Depth of Samples
The type of material flowing over the shaker and the timing of its arrival are fundamental to the mud logging process. To characterize the lithology and fluid content of a particular interval, the mud logger must account for the transport velocity of the cuttings to determine the time it takes cuttings to travel from the bit to the shaker. This lag time increases as depth increases, taking just a few minutes while the upper section of a well is drilled but extending to several hours in deeper sections. Lag time, a function of depth and mud pump rate, is usually measured in terms of pump strokes, which are counted by a pump stroke counter at the mud logger's console.
The lag time dictates when formation cuttings from a given depth will arrive at the shaker. Lagged cuttings samples are collected at regular depth intervals—typically every 3 m [10 ft] or 10 m [30 ft] of drilling—and prior to tripping out of the hole. Lagged samples are also collected to examine changes in formation characteristics, as indicated by significant changes in drill rate or gas curve trends.
Inside the logging unit, the mud logger rinses and dries cuttings samples before examining them under a binocular microscope. The mud logger describes each sample in terms of lithology, color, grain size, shape, sorting, porosity, texture and other characteristics relevant to rock type. This information is plotted in the lithology column of the mud log, which displays an estimate of gross lithology as a percentage of cuttings, reported in 10% increments. Because the presence of hydrocarbons may not be obvious—even under a microscope—each sample is examined for fluorescence under ultraviolet (UV) light.
Fluorescence can be an extremely sensitive indicator of the presence of hydrocarbons in drill cuttings. Sample fluorescence is evaluated in terms of color (ranging from brown to green, gold, blue, yellow or white), intensity and distribution. Fluorescence color may indicate oil gravity; dark colors are suggestive of low API gravity heavy oils, and light colors indicate high API gravity light oils. Following application of a solvent on the samples, hydrocarbon fluorescence will appear to flow and diffuse into the solvent as the oil dissolves. This diffusion is known as cut fluorescence, or more commonly just cut. Under UV light, hydrocarbons may be seen to stream from the rock pores into the surrounding solvent, turning the solvent cloudy.
To measure gas, the mud logger relies on an automated gas detection system. Suction lines transport a constant stream of air and gas from the gas trap, located at the shale shaker, to the logging unit. There, sensitive instruments process the gas samples extracted from the drilling mud. The primary gas measurement tool is a flame ionization detector (FID), which can sense hydrocarbon gas concentrations as low as 5 parts per million. From FID measurements, a total gas curve can be plotted on the mud log. Background gas—a more or less constant, minimum level of gas—establishes a baseline on the total gas plot. A gas show is any significant increase in detected gas, which is usually associated with a zone of increased porosity or permeability.
For more detailed hydrocarbon analysis during shows, the mud logger employs a gas chromatograph. The chromatograph separates the gas stream into fractions according to molecular weight. Commonly detected components fall within the alkane group: methane [CH4]—denoted as C1—as well as the following constituents: ethane [C2H6] or C2, propane [C3H8] or C3, the normal and isopolymers of butane [C4H10] or nC4 and iC4 and pentane [C5H12] or nC5 and iC5. The measurement of these light hydrocarbons helps geologists characterize reservoir fluid composition while drilling. The quantity of gas recovered and the ratios of the various gases are useful in identifying zones of producible oil or gas.
Coordination with the Drill Floor
Gas monitoring is also important to the driller and company representative. Mud gas trends that develop while drilling are integral to the evaluation of mud balance and identification of potentially overpressured formations. By carefully tracking gas and drilling parameters, the mud logger can recognize deviations from normal trends and give advanced warning so the driller can mitigate impending problems. Thus, the success of a well and the safety of the drilling operation may hinge on how quickly a mud logger can synthesize and interpret myriad pieces of data.
A sensor mounted on the drawworks tracks the drill rate, or rate of penetration (ROP), to determine the amount of time spent drilling each meter or foot of depth. The mud logger's role takes on added importance when a drilling break, or significant increase in ROP, is encountered. Then the mud logger alerts the company representative to request that drilling be stopped until mud and cuttings from the bit face can be circulated to the surface. If these cuttings are accompanied by an increase in gas, or if sample analysis reveals the presence of oil, the mud logger notifies the company representative and geologist of a show of gas or oil. The operator then has the option to further evaluate the potential pay zone through coring or testing.
The mud log serves a variety of functions (Figure 2). The ROP curve is plotted as a step chart or a continuous line, increasing from right to left. When displayed in this manner, the ROP curve responds to changes in rock type or porosity in a manner similar to that of a spontaneous potential or gamma ray curve, making for easy correlation between LWD or wireline curves. As a correlation tool, the mud log's ROP and total gas curves often exhibit a remark-able correspondence to gamma ray and resistivity curves, respectively. Throughout the drilling process, mud logs provide real-time correlations with logs from offset wells and help the operator track the bit's position in relation to target formations. Because the mud log is based on physical samples, it can provide a direct, positive identification of lithology and indication of hydrocarbon content. This information can be especially useful when formation characteristics make wireline or LWD log interpretation complicated or ambiguous. The mud log provides independent evidence for a more comprehensive understanding of reservoir conditions and geology.