Technology Update: New Method Adds Value to Wolfcamp Porosity, Organic-Matter Measurements

Abstract

Technology Update A new technique that analyzes scanning electron microscope (SEM) images of formation samples has been used to measure porosity and total organic carbon (TOC) in the Wolfcamp Shale of the Delaware Basin in west Texas. The technique’s application has led to methods and findings that can be used to Determine oil-in-place values, which are a key factor in enhanced oil recovery and reserves determinations Improve petrophysical models for log interpretation Better understand submicron porosity in mudstones and other tight samples In particular, the technique was able to identify where the porosity was hosted and quantify the porosity in organic matter (OM), which cannot be specifically determined by standard laboratory methods. The technique was successfully demonstrated in a project undertaken by Anadarko and Ingrain that compared SEM-image porosity and OM content with standard helium porosity and TOC data from pyrolysis. Tests were conducted on as-received plug sample end trims. Solid OM, porosity, and porosity associated with OM (PAOM) were computed from the SEM images. These values have been compared with pyrolysis-based TOC, bulk volume (BV) oil, and total porosity as determined using the Gas Research Institute (GRI) method. Case Study Focus, Methods The case study focuses on stratigraphy that is dominated by microporosity, with permeability ranging from nano-darcies to millidarcies, and is related to the deposition of argillaceous, siliceous-dominated turbiditic sequences (diagenetic overprinting). The stratigraphy includes carbonaceous-dominated basal debris flows in a variable slope to basin setting through time. The project consisted of 42 as-received 1-in.-diameter plug sample end trims. Archimedes bulk density was measured on as-received samples. To aid with subsample selection for SEM imaging, five X-ray fluorescence (XRF) measurements were acquired along the vertical axis of the plug end trim, perpendicular to bedding. Bulk TOC and mineralogy were computed from homogenized powder, using Fourier Transform Infrared Spectroscopy (FTIR). All SEM subsamples were polished with an argon-ion-beam milling system. Images of the polished areas, approximately 1 mm by 0.5 mm, were captured using a SEM secondary electron (SE2) detector at a resolution of 250 nanometers (nm) per pixel. Next, a series of images at a resolution of 10 nm per pixel were acquired simultaneously, using SE2 and back-scatter electron detectors. Both types of images are required for accurate quantitative analysis. These SEM images were acquired perpendicular to bedding, along the vertical axis. Representative samples were also imaged at 5.0 nm and 2.5 nm resolution. Most of the additional porosity detected by the higher resolution images was found to be associated with OM. The multiple resolution images were used to compute final OM and porosity values. The porosity was further analyzed and separated into PAOM and mineral-associated porosity (intergranular plus intragranular). Fig. 1a shows an example of a 10-nm-resolution SE2 image from one sample. Fig. 1b shows the same image with colors indicating OM, PAOM, and mineral-associated porosity. An effective sand screen is designed to allow the larger formation particles to bridge across the openings to offer maximum fluid flow area and reduce plugging. Smaller formation particles are then retained behind the larger “bridged” particles. Premium screens incorporate layers of metal mesh or weave to handle a larger range of particle sizes while increasing the fluid flow area and providing greater mechanical strength and erosion resistance.