Integrated Formation Evaluation in Laminated Reservoirs

Abstract

Laminated units made up of alternating thin sand and mud layers are known to be very challenging in terms of reservoir characterization and evaluation. Alternation of thin sand and mud layers imposes natural anisotropy in the distribution of reservoir properties such as porosity and permeability. This anisotropy has been proved to be a major control on fluid flow within the reservoir which is of paramount importance to understand and consider in reservoir development planning. The aim of this work is to integrate geological information derived from borehole images into interval pressure transient test (IPTT) interpretation to analyze and explain the complex flow behavior resulting from a combination of formation dip and laminations.A wireline formation tester dual packer module was utilized in two offshore exploration wells to conduct an IPTT/miniDST (mini Drill Stem Test) in different lithofacies including laminated units. Several build up tests were performed to obtain valid reservoir pressure and permeability values. Consistency was observed in the log-log diagnostic plot from these build up tests and the pressure derivative behavior suggests multiple possible reservoir scenarios in this complicated geological setting. A detailed study on borehole images and 3D near-wellbore geological model revealed the effects of slanted wellbore, laminated units and dipping beds on the travel of pressure transient away from wellbore. With this information, a representative reservoir model was able to be constructed to match the IPTT data and to obtain the required reservoir parameters.This paper highlights the main challenges of formation characterization in laminated thin sand and mud layers. Special emphasis was given to resolve significant problems encountered in pressure transient test interpretation in highly deviated wells. Slanted wellbore sections are typical in Malaysia's offshore fields as the wells are drilled from offshore platform to intercept the formation at different angles, hence the pay zone and other petrophysical parameters are usually underestimated. Attempts are made to identify and classify these challenges, and recommendations are provided toward better resolution during interpretation.