3D VSP Benefits in Heavy Oil Field in South Oman

Abstract

Occidental conducted a program to test geophysical methods for reservoir surveillance and monitoring in a steam injection operation at a heavy oil field in Oman. This field is a Permian age Gharif reservoir. It consists of three stacked reservoirs with porosity range from 25-33%. The heavy oil is heated by steam to reduce its viscosity and to increase its mobility. Based upon forward modelling of the reservoirs, Occidental thought that the steam injection operations would induce changes in the reservoir properties that could be detectable by geophysical methods. Monitoring steam is believed to be a vital element in managing the steam injection to reduce cost and increase recovery factor. Feasibility studies done on well log data and core measurements showed that the compressional velocity decreases by 10-15 % due to steam injection. In 2005 a pilot geophysical monitoring program was started to test techniques with potential to detect the changes in the reservoir properties due to steam injection. A list of methods containing surface seismic, borehole seismic, passive seismic and geomechanical deformation methods was proposed. From 2005 to 2010 this intense program was conducted by doing three 2D offset VSPs, a tiltmeter survey, microseismic, six cross well tomography profiles and surface 4D seismic, as well as modelling and feasibility work. All of these techniques were tested over an active area of steam injection to maximize the possibility of observing reservoir changes. Results from each geophysical technique were correlated with other geophysical techniques results and with other surveillance results. An ambitious forward plan has been drawn based on the conclusions from this pilot monitoring program. This plan consists of applying the two geophysical techniques that showed encouraging results from pilot mode to implementation mode. Both VSP and cross well tomography showed that there is a change in reservoir properties matching the modelled response in magnitude and correlating very well with other surveillance measurements From 2011 to 2013 Oxy implemented a 3D VSP program aiming to evaluate the technical and operational difficulties that might be faced when using 3D VSP in optimizing steam injection and heavy oil production in this field. There are many uses of the 3D VSP so far. The data is being used to understand the steam distribution in the reservoir. This helped to identify injectors that could cause steam break through or hot fluids break through. This technology helped the engineers to identify the locations of cold areas and helped the planning of infill wells that would assist them draining these areas for better recovery.