Abstract

Deep-water offshore exploration has revealed increasing discovery of unconsolidated and under-compacted reservoirs characterized by high porosity and permeability. Formation evaluation becomes challenging since conventional drilling practices adapted for this environment often results in near-wellbore alteration which can reach deeper into the formation due to relatively low competency of the rock. This characteristic is observed for almost all the deep-water Miocene-Oligocene turbiditic sands across the globe.

The concept of near-wellbore damage is a well-known topic, however, its implication in deep-water exploration and production is being studied now in great detail. Measurements of properties like mobility and formation-fluid attributes have been jeopardized by fines migration, damage and alteration. The effect of near wellbore deformation causes development of skin and stiffness change that has a strong impact on formation fluid sampling and pressure acquisition. Multi-pole multi-array sonic acquisition has shown that damage causes variation of sonic-velocity in the radial direction that could be identified and quantified with the current technology. Innovations in Nuclear Magnetic Resonance (NMR) with greater depth-of-investigation (DOI) and radial-imaging and interpretation techniques indicates that the concept of fines migration causes considerable effect on formation permeability and porosity estimation and has a critical impact on reserve estimation.

The current work focuses mainly on two aspects of wellbore alteration on measured logs. First, effect of alteration has been identified by using multi-DOI NMR and sonic measurements in an integrated manner. Secondly, and subsequently, quantification of the damage was measured along with its impact on other measurements like formation-tester. This integrated technique can significantly and efficiently reduce formation sampling duration by locating damaged-zones and effectively reduces acquisition cost in a complex deepwater operation. The implication of identifying the damaged zone has a great value in designing optimum completion and sand management and exclusion technique at the subsequent stages of development.

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