The challenges of sampling an unconsolidated laminated reservoir

An operator needed to acquire fluid samples from a 12.25-in wellbore in a high-permeability, highly laminated deepwater reservoir in the Gulf of Mexico. A previous attempt to sample a similar unconsolidated laminated sand in a nearby well by using a conventional single probe had not been successful. Only one out of the five stations could be sampled, and that operation took more than 12 h to obtain the sample because the seal could not be maintained, which also adversely affected sample quality. Such extended time on station at expensive deepwater rig rates greatly increased the cost of the evaluation and also the operational risk.

Self-sealing radial probe with the industry's largest surface flow area

Available in 7- and 9-in diameters, the Saturn 3D radial probe readily creates and maintains circumferential flow around the borehole at four elliptical ports, which have the industry's largest surface flow area of 79.44 in². Because the ports self-seal to the borehole wall, the drainage assembly helps support unconsolidated formations, and the large flow area reduces drawdown while providing high flow rates for efficient sampling that maintains single phase. Unlike conventional sampling with a probe and straddle packer, the Saturn probe does not require isolating a large interval, which minimizes storage volume effects for quicker fluid cleanup times and the efficient acquisition of high-quality pressure measurements.

Successful ultralow-contamination sampling and connectivity evaluation

The large circumferential inlet area of the 9-in Saturn 3D radial probe deployed in the 12 1/4-in open hole made it straightforward to locate a high-mobility (200-mD/cP) layer for sampling. The Saturn probe reliably maintained the seal with no failures while providing mechanical support to the unconsolidated formation to prevent collapse. The pressure differential from the formation pressure was only 35 psi, which kept the near-saturation hydrocarbon in single phase and further avoided failure of the formation.

In only 2.5 h for cleanup and sampling—one-fifth the time required by the conventional probe—the Saturn probe collected single-phase samples with only 1.2-wt% contamination measured by DFA, which was subsequently confirmed by laboratory analysis.

To evaluate the connectivity across the highly laminated reservoir, transient testing and vertical interference testing were performed with a probe set at a 4-ft interval above the Saturn radial probe. These tests quantified the vertical communication across the sands.