Poroelastic Modelling of Production and Injection-Induced Stress Changes in a Pinnacle Reef using Semi-analytical and Numerical Methods

Rock Engineering 2009 - Rock Engineering in Difficult Conditions
Hamidreza Soltanzadeh, Christopher Hawkes, Asanga Nanayakkara, Patrick McLellan, Steven A. Smith,
Abstract Pore pressure changes associated with fluid production from hydrocarbon reservoirs or fluid injection for enhanced oil recovery, greenhouse gas sequestration and/or waste disposal result in stress changes within the reservoir and the rocks that surround it. These stress changes can potentially compromise the hydraulic integrity of the reservoir if they are sufficient to induce new fractures and/or re-activate existing discontinuities in the caprock. As such, the prediction of pressure-induced stress changes can be critical for identifying the range of operating pressures that mitigate leakage risks. Semi-analytical modelling tools are useful for induced stress analysis due to their relative ease of implementation and computational efficiency; attributes which make them ideally suited to the analysis of large spatial domains and for parameter sensitivity analyses. Numerical modelling tools are useful for their capabilities to analyze more complex reservoir geometries, geological heterogeneity and material behaviour models. The value of jointly using both types of modelling tools will be illustrated with a case study of the Acid Gas Enhanced Oil Recovery (EOR) Pilot Project that is being conducted in Apache’s Zama Oil Field in northwestern Alberta, Canada.

A stream of acid gas (approximately 70% CO2 and 30% H2S) has been injected into a Devonian pinnacle reef structure in the Zama oil field since December 2006, at an average rate of approximately 25,000 m3 of acid gas per day. The pilot project includes a variety of efforts focused on examining the effects that high concentrations of H2S can have on enhanced oil recovery and carbon sequestration operations, particularly with respect to monitoring, mitigation, and verification. The results of investigations of initial in-situ stresses, geomechanical properties and petrophysical properties based on laboratory testing, geophysical log analysis, regional datasets and field testing programs will be presented for the Keg River Formation reservoir and the Muskeg Formation caprock. Results generated with the aforementioned semi-analytical and numerical modelling tools will be presented, in which stress changes in the reservoir and cap rock are examined throughout the history of the field’s initial oil production, water-flooding and, most recently, acid gas injection. Learnings and recommendations for modelling carbon storage or EOR operations in this geological environment will be described.
Keywords: caprock integrity, enhanced oil recovery, poroelasticity, reservoir geomechanics, acid gas injection, Zama, in-situ stress, CO2 sequestration
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