Thinly laminated, low permeability carbonate rocks of the Turonian Abu Roash “D” (ARD) member, located in the north Western Desert of Egypt, provide a challenging environment for recording formation pressures and acquiring clean formation fluid samples. The challenges are partly attributable to reservoir lithological heterogeneity and low permeability in addition to the microporosity pore system. From the operations perspective, locating the probe on the sweet spot, and the difficulty associated with sealing probes on testing tools present major challenges.
To increase the success rate of obtaining valid pressure measurements and fluid samples, a proper selection of the formation tester probe geometry is essential. A series of prejob simulations using various probe types indicated that the use of oval-shaped pads provided distinctive advantages over standard probe types.
This paper includes field examples from the Abu Roash D carbonate reservoir where the oval-shaped pad was used. The wireline microresistivity imager was used to characterize the zone of interest and facilitate the test point selection.
The use of the image log for the pressure point selection in a thinly laminated reservoir and the oval-shaped pad positioned to cover a vertical section of the wellbore enabled the detection of a relatively permeable thin lamination with a mobility of 28 md/cp. This process facilitated the collection of a formation fluid sample for the first time from this reservoir after pumping for more than 1½ hours. A high resolution fluid density sensor was used for fluid identification, which was able to detect low concentrations of residual oil during sampling, unlocking, and eliminating the ambiguity in the understanding of the residual fluid in the Abu Roash D reservoir.
This paper presents the method, selection criteria, and workflow used to acquire formation fluid samples in challenging, low permeable carbonate reservoirs.