Net Pay Cutoff Determination Using In-Situ Permeability Measurements with Advanced Formation Testers

摘要

For a reservoir rock to be classified as net, a basic requirement is for it to contain sufficient storage capacity (porosity) and demonstrate fluid flow potential (permeability). Most common methodologies in the industry today for applying net pay cutoffs are developed based on petrophysical measurements. This is especially the case in clastic reservoirs, where a simple approach using a porosity-permeability correlation derived from core or logs is often employed. The ideal approach is where measurements of both porosity and effective permeability are made in situ. It is worth noting that effective permeability can often be quite different from absolute permeability, hence this measurement must be made for the reservoir fluid and not for the invading mud filtrate and taking into account the presence of irreducible water in a water-wet system. By conducting an in-situ permeability measurement, especially in thin layers or heterogeneous zones, the accuracy of net pay cutoff determination is significantly improved. In-situ permeability can either be inferred from petrophysical evaluation or measured directly with formation testers. Generally equipped with a probe-type device, formation testers can be combined in a drillstring assembly (formation testing while drilling, FTWD) or run on wireline (wireline formation testing, WFT). In either case, other petrophysical logging tools can be combined and measurements made in the same run. One of the challenges in accurately evaluating net pay with petrophysical measurements or single-probe-type formation testers is their inability to resolve or find thin layers. In laminated formations, logging devices with higher vertical resolution are required and formation testers, rather than attempting to communicate with the formation through a single point source, use devices with large surface areas open to flow, which have proven to provide much more representative measurements. In this paper the benefits of advanced formation testers and the role they play in net pay cutoff determination in challenging environments are presented.