They vary in size from a single diamond cage constituting adamantane up to at least eleven cages. Due to their thermal stability and a variety of sizes and structures, they are excellent indicators of thermal stress, mixing of oil sources of different maturities and as tools for oil to oil and oil to source-rock correlations of liquids of any thermal maturity including extremely mature fluids.

Diamondoids have many applications to unconventional oil and gas plays. A moderate amount of thermal oil to gas cracking is desirable for unconventional oil production due to an increase in reservoir pressure and the cracking of large molecules, e.g. asphaltenes to smaller, more mobile species, and diamondoid concentrations are a direct indicator of the extent of oil to gas cracking. Mapping diamondoid concentrations areally is an excellent way to locate optimum thermal maturity for unconventional oil and gas production. Mapping diamondoid concentrations within a vertical section of rock extracts from core often leads to the recognition of zones in which highly mature liquids and gas have migrated laterally within the section. Such zones must be of relatively high porosity and permeability in which fluid can flow and would be presumed to be excellent targets for unconventional production.

Understanding where production is coming from post-frac is an important question for which diamondoids can provide definitive answers. For instance different oil families within the Permian Basin have distinct large diamondoid distributions. Furthermore, different horizons within a single unit, i.e. the Eagle Ford, can have different distributions of large diamondoids. These distributions can be used in produced Eagle Ford oils to determine the producing horizons and then monitor them over time. An advantage of using diamondoids for this type of correlation work is their stability. Many unconventionals are most productive in higher thermal regimes in which more commonly-used correlation methods like biomarkers are absent.

Another use of diamondoids is to optimize rates of production. It has been shown that by measuring the ratio of small to large diamondoids, it is possible to determine the maximum rate of production possible without leaving stranded liquids behind.