Morten Kristensen

Flow Modeling and Comparative Analysis for a New Generation of Wireline Formation Tester Modules

Wireline formation testing (WFT) is an integral part of reservoir evaluation strategy in both exploration and production settings worldwide. Application examples include fluid gradient determination, downhole sampling, fluid scanning in transition zones, as well as interval pressure transient tests (IPTTs). Until recently, however, formation testing was still challenging and prone to failure when testing in low-mobility, unconsolidated, or heavy-oil-bearing formations, especially with single-probe type tools. A new-generation WFT module with a 3D radial probe expands the operating envelope. By using multiple fluid drains spaced circumferentially around the tool, the new module can sample in tighter formations and sustain higher pressure differentials while providing mechanical support to the borehole wall. We performed a detailed flow modeling-based analysis of the contamination cleanup behavior during fluid sampling with the new module. Using both miscible (sampling oil in oil-based mud) and immiscible (sampling oil in water-based mud) contamination models we studied the cleanup behavior over a wide range of formation properties and operating conditions. Comparison of the cleanup performance of the new module with the performance of conventional single-probe tools demonstrates that the new module is 10 to 20 times faster than the single-probe tools when sampling in tight formations. Finally, we also compared the new module against the sampling performance of dual packers and a focused probe. This work is directly relevant to the planning and fundamental understanding of wireline fluid sampling. The key contributions are miscible and immiscible contamination cleanup models that include the effect of tool storage, a comprehensive analysis of contamination cleanup behavior for the new-generation WFT module with comparisons against conventional single-probe, focused probe, and dual-packer tools, and a characterization of fluid sampling conditions versus the preferred type of sampling tool. Introduction A logical start for any wireline formation testing (WFT) operation is a tool string design that considers the formation evaluation objectives and expected formation and fluid properties. With the current availability of an arsenal of probes having different shapes, focused probes of circular or elongated design, and dual packers, this planning stage has now become a more complex process. The recently introduced 3D radial probe (Al Otaibi et al. 2012; Flores de Dios et al. 2012) adds another choice for the engineers in planning WFT surveys. Successful WFT operations demand that toolstrings be designed to meet specific formation and fluid challenges (Weinheber et al. 2008). The selected downhole pump and probe or dual-packer combination must be able to induce and maintain flow from the formation without causing excessive drawdown to stay above expected phase-separation envelope. It must achieve and keep a seal with the borehole face and must not plug during the cleanup operation. While achieving these performance goals, it must work effectively with the downhole pump to deliver high rates that reduce cleanup time. For transient testing, the tool system must have a small storage volume, flow should be smooth and stable, and buildup transients should be free of bore-hole or tool-induced noise. A major challenge for downhole sampling and downhole fluid analysis (DFA) is mud filtrate contamination. Acquired samples must be of sufficiently low contamination for reliable laboratory analysis, as well as better DFA. High miscible contamination (oil/gas and oil-based mud filtrate, water and water-based mud filtrate) or emulsions formed in immiscible fluids (oil and water-based mud filtrate, or water and oil-based mud filtrate) can make acquired samples unusable and reliable DFA may not be possible. Knowledge of tool interaction with the sandface, pump selection, flowing time and rate, and