Petrophysics of Tight Reservoirs - Challenges and Opportunities

Tight reservoirs pose a lot of challenges in formation evaluation due to low porosity and permeability, complex fluid fill driven by drainage and imbibition cycles, variation in rock types, long transition zones and uncertainty in determining free water level. Due to the uncertainty in reservoir properties and fluid fill, it is challenging to select completion zones to maximize hydrocarbon production. Integration of core, open hole & cased hole logs and well test is required to narrow down the uncertainty in reservoir properties and to predict zones which are prone to water production and/or do not contribute to the fluid flow. Water zones are identified by drainage and imbibition log response calibrated with residual hydrocarbon measured on core plugs. Reservoirs with bitumen pose a challenge to accurately quantify its presence and impact on reservoir producibility. Due to the presence of bitumen in the reservoir, which can be in the form of pore lining, pore fill and/or pore throat blocking, reservoir connected porosity and permeability is impaired. If volume of bitumen is not correctly quantified then hydrocarbon in place, completion decisions, reservoir producibility and ultimate recovery is impacted. Basic logs often do not have sufficient contrast to differentiate bitumen from hydrocarbon in the reservoir, hence, advanced logs viz. NMR and spectroscopy logs are required to identify bitumen and its quantification. This paper addresses challenges seen in tight reservoirs through integration of core and logs to understand cause of rock type variation, determining accurate water saturation with improved saturation exponent measured in tight rocks and calibration of log and core NMR, identify water producing zones through integration of open hole and cased hole logs, prediction of low permeability zones with sonic log-based workflows, and identify bitumen rich zones to assess the impact on petrophysical properties.