Residual Oil Saturation Analysis of The Burgan Formation in the Greater Burgan Field, Kuwait

Abstract

Residual Oil Saturation (Sorw) is a critical reservoir model parameter for evaluating reserves in the Greater Burgan Field. Past Sorw studies in Greater Burgan Field either looked only at core test data, or only looked at cased-hole log data. None of the past studies considered areal position, different rocktypes, or changes in remaining oil saturation with varying amounts of water sweep. This study includes analysis of Sorw from open-hole water saturation, Time-Lapse PNC data and Special Core Analysis water flood experiments. The majority of the log data in Greater Burgan Field water - swept zones are concentrated in the 3rd sand middle, 3rd sand lower and 4th sand formations. The comparisons of the results from all three methods used in the study to measure remaining oil saturation (ROS) are limited to these reservoirs. Results from these methods were remarkably consistent. All reservoir sand with extensive PNC log data showed that zones encroached by water for 22+ years tend to be at or near residual oil conditions. Measurements in the zones with water encroachment for less than 22 years have about a 50% chance of being incompletely swept. Analysis of the 22+ year data allowed reasonable ranges of Sow were estimated from this data. Investigations of ROS spatial variations in the Magwa, Ahmadi and Burgan sub-fields were made. 3rd sand middle was the only reservoir with both adequate PNC and open-hole coverage in ROS from these three areas in Greater Burgan Field. ROS by rocktype was reviewed in three categories of reservoir rock (excellent, medium and poor quality reservoir) as currently defined by log analysis in Greater Burgan Field. The vast majority of log data occurs in rocktype 1, the highest quality reservoir rock. Only 3rd sand lower formation contained sufficient data in all three reservoir quality rocktype to make valid comparison. Both core flood Tests and PNC Time-Lapse methods also showed no difference in ROS based rocktype. Introduction Background Definitions Residual Oil Saturation (Sorw) is defined as the lowest oil saturation that a reservoir can achieve (technically, a fixed value for a given reservoir and recovery mechanism). With logging tools, we can measure Remaining Oil Saturation (ROS), the oil saturation calculated from a reservoir after it is swept by water due to production. Eventually, these two oil saturations become the same. This study will show evidence to suggest that in Greater Burgan Field, the remaining oil saturation may be changing (lowering) through time, and that true residual oil conditions are frequently not met until a reservoir has been swept for many years. Commonly, the term residual oil saturation is loosely defined as both the true residual oil saturation and the oil saturation that can be measured today with a logging tool. For reserve estimations, the value of the true residual condition is used. When the topic is oil saturation as measured by logging tools, it is also commonly referred to as residual oil, even though the term remaining oil would be more precise. This report will attempt to keep the definitions of Sorw and ROS distinct. ROS Is Important To Reserve Estimation Sorw is a critcal parameter for the accurate estimation of reseves in Greater Burgan Field. Determination of residual oil saturation can provide the basis to refine predictions of a reservoir's recovery factor, and thus possibly increase a field's reserve estimates. There are many ways to measure ROS (and by analogy Sorw) in a dynamic reservoir. Unfortunately, frequently these different methods will not yield consistent answers.