The Evalua1Ion Of Cyclic Steam Sltimulation Of An Oil Sand Reservoir Using Post-Steam Core Analysis
Authors: Fialka Bob N. , Robb Greg A. , McClanahan Richard K. , Longstaffe Fred J. 
Published: March 2010

A post-steam core was recovered from a Cold Lake area oil sand reservoir subjected to cyclic steam stimulation. The objective of the program war to recover competent core from a steamed portion of the reservoir and, via detailed core analysis, determine the effect of steam injection on the deteriorating productivity levels experienced at the pilot test wells.

The core was successfully retrieved by sidetracking from an existing expendable wellbore which had received four cycles of steam injection. The coring program was accomplished at a cost considerably less than that of a new well. Core recovery war satisfactory at 84%, considering the approach used.

Mineralogical analyses performed on the recovered core provided an improved understanding of the recovery mechanisms involved in steam stimulation of the reservoir, Some of the major findings were: 1. Vertical sweep efficiency was determined to be 60%. 2. Mineral reactions were confirmed and found to be a factor responsible for decreased absolute permeability. 3. The dissolution of feldspar may have contributed to suppressing the degree of swelling clay formation. 4. Thin impermeable calcite layers within the reservoir contributed to steam confinement within specific zones. These observations helped to explain the production performance recorded at the test wells and the knowledge was adapted to the development of future thermal operations at the pilot.

Introduction

Reservoir sweep efficiency is an important parameter to determine when evaluating a cyclic steam process. Conventional methods generally used to estimate sweep efficiency include observation wells, temperature logging, seismic profiling and post-recovery coring. Observation wells can be risky because they may not intersect the steamed zone. When the steam zone reaches an observation well, vertical sweep efficiency can be estimated from temperature logs run in the well, Temperature logs, however, have been shown to overestimate the thickness of the steam swept zone as a result of profile smearing(1). Seismic profiling can provide 3-dimensional seismic images which can be used to track the movement of induced heat fronts(2). This technology is relatively new and very expensive; these aspects tend to preclude its use in thermal projects. The most accurate method of determining vertical sweep efficiency is by extracting post-steam core(s) from a steamed portion of the reservoir. This is the only method that can provide tangible evidence which accurately reflects the portion of the reservoir actually swept by steam.

Another important factor associated with cyclic steam stimulation is the effect of steam injection on the mineralogy of the reservoir(3–6). Perhaps the greatest benefit of obtaining a post-steam core is the opportunity to provide unequivocal mineralogical evidence concerning the defects of steam injection on the reservoir mineralogy. Such information could also provide a valuable test for geochemical models that utilize produced fluid compositions from thermal recovery projects to monitor mineral reactions occurring in situ(7, 8).

This paper will deal with how a post-steam core program was used to provide the evidence necessary to help understand deteriorating productivity levels observed at an experimental cyclic steam pilot in the Cold Lake area of Alberta. A proprietary steam additive designed to control the swelling of smectitic clay minerals formed during steam injection was also being tested at the pilot in all attempt to limit reservoir damage that was believed to be occurring. Recovery and examination of post-steam core was necessary to confirm that reservoir damage had