Study Explores Best Practices in Formation-Sand Particle-Size Distribution
Chris Carpenter_
This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 230564, “Is There Really Anything New In PSD?,” by Andrew LaCassa, SPE, and Steven Tinker, SPE, Woodside Energy, and Christine Fischer, SPE, Constien & Associates, et al. The paper has not been peer-reviewed.
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Accurate formation-sand particle-size distribution (PSD) is critical for sand-control design. The complete paper aims to establish a set of best practices for generating PSD data from core samples. Historically, PSDs have been measured by one or both methods of either sieve analysis or laser-particle-size analysis (LPSA). Both methods should theoretically yield the same results; however, the results can frequently differ. By addressing common shortcomings in sample selection and preparation, and by evaluating the capabilities of both sieve analysis and LPSA, this work provides clear guidance for confidently conducting PSD measurements.
PSD Best Practices and Procedures
More than 80 cores were used for an in-depth study of particle-size analysis. Initially, approximately 12 cores of varying permeability were selected. A subset of three cores was cut into various lengths to correspond to different weights to determine the optimal weight for use in PSD measurements for both LPSA and dry-sieve (DS) methods. These weights ranged from approximately 10 to 50 grams of solids per slice. Once the minimum weight for representative analysis was determined, subsequent cores were cut to a length that would provide the minimum amount of material for particle-size analysis while preserving the remainder of the core. All core pieces were individually disaggregated, and PSD was measured by both LPSA and DS.
The most accurate PSD results came from samples that had no less than approximately 20 grams for DS and approximately 13 grams for LPSA. The larger sample size ensures that the percentage of any potentially fragmented particles is lower as a percentage of the whole core section. Careful handling of the core material during cutting is imperative. The procedures for core handling, sieve analysis, and LPSA are provided in the complete paper.
Because LPSA ultimately was found to be the best method to evaluate the PSD of remaining core samples, this synopsis concentrates on LPSA-related experimental results.
LPSA Error.
LPSA has three sources of possible error.
- Long Axis Grains: Long axis grains are characterized by one of the three axes being longer than the other two. LPSA will measure the longest axis and use it to calculate the diameter of a sphere and assign it as the grain size. The caclulation can result in a greater volume for a given grain and introduce error into the PSD. Furthermore, if a particular formation has a large percentage of long axis grains identified through microscopic photography and LPSA data, it could make accurate definition of the PDF challenging.
- Sampling Error: The amount of sample used in LPSA is usually less than 1 gram. It is possible to introduce error into LPSA if the small amount of sample is not representative of the overall sample.
- Constant Density Assumption: LPSA assumes all particles are the same density. If particles with different mineralogies are present in the core sample, they may possess a density either lower or higher than sand. Usually, the amount of other minerals is small and is not likely to introduce much error.