Core Analysis Reconciles Disparate Permeabilities Derived from NMR, Stoneley and Formation Tester Providing Insight to Reservoir Characterisation

Abstract

Abstract Acoustic and nuclear magnetic resonance (NMR) logs are commonly used techniques to derive continuous permeability index profiles grounded in theory (e.g. Stoneley wave attenuation and Coates – Timur methods). A formation testing device may also be used for dynamic measurements (point or interval). Furthermore, the permeability of side-wall-rotatory cores or conventional cores can be obtained using conventional laboratory measurements. Of these methods, the NMR and/or the Stoneley permeability are most useful for real-time formation testing or completion decisions. The expectation is that the NMR and Stoneley permeability profiles will help to optimally select depths to acquire dynamic information and rotatory coring. However on occasions there may be a significant mismatch between the permeability profiles. This case study presents and reconciles why there is a discrepancy between the NMR, Stoneley and the measured dynamic permeability derived from a formation testing device. NMR permeability was calculated, with conventional parameters for this basin, using the Coates-Timur method. Acoustic permeability was derived using the attenuation of Stoneley wave amplitude. Formation testing points were selected at the depths with relatively high movable porosity and high permeability values guided by the NMR data. Mobility was measured at these points with the formation tester and the dynamic permeability was calculated using the mobility and fluid viscosity. Rotatory cores were cut with reference to these datasets and later analysed for porosity, permeability, thin sections, XRD and SEMs study. The dynamic permeability information proved to be lower than the NMR by almost two orders of magnitude. These disparate results generated some doubt concerning the parameters used in the NMR Coats-Timur method. In contrast to the NMR, the Stoneley permeability is consistently lower than the NMR permeability and close to the dynamic measurements. The core results showed reservoir quality is affected by the cementation (quartz overgrowth at pore throats) and diagenetic clay (illite and smectite). This means the reservoir has a complex porosity-permeability relationship that violates the basic assumptions of the Coates-Timur equations. The result was an overestimate of formation permeability due to the large pores and small pore throats (cementation, quartz overgrowth and digenetic clays). The Stoneley permeability technique involves the movement of fluid in these small pore throats and produced permeability comparable with the formation tester and core results. After calibration with the core, the NMR and Stoneley permeabilities are reconciled with the dynamic test results. In general, coherent results provides confidence in an answer, however disparate results, as in this case study, may also provide exceptional information about the reservoir quality. The case study shows it is critical to understand the underlying assumptions of different techniques to explain disparate results and realise the value of the exceptional information.