Collecting representative, single-phase retrograde gas samples from heterogeneous fractured carbonate formations is challenging, especially for cases in which the reservoir pressure narrowly exceeds the dewpoint. Conventional wireline probe/packer assemblies are undesirable in this situation because their small diameter probes increase the pressure drawdown, compared to other available intake configurations. Furthermore, the likelihood of establishing hydraulic communication with an open fracture or vug is low with a ½-in. diameter probe, compared to setting straddle packers spanning a 1-m formation interval. The straddle packer intake geometry reduces the drawdown pressure required to establish flow, thus maintaining the flowing pressure above the dewpoint and preventing the onset of unwanted condensation.

Inflatable straddle packers can be run with pump-out test tools, but they have temperature, pressure, and operational limitations. The new Oval Pad* design, an elongated oval-shaped probe/pad assembly, possesses many of the advantages of dual packers without their limitations. Like a straddle packer, the Oval Pad tool is effective when testing thinly laminated and tight formations as well as fractured or vuggy carbonates. The oval shape enables the pad to encompass a significant vertical length of the borehole while establishing a pressure seal with the formation.

A retrograde gas reservoir is contained within the vuggy carbonates in the Jambi Merang area of South Sumatra. Here the reservoir pressure narrowly exceeds the dewpoint. Pressure must be maintained above the dewpoint during sampling operations to prevent unwanted phase separation. The RDT pump-out tester was fitted with Oval Pad and nitrogen-charged sample bottles to collect single-phase samples and to keep the phase intact until PVT properties can be analyzed in the laboratory. These samples were necessary to determine laboratory PVT properties and to define a production strategy.

This paper presents the details and challenges of the successful collection of single-phase samples in this environment.


The Jambi Merang block is situated in the north of the south Sumatra basin (Fig. 1). The block was awarded in February 1989; and since then ten wells have been drilled, of which six were exploration and four were delineation (Fig. 2). The carbonate in the South Sumatra basin comprises build-ups on carbonate platforms along with the Marine Shale equivalent. The platform limestones are micritic and tight, while the buildups form the reservoirs. Porosity is predominantly created by meteoric waters at the top of the build-ups, hence the traps are typically stratigraphically controlled by the diagenetically developed porosity distribution. The platform carbonates generally form on the shelf of the Sunda Craton with the buildups developing on the platform. In the Jambi Merang block, the carbonate platform is isolated from the palaeo-coastal fringing platform limestones and is situated high on the Merang Block. On this isolated carbonate platform, there is a series of buildups, one of which is Sungai Kenawang. 1

Many operators have worked in this area, which is mainly a gas and condensate producer with the gas transported mostly to Singapore LNG Plants via undersea pipe lines and in west Java Power Plants. The carbonate reservoir is a 700–1000 ft thick retrograde gas reservoir with reservoir pressure very close to the dewpoint and with a bottom aquifer drive. JOB Pertamina-Hess Jambi Merang drilled two exploratory wells with the primary objective of enhancing formation evaluation. The evaluation program involved openhole logs, core analysis, and sampling. Cores were taken for laboratory studies of the carbonate reservoir rock. Openhole logs were run including standard resistivity, density, neutron, sonic, and spectral gamma ray in addition to electrical micro imaging.

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