Representative formation pressures and fluid samples are critical to characterize reservoir fluid distribution, contacts, permeability and for compartmentalization evaluation. High mud overbalance, imperfect mudcake or long borehole exposure to drilling mud results in deep invasion requiring long clean up during fluid sampling to obtain high purity formation fluid. Oil based mud filtrate is miscible with hydrocarbon and accurate fluid identification and clean up estimation during sampling has to integrate various insitu fluid measurements for representative fluid sampling.
High mud overbalance results in deep invasion that requires high pressure pumps and focus sampling method to successfully obtain samples with low contamination. In low permeability formation, pumped volume could be low for allowed station time. This requires reservoir fluid properties to be inferred from insitu analysis of fluid with high mud filtrate contamination; ratio fluid analysis can complement insitu fluid composition to better characterize fluid at high mud filtrate contamination.
Often times fluid gradient could not be inferred from pressure due to insufficient number of pressure points or low quality of acquired pressure data due to supercharge of partial sealing effects. Insitu fluid density helps to enable contacts derivation in these conditions. Insitu fluid viscosity also could also be obtained to characterize fluid viscosity and derive permeability from wireline formation tester and well test interpreted mobility.
Compositional and fluid density grading could be identified using excess pressure technique. Excess pressure analysis can be supported by gradient analysis. Asphaltenes are now being modeled from first principles and are frequently shown to exist in complex gradients, factors that give rise to fluid complexities include current/multiple reservoir charging, biodegradation, water/gas washes, and leaky seals. Asphaltenes gradient can also be used to complement pressure for vertical compartmentalization and lateral continuity analysis.
Efficient probe pretesting tool with capability to perform multiple small volume pretests provide higher pretest success rate in low permeability formation or in depleted reservoir. At borehole condition when probe could not attain good seal, obtain representative formation pressure or sample representative formation fluid, dual packer can be used in openhole or inside casing after perforating the interval to be tested.