Predicting Recoverable Liquid-Rich Sweet-Spots With PVT Phase Kinetic Modeling: Vaca Muerta Shale, Neuquén Embayment

Abstract

Prediction of hydrocarbon composition and fluid phase behavior, in function of dew and bubble points, is essential to appraise shale play resources and delineation of recoverable sweet-spots. Sweet-spot detection is based on shale play fairway analysis in order to identify areas with significant retained hydrocarbons in-place but also specific fluid behavior at reservoir conditions (close to critical point) optimal liquid-rich maximum recovery.

For this purpose we developed a customized household compositional kinetic model for hydrocarbon generation coupled with 1D PVT reservoir fluid modeling (Peng Robinson EOS) for detailed fluid phase behavior properties. The defined kerogen and the associated source rock type fractions react as a function of thermal stress throughout primary and secondary cracking, being possible to quantify the compositional yields (mole fraction) of in-situ retained gas and liquid fractions. Hydrocarbons in-place at reservoir conditions were estimated by net source rock thickness organic porosity development, taking into account the generation of immobile fractions during secondary cracking.

Calculated mole fractions and reservoir conditions (P-T) were used as inputs for PVT modeling to predict fluid phase diagrams as a function of critical point and to create maps of volumetric properties such as Bo, Bg, initial GOR and CGR. Volumetric factors maps were used to quantify hydrocarbon volumes in place at surface conditions. Finally, based on existing production data and forecast profiles (using adapted Arps equation); it was possible to define recovery factors to map recoverable resources in MMstbl/km² and Bscf/km²

This workflow has been applied using public data from the Lower and Upper sections of the marine organic-rich Vaca Muerta Shale Play in the Neuquén Embayment. Calibration with production data comes from developed YPF’s Loma Campana concession block. Results provide a precise position of fluid type limits and detailed PVT properties in less developed areas for volatile oil and retrograde gas condensate windows. Higher potential of recoverable liquid-rich sweet-spots are located in areas where fluid behavior at reservoir conditions is near the condensate-volatile oil limit, close to critical point. Herein ultimate recoverable oil and gas resources vary from 0.7-1.1 MMstb/km² and 3.0-6.5 Bscf/km² Detecting recoverable sweet-spots provides, thus, a significant step forward for shale resource assessment and bidding block ranking.

AAPG Datapages/Search and Discovery Article #90323 ©2018 AAPG Annual Convention and Exhibition, Salt Lake City, Utah, May 20-23, 2018