CORE PLUG NUCLEAR MAGNETIC RESONANCE (NMR) ANALYSIS AS A METHOD TO ESTIMATE PERMEABILITY ANISOTROPY

摘要

Porosity and permeability data are two of the most important petrophysical properties typically measured from core plugs taken from cored wells for reservoir characterization. Laminated reservoir facies can impart permeability anisotropy within reservoir intervals. Permeability anisotropy is commonly characterized by permeability measurements collected from horizontal (parallel to bedding) and vertical (perpendicular to bedding) core plugs taken adjacent or close to each other along the cored section. However, more horizontal core plugs are typically collected than paired horizontal and vertical core plugs. As a result of this, any k_v/k_h data derived may not be representative of the entire cored interval. In addition, it is often not possible to retrospectively collect vertical plugs from cored sections of older wells. This study examined whether nuclear magnetic resonance (NMR) could be utilized to determine core plug permeability anisotropy (k_v/k_h) from a single horizontal (or vertical) plug. The potential advantages of NMR are that: it is non-destructive, so the core plug is still available for further analysis; it would be possible to retrospectively collect permeability anisotropy data from existing horizontal core plugs; and a more representative k_v/k_h can be derived from a single plug than by deriving k_v and k_h from adjacent horizontal and vertical plugs. The approach investigated focused on using an NMR tomographic core measurement on the scale of individual lamina within cores. We simultaneously probe two size scales to estimate permeability at the pore scale and the layering at the laminar scale using spatially resolved T_2 relaxation measurements. By digitally slicing the rock down to 1 mm using advanced pulsed gradient NMR methods, porosity and pore size distributions can be quantified at the laminar scale in strongly laminated sediments. This information is used to estimate slice permeability via the T_2 log mean (k_(SDR)) model, which can then be used to estimate the permeability anisotropy by evaluating the sliced permeability values in either parallel or series. The ratio of these estimates is the k_v/k_h permeability anisotropy. In addition, perpendicular permeability can be predicted from k_v/k_h, if standard core analysis permeability data exists for the measured core plug. To enable calibration of NMR results with ‘hard data’, suitable oil well core plug samples from a number of stratigraphic intervals have been used. Core plugs were selected with a range of existing analyses such as, mineralogical and textural measurements, along with porosity and permeability, determined by standard core plug analysis techniques. Core plugs were selected in pairs with one plugged horizontal and one vertical. The traditional core plug data and our new NMR method were used to assess the application. Initial results showed that comparable permeability anisotropy data could be collected from single plugs by NMR when compared to k_v/k_h from horizontal and vertical core plug data. In some cases where there were differences between the NMR technique and the traditional core k_v/k_h data, the vertical and horizontal core plugs sampled slightly different intervals or degrees of lamination varied between the plugs. Hence, the NMR permeability anisotropy data may be more representative than the paired vertical and horizontal core plug data.