Mudlogging is the creation of a detailed record (well log) of a borehole by examining the chips of rock or sediment brought to the surface by the circulating drilling medium (most commonly mud). Mudlogging is usually performed by a third-party surface logging service provider. This provides well owners and producers with information about the lithology, fluid and gas content of the borehole while drilling. Historically it is the earliest type of well log. A mobile laboratory is placed by the surface logging company on the drilling platform, either an on-shore rig pad, or the deck of an offshore drilling rig or ship.

Drill cuttings are small pieces of rock produced by a drill bit moving through subsurface rock formations while drilling wells. These cuttings are brought to the surface by the circulating drilling fluid and are removed from the borehole by screening the drilling mud over shale shakers. As drilling is more economical than coring, cuttings increasingly replace core samples as the only direct evidence of subsurface geology. At the same time, high-performance polycrystalline diamond compact (PDC) drill bits are gradually replacing roller cone bits, producing ever smaller cuttings in some rock formations. As a result, there is an industry requirement for more advanced cuttings analysis and reporting. The analysis of cuttings is referred to as mudlogging and plays an important role by improving on-site interpretation of downhole tool data including logging while drilling (LWD) and wireline logs. In circumstances where downhole data is limited, such as in deep and horizontal drilling, cuttings data may be the only direct data from the borehole.

Cuttings descriptions at the rig site are conventionally based on manual optical inspection under a microscope which identifies mineralogy, rock types (lithologies), grain size, shape and colour. The presence of basic lithologies such as sandstone, siltstone, clay stone and carbonates is reported. This process is necessarily subjective, qualitative, and limited by the lack in ability to accurately describe the mineralogy, including the type of clay minerals, feldspars and carbonates.

As a result, oil and gas companies are increasingly asking for advanced mudlogging services delivering more detailed, consistent and quantitative data. Advanced Mudlogging approaches generally involve bench top versions of established laboratory techniques reporting mineral and chemical assays, such as x-ray diffraction (XRD) and x-ray fluorescence (XRF). However, the nature of cuttings production and sampling effectively averaging cuttings from several meters long drill intervals often involving multiple lithologies, swarf, cement particles, and solid drilling fluid additives, significantly limits the value of any bulk sample reports. Numerical bulk sample reports also lack contextual information. However, it is not always the amount of a mineral but its spatial occurrence that can have important implications to downhole data interpretation and reservoir behavior.

In contrast, the QEMSCAN WellSite automated petrography solution is designed to identify and quantify both mineral and textural properties in a sample. Applied to drill cuttings, micron-scale compositional mapping provide the unique ability to categorise cuttings separate from contaminants and additives, and sort them into discrete lithology classes. Mineral, textural, and physical properties can be reported for any selected sequence of samples or on the basis of selected lithologies. Until recently, samples had to be shipped to central labs, resulting in logistical challenges and poor time to data, significantly limiting the value of QEMSCAN data for well site applications. This is where QEMSCAN WellSite enters the world of advanced mudlogging, providing near real-time onsite information supporting downhole instrument data interpretation and time-critical drilling decisions.