Collecting high quality fluid samples with a pumpout wireline formation tester (PWFT) configured with a conventional probe in carbonate reservoirs is often a challenging operation. Straddle packers are far more suitable than probes for sampling low permeability, heterogeneous carbonate reservoirs. A large volume of whole mud is trapped between the packers when they are initially inflated. The mud must be partially voided before formation fluid begins entering the packed-off interval. Before pumping begins, the near borehole region is occupied with contamination as a result of drilling overbalance and mud filtrate invasion. Uncontaminated reservoir fluid begins entering the packed-off interval after an adequate volume of contaminated fluid has been removed. Water-based mud filtrate (WBMF) and crude oil tend to separate in the packed-off interval as a result of fluid density difference and immiscibility. Moreover, a water oil interface forms at some level within the packed-off interval. As pumping continues, this oil/water fluid contact levels off at the inlet port, and both water and oil will continue to flow through the inlet throughout the sampling process. This complicates fluid identification and makes obtaining a low contamination single phase sample problematic. By taking advantage of a dual inlet port straddle packer and a new fluid density sensor, it is possible to verify the occurrence of fluid segregation within the packed-off interval. The new dual port straddle packer design enables independent opening and closing of the top and bottom ports to observe fluid segregation. By sequencing these port valves and sample chamber valves, we demonstrate how low contamination samples are obtained faster than with a single port system.

This paper provides a description of the new sampling techniques based on samples collected in a Saudi Aramco oil field, as well as examples that show the fluid segregation effect between the packers. It compares the pumping times of a straddle packer vs. an oval pad pumping in similar reservoir conditions. This paper also describes the process used to collect both water and oil samples and the challenges associated with sampling at the oil/water contact (OWC). It also presents the lab results that verify the low contamination level samples obtained by using the new methods.

You can access this article if you purchase or spend a download.