Abstract
As the depth of exploration wells increases, operators are encountering reservoirs with increasingly low permeability. The accurate assessment of such tight reservoirs is critical for making informed decisions about field development, as operators must demonstrate adequate reserves and producibility. However, evaluating these challenging reservoirs presents significant difficulties within existing workflows. This study showcases an integrated approach to formation evaluation applied to two prospects in Southeast Asia, both containing clastic formations with low to ultralow permeability.
For both cases, advanced and comprehensive logging programs were implemented, combining petrophysics, geological data, downhole testing, and acoustic measurements. These efforts aimed to assess crucial rock properties such as porosity, fluid distribution, lithofacies, saturation, and permeability under reservoir conditions. The petrophysical logs were analyzed in near real time to optimize subsequent testing and coring operations. Formation testers were deployed in both wells to measure formation pressure, evaluate mobility, permeability, and obtain high-quality fluid samples. The paper details the workflow that enabled these objectives to be achieved efficiently.
The study uses realtime data from actual field operations and incorporates state-of-the-art logging technologies, including nuclear magnetic resonance (NMR), elemental spectroscopy, dielectric dispersion, image logs, and downhole formation testers. The integrated analysis of these diverse data sources aids in rock characterization and reserve calculation within a critical operational timeframe. Distinct workflows were developed to supply petrophysical input for selecting and interpreting formation tester (FT) and drillstem test (DST) results. These workflows provide detailed reservoir insights, such as rock and fluid volumes, porosity, saturation, free fluid, gas-corrected porosity, permeability, producibility, net thickness, rock facies, and rock typing.
The challenges specific to formation testing in low to ultralow permeability reservoirs are also addressed in this paper. Key considerations include selecting the appropriate inlet geometry (sampling point) and area for connecting the FT to the reservoir sand face. To minimize pressure drop and maintain single-phase flow, ultralow-rate pumps are essential. Furthermore, the paper introduces an innovative wireline FT platform integrated with an advanced downhole fluid analyzer (DFA). This platform captures reservoir fluid composition, saturation pressure, fluid compressibility, density, and viscosity, all of which contribute to generating an equation of state (EoS).
The results from the Southeast Asia prospects highlight the effectiveness of the integrated approach, showcasing lessons learned and best practices for achieving comprehensive logging and testing objectives, even under challenging reservoir conditions. The paper provides insights and methodologies that can be adapted for similar environments in other regions, enhancing the accuracy and efficiency of reservoir evaluation and testing processes globally.
