Mudlogging: drill cuttings analysis
|Development Geology Reference Manual|
|Series||Methods in Exploration|
|Chapter||Mudlogging: drill cuttings analysis|
|Author||Alun Whittaker, Diana Morton-Thompson|
Cuttings are the small pieces of rock that are chipped away by the bit while a well is being drilled. The rock fragments are transported by the mudstream from the bit to the surface where they can be “caught” and analyzed (see Land rigs).
Drill cuttings are important because they are often the only physical lithological data that are recovered from a well. These data are used as supportive information for subsequent show evaluation, correlation of lithology to wireline logs, and other special geological, geophysical, or engineering analyses (see Show evaluation). Some of the activities that utilize data obtained from drill cuttings are summarized as follows:
- Show evaluation
- Reservoir and lithological description
- Geological correlation and formation identification
- Verification of wireline log response
- 1 Catching representative samples
- 2 Sample lag
- 3 Sample preparation
- 4 Basic sample analysis
- 5 Sample packaging
- 6 See also
- 7 External links
Catching representative samples
Both the sample interval and the number and types of sample sets are usually determined at the start of the well (see Well planning). Typically, drill cuttings are caught or collected as composite samples that reflect the various lithologies drilled over a 10-ft interval. The interval length can vary depending on the drilling rate and the detail needed for reservoir and lithological descriptions. When the drill rate is low, it is important to collect a sample that is representative of the whole interval drilled and not just the final few feet. It is also important to collect representative samples when drill rates are excessively high and it is physically difficult for the mudlogger to collect and analyze all of the samples accurately. In this situation, the sample interval can be increased or an additional mudlogger can be assigned to the tour. Also, it is not unreasonable to request that the drill rate be slowed so that better samples can be obtained and used to evaluate prospective reservoir zones and casing or kick-off points.
When a drilling break or change in background gas occurs, supplemental samples should also be caught to aid in show evaluation.
Drill cuttings are typically collected from the rig shale shakers, a sluice box in the possum belly, or the desander/desilter outlets (see Land rigs).
Most rigs have one or more shaker screens for separating the cuttings from the mud. As returning mud travels across the screens, the mud fluid falls through the mesh, while solids travel to the end of the screen. A board or catching box is placed at the foot of the screen to catch the composite sample and prevent the samples from falling into the mudpit or onto the ground.
When a “double deck” shaker is used, cuttings on both the upper and lower screens should be sampled.
Another method of catching drill cuttings is to place a bucket or metal box in the possum belly near the entrance of the flow line. This method is thought to help prevent the disaggregation of sandstone and conglomerate cuttings.
Unconsolidated samples can be taken from the desilter or desander outlets if they use the underflow from the shakers and are not supplied from a pit. Samples from the desander contain both formation particles and solids related to the drilling fluid. It is the responsibility of the mudlogger to discriminate between the two.
Because there is a time difference between when the rock is first broken away from the formation and when the sample is caught at the surface, all cuttings samples should be “lagged” or adjusted to the proper depth (see Wellsite math). This process is critical in showing evaluation and lithological correlation.
Once the composite sample of drill cuttings is retrieved from the mud system, it is typically split or subdivided into a bulk, unwashed wet-cut sample and a washed and sieved dry-cut sample.
A small portion of the unwashed wet-cut sample is set aside and used for blender gas analysis (see Mudlogging: gas extraction and monitoring). Although this sample should not be rigorously washed, discretion can be used in lightly rinsing the sample to remove surface drilling mud film. The remainder of the sample is packaged for later analysis and archiving.
These samples are washed and sieved before they are analyzed. Consolidated cuttings are cleaned by washing or hosing the sample in a container of water to remove the mud film. Washing of poorly consolidated sandstones and shales is more difficult and requires several precautions. Clays and shales are often soft and tend to be washed away. In samples with swelling clays, it is often helpful to wash the sample in formation water or a 2% solution of KC1 to prevent defloculation.
Cuttings from wells with oil-based muds are usually more representative of the formation than cuttings from water-based mud because the oil emulsion prevents sloughing and dispersion of clays and shales. However, these samples cannot be cleaned by washing in water alone. It is usually necessary to wash the cuttings in a detergent solution to remove the drilling fluid. In extreme cases, it may be necessary to wash the cuttings first with a nonfluorescent solvent and then wash them in a detergent solution. Use of a solvent is not advisable unless absolutely necessary because of the risk of removing any oil staining present. Solvents and material contaminated by solvents should be properly disposed of in accordance with safety and hazardous materials procedures.
After washing the cuttings to remove any drilling mud, the samples are typically washed through a 5-mm sieve. Particles greater than 5 mm0.005 m
0.197 in usually contain a high percentage of lost circulation material and cavings. This material should be cursorily examined and discarded.
Basic sample analysis
A split of the washed and sieved cuttings is examined wet under a binocular microscope. From this sample, the mudlogger estimates the percentages of the various sample constituents and records the following information:
- Rock type and lithological composition
- Hardness (induration)
- Grain size
- Grain shape
- Cementation or matrix
- Sedimentary structures
- Hydrocarbon show
- Gas (total and petroleum vapor)
The quality of the description is directly related to the quality of the cuttings. Factors that adversely affect cuttings quality are summarized as follows:
- Excessive weight on bit, which results in cuttings being ground into a fine powder
- Insufficient mud viscosity, which results in cuttings not being transported to the surface; can be a significant problem in highly deviated and horizontal wells
- Improper mud chemistry, which results in (1) a high percentage of cavings that can mask the true drilled lithology, (2) loss of soluble minerals such as gypsum and salt and some shales, and (3) a high percentage of contamination by cement, lost circulation material (LCMlost circulation material), and metal.
Drill cuttings samples can be laid out in small partitioned trays so that vertical changes in rock properties are easier to observe and interpret. The first occurrence of a specific lithology reflects the highest possible position of that bed. However, because the borehole may tend to cave or slough during drilling, a lithology may continue to be present in samples from deeper drilled depths.
Wet-cut samples are packaged in cloth bags and labeled with the well name, operator, APIAmerican Petroleum Institute number or well location, and sample depth. Dry-cut samples are either dried in the air or under heat lamps. Each dried sample is then placed in a small paper envelope which is labeled. All labeling should be done with a dark, waterproof marker because nonwaterproof markers will inevitably smudge and streak, and pencil marks will fade with time.
The packaged samples may be sent to the operator at regular intervals during the drilling of the well or sent en masse at the end of the well. Sample sets are usually required for all partners and may be required for the appropriate state and/or federal agencies, depending on the location.
Samples for special analysis
In general, samples used in special analyses should not be artificially dried or stored near intense heat (such as in the “dog house” near the generators). This is because the heat causes more of the light hydrocarbons to volatilize, which will bias subsequent analyses. If precision is required, the bulk wet-cut samples should be placed in glass jars and covered with formation water (or a compatible fluid). Bactericide should be added and the containers sealed and labeled.
- Introduction to wellsite methods
- Mudlogging: equipment, services, and personnel
- Show evaluation
- Rate of penetration
- Mudlogging: the mudlog
- Mudlogging: gas extraction and monitoring