Integrated Dynamic-Flow Analysis To Characterize an Unconventional Reservoir
Authors: Denney Dennis 
Published: February 2015

This article, written by Senior Technology Editor Dennis Denney, contains highlights of paper SPE 156163, ’Integrated Dynamic-Flow Analysis To Characterize an Unconventional Reservoir in Argentina: The Loma La Lata Case,’ by Matias Fernandez Badessich and Vicente Berrios, YPF, prepared for the 2012 SPE Annual Technical Conference and Exhibition, San Antonio, Texas, 8-10 October. The paper has not been peer reviewed.
In November 2010, YPF brought the first shale-oil well on line in Argentina in the Loma La Lata field after fracturing the Vaca Muerta formation, the main source rock in the Neuquen basin. Production forecasting and reserves estimation in this kind of reservoir are fraught with challenges. Several reservoir- and production-analysis techniques were applied, including pressure-transient analysis (PTA), rate-transient analysis (RTA), interpretation of available diagnostic-fracture-injectivity tests (DFITs), and time-lapsed production logging. YPF implemented a workflow to analyze dynamic data that capture the physics of the flow process, to explain the observed data, and to forecast reserves for a range of assumed in-place volumes.
Introduction
The formation in this field has a thickness range of 25 m in the proximal areas to 450 m at the basin center. Formation depth ranges from less than 1000 m at the basin margin to 4000 m near the basin center. With the wide areal distribution and variable thickness and over-burden, the Vaca Muerta has produced low-gas/oil-ratio liquid hydrocarbons, volatile oils, gas with condensate, and dry gas.
According to log and core analysis, the matrix porosity of the Vaca Muerta shale varies from 4 to 14%, with an average of 9%, while matrix permeabilities span from hundreds of nanodarcies to production rates. At the time this paper was written, 27 vertical wells and three horizontal wells in this field had been placed on production.
Most of the wells required massive hydraulic fractures to achieve commercial rates. Typically, four fracture stages were performed in the vertical wells, while 10 stages were completed in the approximately 1000-m-long horizontals. More than 100 fracture treatments were completed, with a very small percentage of screenouts. Approximately 30% of the wells penetrated “sweet spots” where the upper Vaca Muerta appears to be naturally fractured, and these wells produce without stimulation. After these high-pressure/high-productivity zones are penetrated, drilling operations can hardly continue because the wells become very difficult to control. At that point, the bottom-hole assembly is retrieved and the well is completed open hole.
Data-Acquisition Plan
The data-gathering strategy was to capture the right information at the right time. The collection plan involved acquiring the following static and dynamic data.
Core and log data
Geomechanical studies
DFITs in key wells
Microseismic monitoring
Wellhead pressures and temperatures during production
Downhole pressures and temperatures by use of retrievable downhole gauges
Time-lapsed production-logging surveys
Full chemical analysis of the oil, gas, and water
Tracer analysis
With this information at hand and with the goal of integrating all the available information, several reservoir-engineering techniques were applied to estimate the expected range of ultimate recovery per well.