Hydrocarbon exploration is a challenging, costly and high-risk investment and success often requires further drilling of one or more appraisal wells to acquire data of the highest quality for reservoir evaluation and characterization before development plans can be put in place. During the exploration and appraisal stages it is important to define the reservoir fluid properties and reservoir parameters to enable accurate economic assessments.

Three appraisal wells in two gas fields offshore Trinidad and Tobago were drilled in reservoir formations that are mainly unconsolidated sandstones with dry, biogenic gas. An extensive wireline logging program was planned which ranged from basic and advanced petrophysical logs to images and formation testers including the dual-packer module to perform interval pressure transient tests (IPTT) or ‘Mini-DSTs’ as these are also known.

This was the first time in Trinidad and Tobago that IPTTs were conducted on appraisal gas wells. Technical factors influencing the decision to use an IPTT as opposed to a full Drill Stem Test (DST) included access to data from existing full DST's acquired in a nearby offset wells and a specific need to target thin-bedded and laminated sands so as to evaluate them individually and to observe vertical interference or connectivity between sand units.

A combination of IPTT analysis and real-time downhole fluid characterization was applied to the appraisal wells which resulted in an improved understanding of the reservoir.

The IPTT planning, methodology and applications, along with insights on some of the challenges encountered during operations which included changes to drilling operations in order to achieve the planned formation evaluation are presented.

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