Extracting Reliable Density and Porosity Data From Cuttings

Abstract

This article, written by Technology Editor Dennis Denney, contains highlights of paper SPE 96918, "Techniques for Extracting Reliable Density and Porosity Data From Cuttings," by S. Siddiqui, SPE, Saudi Aramco; A.S. Grader, SPE, Pennsylvania State U.; and M. Touati, SPE, A.M. Loermans, SPE, and J.J. Funk, SPE, Saudi Aramco, prepared for the 2005 SPE Annual Technical Conference and Exhibition, Dallas, 9-12 October. Reservoir samples are essential for proper formation characterization. The use of drilling cuttings has been proposed as an alternative to coring. Despite some loss, contamination, and depth uncertainty (typically within 10 ft), cuttings show strong potential to represent all the formations encountered during the drilling of a well. This paper discusses the capabilities of various laboratory instruments to extract reliable density and porosity data from cuttings. Introduction Subsurface geologic samples usually are of two types—cores or cuttings. Coring involves lowering a special bit and core barrel into the well to cut and retrieve rock samples. Wireline percussion coring provides an alternative to rotary coring operations, but its success is not well established, and except for hard and consolidated formations, the recovery is generally very poor. Cuttings are always available. Disadvantages of the use of cuttings include the following.Small samples mix with drilling mud and pass through the coarser screens (especially true for cuttings generated by diamond or polycrystalline-diamond-compact bits).Contamination by drilling fluids.Risk of cavings.Risk of moving cuttings with those of different specific gravity moving at the same speed.Difficulty of actual lag-time calculations (usually a correction of 10 ft per 1,000 ft of well depth is used), especially if drilling rates change often.Largest cuttings do not exceed 3/8 in. In spite of these limitations, cuttings (in some cases) provide the only direct source of information about the rock layers being drilled. Some of the disadvantages can be minimized with today's improved data-acquisition and -analysis technology, especially for the efficient removal of contaminants from cuttings. Experienced mud loggers and field geologists can identify the depth of cuttings accurately to within 5 to 10 ft. Tools and Techniques Optical Microscope. Field geologists can examine the cuttings with an optical microscope to obtain properties such as color, grain size, shape, roundness, texture, and composition. Cuttings provide valuable input for rock typing. Porosity and permeability are two of the most desirable parameters obtained from drilled cuttings. While efforts are under way for direct measurement of these parameters from cuttings, more validation work is needed for these techniques to become widely accepted. Porosity data on tiny samples can be obtained with thin-section petrography. To measure porosity from thin sections, the sample is impregnated with a blue dye under vacuum before making the thin section. The dye penetrates the pore space, and porosity usually is calculated by use of a point-count technique.