Geomechanical Characterization of an Acid Gas EOR, CO2 Sequestration and Monitoring Project, Zama Field, Alberta

A comprehensive monitoring, mitigation and verification (MMV) plan is critical to the success of any geological carbon
sequestration project utilized as a method of reducing CO2 emissions to the atmosphere. Beginning in October, 2005 and
running through September, 2009 the Zama Oil Field in northwestern Alberta, Canada has been the site of acid gas
(approximately 70% CO2 and 30% H2S) injection for the simultaneous purpose of enhanced oil recovery (EOR), H2S
disposal, and sequestration of CO2. The Plains CO2 Reduction (PCOR) Partnership has conducted MMV activities at the
site throughout this period while Apache Canada Ltd. has undertaken the injection and hydrocarbon recovery processes.
This project has been conducted as part of the US Department of Energy (USDOE) and National Energy Technology
Laboratory (NETL) Regional Partnership Program and includes the participation of Natural Resources Canada, the
Alberta Department of Energy, the Alberta Energy & Utilities Board and the Alberta Geological Survey.
In an effort to research caprock integrity and the risk of leakage during these field operations a first order geomechanical
characterization has been undertaken of the injection reservoir, comprising the Keg River Formation and its Zama
Member, and the overlying Muskeg Formation caprock. This poster will summarize key data obtained from a laboratory
and wireline log-based analysis of the petrophysical and mechanical properties, and the in-situ stress state in this setting.
Vertical stress estimates were determined by integrating bulk density logs in the area, while accounting for the unlogged
portion above the surface casing shoe. Horizontal stress magnitudes in the caprock and reservoir were estimated from
regional and local stress data for this part of Alberta. Dedicated stress tests such as a mini-frac, a microfrac profile, or an
extended leak-off test have not been conducted in the caprock to date in this field. Minimum and maximum in-situ
horizontal principal stress orientations in the Zama field and surrounding area, measured within and above the injection
interval, were determined from borehole breakouts.
Vertical and horizontal in-situ stress changes have occurred within the reservoir and surrounding caprock due to initial
production in the pinnacle reef, subsequent water flooding, and most recently acid gas injection. The prediction of these
stress changes is a complex function of the reef geometry, the poro-elastic response of the reservoir, pore pressure
changes over time in the reef and reservoir, and possibly temperature changes. For this poster, only the horizontal stress
changes due to poro-elastic effects have been considered. 3D geomechanical modelling will be used to simulate the more
complex problem once the mechanical properties and in-situ stresses are adequately constrained.
Basic porosity and unstressed permeability distributions from two cored intervals through the Zama Member and Keg
River Formation in two pinnacle reefs in the setting are summarized. Ultrasonic shear and compressional wave velocity
measurements have been made under unconfined and confined stress conditions on anhydrite and dolomite from the
Muskeg Formation caprock. Triaxial rock strength and unconfined compressive strength (UCS) tests are summarized
using Mohr Coulomb and Hoek Brown failure criteria. Static and dynamic elastic properties measured under anisotropic
stress conditions are compared. A Schmidt rebound hammer was used to develop a profile of pseudo-static Young’s
moduli and UCS though the Muskeg Formation caprock and portions of the Keg River Formation in two wells.
Dynamic log-derived elastic properties and their static equivalents were determined for the Muskeg and Keg River
Formations in two wells. In order to do this a synthetic shear velocity relationship was developed using recent data from
an offset well in the region. These log-derived properties are compared to the static laboratory and Schmidt hammer
derived data. Pore volume compressibility tests were also made on a select number of core plugs of the Keg River
Formation under relevant reservoir pore pressure and stress conditions, along with stress-dependent permeability and
elastic properties. Statistical relationships describing the petrophysical and mechanical properties of the rocks
investigated in this study are presented. Key learnings with regard to the heterogeneity of the vuggy dolomitic reservoir
versus the evaporitic caprock are highlighted.
The data presented in this poster have a variety of applications to EOR and CO2 sequestration in pinnacle reefs of the
type being investigated in the Zama field. In addition to caprock integrity, the data can be used to assess optimal
injection strategies, design well drilling, completion and stimulation programs, develop and interpret reservoir
monitoring data, and conduct coupled geomechanical-reservoir simulation studies of acid gas injection.

Tags: CO2, EOR, PCOR, Phase II, Zama, acid gas, characterization, geomechnical, sequestration