Summary
EnCana Corporation's Christina Lake Thermal Pilot Project located 170 km south of Fort McMurray, Alberta, Canada, uses steam-assisted gravity drainage (SAGD) technology to recover bitumen from the Lower Cretaceous McMurray formation. This paper presents an analysis of time-lapse and crosswell seismic data, as part of an overall study integrating different disciplines and technologies, to understand the effects of geology on SAGD-process performance in the pilot area.
A 3D baseline survey was conducted at the start of the pilot in 2001, and two follow up surveys were conducted in 2004 and 2005. In addition, six crosswell seismic profiles were acquired by placing both sources and receivers in the vertical wellbores. The goal of the seismic surveys was to better understand steam-chamber growth and reservoir architecture by detecting lithology changes, including the occurrence and distribution of mudstone stringers. Data from the surveys, especially from the crosswell profiles, indicated significant reservoir heterogeneity, and helped to characterize reservoir architecture in the pilot area more accurately.
Analysis of seismic data (both 4D and crosswell) showed steam-chamber growth and oil recovery to be influenced strongly by reservoir geology. Steam-chamber growth is especially affected by the presence of low-permeability facies in the vicinity of the SAGD well pairs. Our analysis indicates that these reservoir heterogeneities have contributed to the creation of areas within the reservoir that have been unaffected by steaming operations to date. These findings are in agreement with flow-simulation results and collectively contribute significantly to the planning of future developments.
Introduction
The SAGD process was developed conceptually and investigated experimentally by Butler (1994). Main features of the original SAGD model for the lateral spread of the steam chamber included thermal conduction ahead of a steady-moving steam-chamber interface; countercurrent gravity drainage of mobilized bitumen, or heavy oil; and vertical rise of the steam chamber. This recovery process was field tested at the Underground Test Facility (UTF) near Fort McMurray through a number of different phases of pilot operation (Edmunds et al. 1989; Komery et al. 1993). Field applications of the SAGD process have revealed several issues of considerable importance to the recovery performance, including wellbore hydraulics, reservoir heterogeneity, effects of solution gas, and production of solids (Edmunds and Gittins 1993; Ito and Suzuki 1999; Suggett et al. 2000; Ito 1999; Edmunds 1999; Birrell 2003). This paper relates initial efforts undertaken by the Christina Lake Project team to integrate geology, geophysics (specifically, seismic technology), and reservoir engineering to further the understanding of steam-chamber growth in the McMurray reservoir for the Christina Lake SAGD project.
Phase 1 of the SAGD pilot was implemented in 2001 at Christina Lake with the drilling and completion of three SAGD well pairs (A1, A2, and A3). Since then, three additional well pairs have been added (A4 well pair in October 2003 and A5 and A6 well pairs in August 2004). Fig. 1 illustrates the project area (TWP 76, R06 west of 4th Meridian), which now includes six SAGD well pairs along with observation wells and disposal wells. The following discussion will be limited to A1 through A4 well pairs, as production histories for the A5 and A6 well pairs are rather limited.
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