Application of Quantitative 2-D Image Analysis to Geologic Core Description: Example From Steamflooded First Eocene Reservoir, Wafra Field, Kuwait-Saudi Arabia Partitioned Zone

Abstract

Due to poor recovery of heavy oil in the 1st Eocene Dolomite reservoir in Wafra field, a steamflood pilot test was initiated in 2006. The reservoir consists of cyclic-bedded dolostones with subtidal peloidal facies that shoal upward into algal laminites. Intercrystalline and moldic pores within the subtidal facies comprise the reservoir, with porosity and permeability ranging up to 50% and 6000 mD. Tight finely crystalline algal laminites at the cycle tops are flow barriers. Nodular and bedded gypsum and anhydrite are dispersed throughout the interval. In the primary reservoir, prior to steamflood, evaporite nodules display an outer rind, up to 1 cm thick, of coarse blocky gypsum. To evaluate steamflood-induced changes in fluid saturation, reservoir quality, and mineralogy, after 6 years of intermittent steamflood, a core was acquired at a lateral distance of < 100 ft from a well that was cored prior to steamflood. Petrophysical and petrographic comparison of the two cores demonstrates that an effective 50 ft thick steamflooded interval is delineated by a decrease in oil saturation and oil stain. Gypsum is absent in the steamflooded interval due to dissolution, creating porosity along the outer rim of evaporite nodules. Because this fine-scale alteration is beyond log resolution, image analysis of core photos and CT scans was conducted to quantify steamflood-induced changes in oil stain and mineralogy. Image analysis of oil stain is consistent with plug and log data, indicating up to 75% recovery during steamflood. The amount of gypsum and gypsum dissolution was quantified from CT scans and core photographs based on density and color contrasts between gypsum and anhydrite. Quantitative estimates of gypsum dissolution are consistent with core and log data. The leached gypsum coincides with thin zones that display 5-10% porosity increase. Because porosity enhancement is confined to the outer rinds of the evaporate nodules, this could potentially channel steamflood and reduce matrix sweep. Scale derived from gypsum dissolution also creates a production problem. Core description assisted by 2D image analysis is a viable technology to generate quantitative data to calibrate petrophysical interpretations and provide a detailed understanding of reservoir heterogeneity.

AAPG Datapages/Search and Discovery Article #90259 ©2016 AAPG Annual Convention and Exhibition, Calgary, Alberta, Canada, June 19-22, 2016