As interest grows in formations with permeability ranging from tight (<1.0 md) to ultralow <0.1 md), formation testing and conducting drill stem tests (DSTs) become increasingly challenging and more costly. Formation testing using conventional probes has low success rates for pressure surveys and sample collection, long pumping times, and poor data quality that cause difficulty in fluid identification because of incomplete cleanup of filtrate. DSTs are often unsuccessful without sufficient inputs of permeability and fluid properties for optimization of testing plan. This paper presents case studies of a revolutionary new 3D radial probe that makes effective evaluation possible for unlocking tight gas reservoirs in the South China Sea.

The tight gas formations have poor borehole condition for conventional probe sealing and testing. A total of 25 attempts by two different conventional probes fail to seal. However, three stations for the 3D radial probe took less than 4 hours per station to successfully collect PVT-quality samples. Fast cleanup to flow pure formation fluids enabled better accuracy in real-time fluid identification, sampling assurance, and compositional analysis including CO2. Transient analysis indicated that all stations have <0.1-md/cp effective mobilities, with the lowest at 0.03 md/cp. Production prediction was conducted to assess the natural productivity. Comparison with DST results from neighboring wells show that this prediction has acceptable accuracy and is sufficient for crucial decision making. This enables the operator to avoid unnecessary DST stations and ensure optimal completion and well testing designs.

Performance was compared against conventional modules. The advantages of the 3D radial probe over elliptical probes and dual-packer module in this kind of tight gas reservoirs are obvious and significant, with better efficiency, quality and operability such as applicable differential pressure, sealing capability, minimum storage, no exposure to borehole mud, and less sticking and plugging probabilities. Numerical modeling of an immiscible water-gas system with representative borehole conditions and reservoir properties of tight gas formations was conducted to investigate flow and pressure transient behaviors for operation optimization. Initial results of the simulations will be briefly discussed in this paper.

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