Feasibility study of time-lapse seismic monitoring of EOR with steam injection in a heavy oil field
Authors: Yuh Sung , Le Ravalec-Dupin Mickaele , Hubans Christian , Lys Pierre-Olivier , Foulon David Jean 
Published: May 2016

Abstract
Thermal recovery techniques such as Steam Drive (SD), Steam Assisted Gravity Drainage (SAGD), or Horizontal Alternating Steam Drive (HASD), have been developed to increase the limited recovery rates in a heavy oil field. One of the key issues during steam flooding process is a monitoring the evolution of vapor chamber since there exist potential risks of leakages of vapor or barrier of vapor growth due to heterogeneities of the earth. Time-lapse seismic monitoring has been one of the efficient methods to detect steam chambers. A reservoir saturated with heavy oil has higher seismic velocity than steam saturated reservoir in most cases. Thus the velocities contrasts are relatively large and we can expect a strong 4D seismic anomalies around the vapor chambers. However the heavy oil in our field is still mobile and it has been produced (cold production) for several decades. Thus there exist a little amount of free gas everywhere in the field. This means the velocities of our reservoir are already low prior to a steam injection. Based on our study, the velocity changes are less than 3 % Thus, seismic monitoring of the steam chamber becomes a challenging issue in such conditions. Our modeling study shows that it requires a high signal-to-noise ratios, greater than 10 dB, to estimate an accurate dimension of the steam chamber. In addition, we observe that the time-lapse seismic images give insights about the variations in hydrocarbon gas saturation.

Introduction
The resources of heavy oil in the world are over twice those of conventional crude oil. It is technically challenge project to development of heavy oil field due to the low mobility of heavy oil. One of the efficient methods is the steam flooding technique to increase the recovery rate (Bulter, 1991). There are several different kinds of techniques depending on the arrangements of injection and production wells. For example in this study, we tested Steam drive (SD), Steam Assisted Gravity Drainage (SAGD), and Horizontal Alternating Steam Drive (HASD). During the steam flooding process, it is necessary to monitor the expansion and migration of the steam chambers. Most monitoring methods are based on well logging but its data are limited to vertical information. Time-lapse seismic monitoring technique can measure the 3D shapes of steam chambers if favorable conditions are permitted (Forgues, 2006, and Bianco, 2008). Previous studies have shown that the seismic velocities decreased considerably due to the large property difference between steam and heavy oil. In our study area, oil is mobile and its gravity is 8°. The field has been developed based on natural depletion for more than ten years. Now, the reservoir becomes below bubble-point pressure and free-gas is release. Seismic velocity of our heavy oil field is already low due to the free gas. Thus, probably we couldn't expect a large velocity drop after steam injection since the seismic velocity of the reservoir saturated with free gas and steam vapor could be similar. This is the objective of this feasibility study to investigate if time-lapse seismic can detect the evolution of steam chambers.
In this study, we tested three different steam flooding techniques: Steam drive (SD), Steam Assisted Gravity Drainage (SAGD), and Horizontal Alternating Steam Drive (HASD). Each simulations yield the pressure, saturation, and temperature at continuous times (every six months). Then, we computed the rock physics properties based on laboratory measurements and well logs. Last the time-lapse seismic modelings are performed based on the results of the flow simulation, for every 6 months.

Theory and method
The flow simulations for the steam flooding process are conducted with the thermal, compositional simulator, STARS, which accounts for temperature and phase changes in the reservoir.