Abstract: Time-Lapse Crosswell Seismic Imaging in the Lost Hills Steam Pilot

LANGAN, R., Chevron Petroleum Technology Company, New Orleans, LA; K. GESTER, Consultant, Bakersfield, CA; M. JERVIS, TomoSeis, Inc., Houston, TX; DALE JULANDER, Chevron North America Exploration and Production, Bakersfield, CA; LOUIS KLONSKY, Chevron North America Exploration and Production, Bakersfield, CA; J. MEYER, TomoSeis, Inc., Houston, TX; MICHAEL MOREA, Chevron North America Exploration and Production, Bakersfield, CA; T. NEMETH, Chevron Petroleum Technology Company, New Orleans, LA 

We acquired three crosswell seismic snapshots between 1997 and 1999 in a steam pilot in Chevron's Lost Hills Oil Field. Our objective was to image the progress of steam injection at a scale finer than could be obtained with 3-D surface seismic data alone. Each snapshot consists of four crosswell profiles forming a 12 m by 30 m rectangular box around a steam injector, and a fifth profile along a diagonal. The injector was perforated and fracture-stimulated over a 45 m interval into relatively clean diatomite.

We assumed that the temperature and saturation changes created by the injection would cause an isolated decrease in velocity that would be simple to capture with time-lapse velocity imaging. We observed instead a spatially broad velocity increase of 5 to 10% over zones that grew between 1997 and 1999. There are isolated decreases in velocity (superposed on the increases) in the 1999 snapshot that correlate with temperature profiles in nearby observation wells.

We computed time-lapse crosswell reflection images in an attempt to further define the zones where heat and hot fluids have flowed. Some bright reflection events do appear within and above the zones where there are isolated decreases in velocity, but there are widespread changes in reflection character as well. Cornish et al. (2000, this meeting) observe similar changes in a time-lapse 3-D VSP acquired nearby. One possible explanation for the broad velocity increases is that injection-induced pressure increases are causing a reduction in residual gas saturation over a region greater than the thin zones swept by steam and heat.

LANGAN, R., K. GESTER, DALE JULANDER, LOUIS KLONSKY, J. MEYER, MICHAEL MOREA, and T. NEMETH

AAPG Search and Discovery Article #90911©2000 AAPG Pacific Section and Western Region Society of Petroleum Engineers, Long Beach, California