Drill Fluid and Return Permeability Tests of South Texas Wilcox Cores

Abstract

Core plugs of argillaceous sandstone from the Wilcox Formation of South Texas were examined by petrographic methods and tested for oil permeabilities before and after exposure to a variety of drilling fluids and filtrates. This was done to determine why some drilling fluids are more damaging to permeabilities than others. The clay minerals in these sandstones, which are mostly diagenetic, include chlorite, kaolinite, and pore -bridging mixed-layer illite-smectite. The greatest permeability damage was caused by gel-lignosulfonate-based fluids, which caused pore throat blockage by dispersion and migration of the mixed -layer clays. Much less damage was caused by the PHPA polymer/KCl-based fluids, which apparently stabilized the fibrous illite-smectite. INTRODUCTION AND PURPOSE OF STUDY This study involved approximately three feet (91 cm) of four inch (10.2 cm) diameter sandstone core obtained from a major operator's well in South Texas. Five intervals ranging in depth from 6254 to 6296 feet (2052 to 2066 m) in the Wilcox Formation were selected. Fossils, sedimentary structures, and regional studies indicate these rocks are shallow marine deposits.' The purpose of this study was:to determine the amount of permeability reduction caused by exposure to various types of drilling fluids under simulated wellbore conditions,to thoroughly examine the mineralogy and texture of these rocks in order to determine any differences among the samples, andto explain the mechanisms of potential and actual permeability loss. The results of these investigations should be helpful to those who wish to select a drilling fluid that will cause the least amount of borehole damage and permeability loss in rocks similar to these. ANALYTICAL TECHNIQUES The samples used in this study are the larger portions of cores slabbed off center, so a total of 16 one inch (2.54 cm) diameter permeability plugs four inches (10.2 cm) in length were taken behind the slabbed faces. The outer 1/8 in (3.2 mm) of each plug was cut off to eliminate the zone of drilling mud invasion, and then they were cut in half. Two 3/8-in (9.5 mm) slices were taken from the inner ends of the halved plugs, and these were used for SEM, thin section, and x-ray diffraction analyses. The inner ends of each plug were the ones exposed to the drilling fluids, so a reasonable correlation between the petrographic data and the drilling fluid tests should exist. Scanning Electron Microscopy A small chip of core with a face perpendicular to the flow test direction was broken off for analysis by scanning electron microscopy (SEM) and energy dispersive x-ray analysis (EDS). The sample was dried in a vacuum oven and coated with 200-300 angstroms of gold in a sputter coater. These were examined with a scanning electron microscope and energy and wave-length dispersive x-ray analyzers. Pore spaces and clay mineral types and occurrences were the principal items studied, and selected plugs that were exposed to drilling fluids were examined for evidence of permeability damage. Photographic documentation of these data are in Figures 5 through 8. Thin Section Analyses The other 3/8-in (9.5 mm) slice was impregnated with blue epoxy, mounted and ground to the appropriate thickness for thin section analysis. A 250-point count was made of each section using a computerized point counting system to determine grain size parameters, framework grain and cement composition and porosity types and abundances. P. 141^

An Environmentally Safe Water-Based Alternative to Oil Muds

“…Droddy et at. 10 describe the procedure for determining formation damage. Table 6 shows the percent of return permeability for producing sands from two different areas to be between 88% and 100%.…”