Mud logging

by Tom Sturman, with edits by Luke Stoeckel

Arguably, the most commonly used data source on any drill site is the rock cuttings and their associated mud logs; the reason being that there is an abundance of available information brought up within the cuttings which is already isolated during mud cleaning.

During mud circulation the mud becomes ‘contaminated’ by the cuttings as well as the formation fluids, both of which are obtained at the depth where the bit meets the formation. When the mud reaches the surface all contaminants must be completely removed in order for the mud to be recycled; by combining the arrival time of the mud cuttings with the current driller’s depth, a log of the lithology vs. depth can be generated. This log contains a detailed description of the cuttings in terms of their lithological composition and formation fluid contents. The lithologies themselves are given a brief description, outlining their colour, grain size, strength and any apparent porosity. Additionally the abundance of the lithologies is recorded (e.g. 20% shale and 80% sandstone, etc.) This information is displayed in log form (Figure 1), similar to that of a stratigraphical log, having the ground surface at the top extending downward with depth. The driller’s depth is displayed along the left hand side with all of the above mentioned information displayed alongside.

[Figure 1: Sample of mud logging sheet from “Fort Mason Logging”]

Undoubtedly the biggest advantage of using the mud cuttings as a data source is that it saves both time and money by providing a continuous depth related stratigraphical reference of the formations without the need of using extra resources. The cuttings themselves must be removed from the drilling mud in order for it to be recycled, and if they were not to be used they would simply be discarded. In addition, by noting any patterns emerging in the lithology of the cuttings it is possible to infer a depositional environment, and thusly partially predict the location of nearby reservoirs (e.g. if rapidly alternating shales and sands are noted, it is possible the environment was a fluvial system of some sort). Furthermore, by correlating the stratigraphical log created by the cuttings with prior stratigraphical knowledge of the region, it is possible to confirm predicted theories of the subsurface, as well as update any unknown areas.

The process of grinding and ripping out the cutting from the formation makes cuttings the easiest accessible data source on the drill site; however this very process also causes the most valuable down hole data to be altered or destroyed. Specifically knowledge of porosity is difficult to ascertain, and permeability is impossible to gain. Furthermore the cuttings alone fail to recognise fractures within the formation. Whilst the use of cuttings as a data source is without doubt an excellent start in formation evaluation, it is imperative to combine this new found knowledge with data from other sources such as core samples and wireline logs.