It is more effective to capture clean reservoir fluids as early as possible during drilling operations, and with the realization of formation-sampling-while-drilling (FSWD), this goal is fast becoming a reality. This paper summarizes one example from Saudi Aramco and shows the benefits of this evaluation procedure.

Our experience in sampling oil bearing formations drilled with water-based mud, has seen that the data from these instruments, which are now able to withstand the rigors of the drilling environment, can be used in real-time to control the sampling process, capture high quality samples, and also place the well where required. In this case study, a tight carbonate reservoir was the objective with the oil column being near saturation pressure. Under these circumstances, the challenges presented to us while sampling, included maintaining low flow rates and potential three-phase conditions, due to unavoidable drawdown while minimizing the time on the wall.

During sampling, pump efficiency was consistent with low-mobility formations, even with the various multiphase flow regimes encountered. Hydrocarbon breakthrough was significantly faster than historical wireline sampling performed in the same reservoir. FSWD greatly improves our costs by saving rig time. Also, FSWD was utilized to evaluate low-resistivity-pay zones and observe if they hold a movable water fraction. We see this as crucial for geosteering wells to prevent drilling through the zones with movable water, which will enhance the productivity index of the wells.

As a first time experience of FSWD in tight zones, a valuable lesson came to light where additional real-time monitoring improvements can be made, including the need to improve real time data quality, to accurately determine clean-up fractions while sampling. Based on this experience, we have formulated operational guidelines for improving real-time data analysis and determining the most opportune time to sample during the drilling process.

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