Paper: Cuttings An Underused Asset In Formation Evaluation

Formation evaluation is the process of the application of technology to gain or improve understanding the physical characteristics of subsurface rock formations and the nature and distribution of their contained fluids for the purpose of identifying and developing commercial hydrocarbons. Cores and rotary sidewall cores are prized as actual samples of subsurface rock formations and their contained fluids and the opportunity they represent for direct measurements and observations of rock and fluid properties. Because of their expense, these assets are not commonly available to formation evaluators. Cuttings, however, are an inescapable by-product of every well drilled. Cuttings provide petrophysical information such as mineralogy, texture and pore system characteristics of all penetrated formations, as well as stratigraphic information through their appearance and content. They also provide fluid samples for relative hydrocarbon saturation estimates and geochemical characterization. This information can be broadly applicable in all phases of the petroleum industry: exploration, reservoir management, and drilling and completion. The process of assuring cuttings circulated to the surface are properly located on depth requires completion of a circulation lag check. The data gathered to calculate lag time can be used further to determine a lag time openhole caliper in near real time, and an average openhole diameter profile of a new wellbore can be developed during ongoing drilling using this information. A new evaluation element based on openhole diameter trends, the cavings factor, is proposed to quantify anticipated cuttings sample quality during drilling. If value is placed on acquisition of valid formation evaluation data in general, and cuttings samples in particular, real time remediation is possible through this focused awareness on the developing geometry of a new wellbore. The skills and the interest in cuttings espoused by subsurface formation evaluators in the early history of the oil and gas industry has atrophied. Today, the proliferation of digital data and mathematical models permits calculation and generation of impressive volumes of formation evaluation output without ever examining those broadly and readily available natural earth samples, cuttings, to validate the results. Should a binocular microscope sit next to every computer on the desk of every technical professional in this industry? As exploration and development proceeds into the next century, formation evaluators should focus on what is central to their profession and strive to use cuttings classically and innovatively. As a sample of subsurface rock material and surface-retained fluids, this inescapable asset can be used to reduce risk and uncertainty in prospecting and development operations and maximize the value of formation evaluation in the oil and gas industry.