Reservoir fluid characterisation in cased-hole is often challenging especially in thinly laminated and low permeability formations. Obtaining representative reservoir fluid samples is an important step in verifying fluid type and properties in addition to reserves booking requirements. Downhole fluid analysis (DFA) provides critical information on early confirmation of fluid type, while samples are used in the lab to provide PVT and physical properties of the reservoir fluid.
There are two (2) main ways to obtain fluid samples in cased-hole environment: either through subsurface sampling via wireline downhole sampling tool (WFT) or surface sampling via well testing full term (DST). Depending on the test objective, each method has its pros and cons. Subsurface sampling is normally performed in open hole. However, wellbore conditions and formation characteristics sometimes prohibit logging in open hole, hence sampling can only be performed in cased-hole. This also applies to wells which had been cased earlier than expected due to operational constraints. Conventional DST in cased-hole can be prohibitive due to many factors including cost especially if multiple layers have to be tested, not to mention environmental concerns restricting flaring.
This work depicts the successful fluid sampling strategy based on case studies in East Asia reservoirs. The paper will also discuss a proposed best practice that is supported by field data.