Impact of Drilling Technique and Drilling Fluids on Geochemical Sample Analysis

Abstract

Geochemical sample analysis in petroleum industry heavily relies on rock samples and fluid samples collected from well bores and at well‐head. As the samples were often taken after a well had been drilled or while a well was being drilled, drilling technique and drilling fluids can impose significant threat to sample integrity and ultimately impact data interpretation. The threat is particularly amplified by use of oil‐ and synthetic‐based mud, various organic drilling additives, and high‐speed drilling techniques, which are used frequently nowadays to improve drilling efficiency in deep‐water and other challenging environments. This paper will review the evolution of drilling techniques and drilling fluids, with the emphasis on impacts on geochemical sample and data quality. In general, gas samples tend to be less impacted by choice of drilling fluids, being it water‐based mud or organic‐based mud. However, high‐speed drilling and drilling break can change both bulk composition and isotopic composition of mud gas drastically. Therefore, gas data interpretation is best accompanied by an understanding of drilling reports. Rock samples are more susceptible to selection of both drilling technique and drilling fluids. For example, common geochemical analyses such as TOC, Rock‐Eval, vitrinite reflectance analysis can all be affected by both drilling technique and drilling mud used. Depending on the type of geochemical analysis intended, as well as the lithology, porosity and permeability of rocks, the impact of drilling varies. However, in general conventional cores are preferred over side‐wall cores, and side‐wall cores are preferred over cuttings. Rock extracts and oil samples are most likely to be affected by use of organic‐based drilling fluids, particularly as fewer DST samples and more MDT samples are collected in recent years. Although various sample collecting tools and cleaning procedures have been developed over the years to remediate mud contamination, it is not always possible to completely remove the contaminants without altering rock properties. As such, pre‐ drill mud screening and drilling optimization is recommended for high‐stake wells, where high quality geochemical data is critical for decision making. Examples will be provided to illustrate the benefit of pre‐ drill mud screening. Overall, evolution of advanced drilling techniques and increased cost control impose significant challenges for geochemical sample analysis. Meanwhile more options are available to choose from as a benefit of technology advancement. The key to success is often understanding the impact of techniques and align them with drilling objectives through careful planning.

AAPG Datapages/Search and Discovery Article #90349 © 2019 AAPG Hedberg Conference, The Evolution of Petroleum Systems Analysis: Changing of the Guard from Late Mature Experts to Peak Generating Staff, Houston, Texas, March 4-6, 2019