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Technical Paper: Advanced Core Analysis Methodologies Quantify and Characterize Prolific Liquid Hydrocarbon Quantities in the Vaca Muerta Shale

Society: SPE
Paper Number: 1922938
Presentation Date: 2014
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The analysis of self-sourcing liquid-rich mudstones is complicated by the inherent heterogeneity of these ultralow-permeability formations. Determining reservoir and completion quality requires a detailed understanding of how rock properties, fluid saturations, hydrocarbon composition, and fluid thermodynamic properties are distributed throughout the reservoir. In many cases, the rock and fluid properties are intrinsically coupled from both geological and physical chemical perspectives and must be analyzed together. A wireline log–based heterogeneous rock analysis (HRA), coupled with advanced laboratory analyses, was used to characterize the distribution of petrophysical properties along a core spanning the entire section of the Neuquén basin Vaca Muerta shale. A novel quantitative extraction and gas chromatography method was used to measure the total hydrocarbon composition (C1–C30+) at multiple depths throughout the reservoir. The results of this combined rock-fluid study show that the formation contains two distinct reservoir zones connected by a transitional zone. Each of these reservoir sections owes their uniqueness to a change in depositional and diagenetic processes. This leads to local heterogeneity in properties such as mineralogy, transformation ratio, and hydrocarbon composition, which could not be explained by thermal maturity alone. A high degree of correlation was found between the rock and fluids in each of the reservoir zones. Advanced core analysis studies of the Vaca Muerta shale found that the lower zone contains high quantities of liquid hydrocarbons (~45g/kg of rock), but it has the lowest gas-to-oil ratio (GOR) and the highest viscosity and also contains numerous ash beds that may reduce the overall completion quality of the section. The upper zone lacks the overall liquid hydrocarbon volume of the lower (~10g/kg of rock), but it has a higher GOR, lower viscosity, and fewer ash beds—properties that are favorable to production. The transitional zone maintains the high liquid hydrocarbon content of the lower zone (~30g/kg of rock) while maintaining moderate GORs and viscosities and being free of ash beds. This transitional zone represents the highest analyzed reservoir quality section in the Vaca Muerta shale when both rock and fluid properties are considered together. The HRA method can be used for data propagation to other regions of the Vaca Muerta shale.

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