Accurate fluid description in the Tertiary sediments of Mexico is important for PEMEX operating assets because large variations in viscosity over very short intervals have a major impact on well productivity. Well placement and selection of the completion strategy in this kind of reservoirs strongly depend on the characterization of the fluid distribution. This is because the efficiency of steam injection and the artificial lift system can be severely impaired if a preferential path, following high mobility section (k/μ) exists over the producing intervals. Such is the case for the Samaria-Neogeno project, which has fluids with viscosity values up to 40,000 centipoise (cP) at standard conditions.

Wireline openhole formation testing and sampling in this environment is not short of challenges and hence is rarely done in the field. The poorly consolidated formations and the high viscosity of the fluids mean that the pressure differential generated during sampling operations almost invariably causes failure of the wellbore wall, sanding out the tool or breaking the seal needed. Other experiences in the world with straddle packers in similar formations have been mostly unsuccessful because the formation collapses in the unsupported interval between the packers once the pressure differential is applied.

This paper summarizes the recent successful application of the newest wireline formation tester module for testing and sampling extra-heavy oils in the Tertiary sandstone formations of South Mexico. This is the world’s first application of such technology in unconsolidated sandstones. The new tester module provides mechanical support to the formation while allowing the application of the needed pressure differentials to mobilize formation fluid into the tool’s flowline and sample containers. The case presented shows how ~8° API fluid samples with viscosities in the order of few thousands centipoises at downhole conditions were acquired using this new device. Other applications reviewed are pressure profiling and pressure transient tests for permeability evaluation.