Cuttings Petrography

QEMSCAN^® WellSite™ is actually an automated petrography solution developed since the late eighties primarily for the mining industry. QEMSCAN enables access to two primary components of analysis: mineralogy and texture (i.e. microstructure). The primary output of QEMSCAN WellSite is 2D numerical mineral and texture maps of cuttings. These maps form the basis of subsequent image and chemical analysis on a cutting-by-cutting basis. The visual nature of the color-coded petrographic images make QEMSCAN WellSite data readily accessible to a wide range of professionals at the rig (both geologists and petrophysicists).

Field-Tested System Configuration

Based on a product platform consisting of a compact, rugged and mobile Scanning Electron Microscope (SEM), energy dispersive spectrometers (EDS) and QEMSCAN Automated Petrography measurement and petrography analysis routines, QEMSCAN WellSite has been designed for, and tested at, remote on- and off-shore well site locations. The physical dimensions easily permit placement in 30 foot or larger mudlogging units, or even smaller <20 foot units when housing a dedicated QEMSCAN WellSite solution.

One analogy to how QEMSCAN WellSite works is point counting. The system counts points ("pixels" in the numerical mineral map) at a rate of 100-200 measurements per second over a pre-determined grid. Typical measurement times of approximately 30 minutes per cuttings sample can be achieved, with the total number of points up to 400,000 analyzed. At each point, the system determines the mineralogy from backscattered electron intensity (indicative of mineral density), and elemental chemistry (secondary x-rays generated as the electron beam intertacts with the mineral). Once analysed, cuttings descriptions can be made based on a unique rock particle-by-particle basis, as opposed to other cuttings analysis obtained at the WellSite such as LIBS, XRF or XRD, which are bulk methods.

Optimized Sample Preparation Workflow

Samples for QEMSCAN WellSite are presented in the form of 30 mm diameter resin blocks containing an average of 1,000-2,000 cuttings particles/block, depending on the average size of the cuttings. The sample preparation workflow involves: drying the samples, mixing cutting particles with resin, sectioning the block with a diamond saw, and finally coating the sample surface with a conductive carbon film. Each device used in the sample preparation process has a small enough footprint to be placed on desktops found in a mudlogging unit. An additional benefit of the sample preparation process is that the resulting resin blocks improve handling and archiving, making cuttings readily available for complementary analysis at a later stage at a central storage or laboratory facility.

Near Real-time Data Acquisition

The development phase of QEMSCAN WellSite included two extensive field-tests, the first in the highlands of Papua New Guinea, and the second, off-shore Qatar. The field tests were aimed primarily at testing the feasibility and value of near real-time data acquisition. Whereas time-to-data and throughput may vary significantly as a function of primary data required, these field-tests yielded near real-time results. 

From the time wet cuttings are collected at the shale shaker, to wet-screened, dried and mounted samples, it takes less than 30 minutes to prepare a measurement-ready block. Samples can be loaded into the system one-by-one or in batches of up to six. Measurement times are on average around 30 minutes per sample. Fully processed data, therefore, are available one hour after collection of cuttings and the QEMSCAN log data can be updated every 30 to 40 minutes.       

Experience from the first two field-tests has demonstrated that up to 35 cuttings interval samples can be measured in a 24 hour interval, with an average of 22 samples if interruptions in the drilling process are included. Sample preparation efficiency can exceed sample analysis as demonstrated by one trial where 55 samples were prepared in a 24 hour period. At high drilling rates typically >100 foot per hour, a backlog of samples may accumulate; however, during both field-tests, drilling downtime due to e.g. cementation runs, allowed operators to catch up on the backlog.