Wintershall Energía S.A. has been actively involved in the evaluation of the Vaca Muerta Shale in the Neuquén Basin onshore Argentina since 2011 and holds 623 sq km (202 sq miles) of net shale acreage. This coverage in both operated and nonoperated assets provides access to a large amount of data to evaluate the black oil to dry gas trends.

Wintershall has participated in 14 vertical and more than 40 horizontal wells with a maximum lateral length of 2,500 m (8,200 ft). In 2015 the company was granted the 97-sq-km (37-sq-mile) Aguada Federal Block and holds a 90% working interest along with Gas y Petróleo del Neuquén as partner (10%). This block lies in the black oil window, where three vertical wells and four horizontal wells have been drilled. The 107-sq-km (41-sq-mile) Bandurria Norte Block was acquired, and Wintershall holds a 100% working interest. This block lies in the volatile oil window. A vertical well and four horizontal wells have been drilled.

The initial evaluation phase included the drilling of vertical wells with full suites of data acquisition aiming to characterize the play. The next evaluation phase consisted of drilling the horizontal wells to test productivity as well as stimulation and completion parameters. For these purposes a large amount of data, including well logs and various types of seismic, were acquired and analyzed. Both blocks are presently on long-term production testing, and Wintershall is evaluating how to proceed with predevelopment plans.

With the acquisition of the operated Vaca Muerta Shale blocks, Wintershall recognized the value of collaborating with the Reservoir Characterization Project (RCP) at the Colorado School of Mines. Access to this world-class consortium provides additional expertise, insights and opinions that result in a better assessment of the projects and insight into commercial contractors.

Project parameters

Unlike most RCP projects that cover development projects or mature fields, the RCP/Wintershall Vaca Muerta initiative is focused on an early-stage project. The primary focus of this study is to understand the factors that control production using an integrated approach with geology, geophysics, geomechanics and engineering.

In unconventional reservoirs where permeability is low, an understanding of both reservoir and completion quality is critical in driving the project forward. The mechanical behavior of the reservoir as it relates to the ability of natural and induced fractures to sustain hydraulic conductivity pathways is one of the factors controlling the economics. Characterization of the mechanical properties of the formation, the stress state at the reservoir level and analysis of the natural fractures is essential for targeting well locations and horizontal well direction and guiding stimulation programs.

Although this shale play contains a limited number of wells as compared to those in North America, the data available in the RCP study area are comprehensive. Seismic surveys include 3-D conventional wide-azimuth (WAZ) data, 3-D multicomponent data, borehole seismic data and microseismic datasets. Initial surface microseismic was recorded followed by a permanent buried array or downhole seismic. Seismic surveys are complemented by several vertical wells, including full wireline log suites, cuttings, image logs, pressure data and early production data. (Horizontal well data were not made available to RCP.) The completeness of both the geologic and geophysical datasets offer the opportunity to build a fully integrated geoscience workflow to address the question of how geomechanical properties vary throughout an unconventional reservoir.

Initial research

Initial RCP multidisciplinary research covered the following topics:

  • Building a geomechanical model for Aguada Federal by integration of well data with seismic inversion results;
  • Analysis of natural fractures and mechanical stratigraphy of the Vaca Muerta by interpretation of borehole images and rock property modeling;
  • Analysis of microseismic data integrated with seismic and well log data;
  • Parasequence interpretation based on well logs and relating anisotropy of elastic parameters with stimulation and production;
  • Post-stack and prestack deterministic inversion followed by geostatistical inversion integrated with previous microseismic results; and
  • Prediction of brittle zones and hydrocarbon presence from a petrophysical evaluation of well logs and a seismic inversion-based classification, which leads to horizontal well proposals to target the most promising zones for production.

Multicomponent seismic surveys record shear waves in addition to standard compressional waves. This additional information can be vital to obtain a more precise 3-D geomechanical model away from wells. A multicomponent seismic survey was acquired in late 2016 and processed in 2017 as part of the pilot project. The required equipment was not available in Argentina, but Geospace Technologies (seismic equipment manufacturer and RCP sponsor) delivered cableless three-component (3-C) GSX nodal system receivers for this pilot. The 3-C receivers were placed parallel to standard single-component receivers during a conventional WAZ seismic survey. The synergy of utilizing seismic sources and logistics from a conventional survey in association with special 3-C receivers (at academic cost through RCP) made this multicomponent seismic pilot possible, resulting in the first 3-C/3-D survey for unconventionals in Argentina.

An arbitrary south-north line shows the prediction of brittle zones and hydrocarbon presence from the inversion of conventional seismic data. Schematic horizontal wells were placed to target sweet spots. Shmin shows the orientation of the minimum horizontal stress (south-north). (Source: Convers et al. “Elastic Parameter Estimation for the Identification of Sweet Spots, Vaca Muerta Formation, Neuquén Basin, Argentina.” The Leading Edge. https://doi.org/10.1190/tle36110948a1.1)

Processing

The Aguada Federal multicomponent survey was processed by Unified Geosystems. Current work will include this new 3-C seismic survey along with the 3-C vertical seismic profile to determine the added value of multicomponent data. The focus will be on characterizing anisotropy to improve the understanding of natural fractures and hydraulic fracture development and to propose landing locations and orientations for future wells. A joint inversion is planned along with analysis of the available anisotropy attribute volumes, and a differential horizontal stress analysis will study stresses in the reservoir and hydraulic fracture propagation. The applied anisotropy analysis inverts sinusoidal reflector time and amplitude fluctuations to azimuthal anisotropy parameters. Compressional (PP) azimuthal analysis helps describe the horizontal component of anisotropy, which could be related to stress or fractures, and PP-shear data can help characterize fractures.

Good communication and integration are key to success for a joint project between industry and academia. Most RCP students spent three to six months as interns at Wintershall with tasks related to their thesis. Each thesis was supervised by a Wintershall geophysicist, geologist or reservoir engineer. The RCP/Wintershall Vaca Muerta project will be completed at the end of 2019. Learnings will be applied to existing and possible future blocks in the Vaca Muerta.

If the ongoing multicomponent seismic pilot will significantly contribute to the success of Wintershall’s shale development project in a technological or commercial way, this particular technology could be applicable to other areas in the Vaca Muerta and potentially to other early-stage shale projects. In any case, RCP’s integrated and multidisciplinary approach to research continues to be valuable, providing independent opinions in both supporting and challenging the views of Wintershall.