Causes of lost circulation in drilling
Lost circulation of drilling mud, which increases drilling time and costs, is one of the most important issues in the drilling industry. Causes of lost circulation must be prevented in advance in order not to lose money, time and human resources. Lost circulation occurs due to factors such as the difference between the pressure of the drilling column and the formation, the existence of gaps and the properties of the formation. To prevent this problem, there are various solutions such as reducing the weight of the mud, reducing the inlet flow to the formation, using lost circulation control materials and finally placing the cement pad.
In some formations, due to the low pressure of fluids inside the cavities compared to drilling mud, lost circulation occurs. In general, mud loss occurs because of geological reasons and drilling operation.
In drilling processes, by studying daily drilling reports on mud, geology and well diagramming, the causes of lost circulation are investigated. Furthermore, the values of the variables affecting loss circulation and the amount of loss were recorded from the well plotting reports and adjusted as a table. Then the graph of the amount of lost circulation and the factors affecting it in relation to the depth are drawn and the results were matched with lithological data. Then the loss areas are identified and solutions to solve this problem are presented.
The effect of drilling variables on the amount of lost circulation
By reviewing the daily drilling reports and well charting, drilling variables such as weight on bit (WOB), drill string rotation rate per minute, inlet flow to the well, drilling speed and mud weight are extracted. Graphs of these variables are plotted against the depth in the Excel environment to allow the study and comparison of results.
In the field of drilling variables, many factors can reduce or increase the lost, the most important factors are the weight on the bit, drilling speed, the amount of inlet mud current to the well and the amount of rotation of the drilling string in one minute. If the effect of these factors on lost circulation is precisely known, it is easy to control lost and even prevent severe cases. Some of these variables are directly related to the amount of mud lost circulation and some are inversely related.
Where does Lost circulation occurs?
Using information collected from different drilling depths, the depth of loss, geological conditions and the amount of loss are determined. Once the variable affecting the lost circulation of each area is identified, a suitable solution can be provided for each section.
For example, if the weight on the bit is a cause of waste, this factor must be controlled. Because more weight than usual on the bit causes the formation to break and this can even cause complete loss. At lower depths, factors such as drilling speed and weight on the drill are very effective.
Prevention of lost circulation
There are several solution to prevent and manage lost circulation, some of these are outlined below:
- Control of pressures on the well
- Drilling speed control
- Drilling mud weight control
- Use of anti-corrosion polymers
- Drilling mud optimization
- Chemical inhibitors
- Putting cement plugs
Lost circulation materials
A drilling fluid is a mixture of liquids, solids, and sometimes gases that are pumped into a well. If the physical, chemical or rheological properties of drilling mud are not suitable, it affects safety and economic conditions. In the most appropriate drilling mud, they just pay attention to price. This leads to an increase in the overall cost of operations and a decrease the production.
In order to prevent drilling fluid wastage, control rheological properties and gelation, different chemical compounds are added to the drilling fluid. During the operation, the drilling mud may lose its original properties due to collisions with different layers of the earth. Therefor mud properties should be regularly controlled and the properties of the fluid to be constantly improved.
Various materials including minerals and chemicals are added to mud, which are used depending on the type of drilling, rock strength, formation type, drilling depth, gas layers and mineralogical composition of the rock. In general, drilling mud additives are used to create specific physical and chemical properties, including weight gain materials, viscosity control, filtration control, anti-corrosion, pH control, fluid waste control to Inside the formation and other chemicals that are effective in improving drilling fluid, he pointed out.
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Intensity of lost circulation
Intensity of loss circulation for water-based drilling, oil-based drilling has different criteria. Drilling fluid lost is divided into three categories in terms of intensity: partial, medium and complete. The number of barrels lost per hour is the criterion for this division.
| Water base drilling | Oil base drilling | |
| partial | <25 barrels | 10 barrels |
| medium | 25 to 100 barrels | 10 to 30 barrels |
| Complete | >100 m3 | 30 barrels |
Formation with lost circulation risk
There is risk of lost circulation in porous formations as well as formations with natural or induced gaps. In these formations loss causes an increase in costs and drilling time and decrease in the quality of drilled wells.
Mud loss usually occurs in formations with the following characteristics:
- Loose formations
- Stones with natural cracks
- Stones with inductive slits
- Channel formations
It should be noted that drilling fluid lost circulation sometimes occurs due to incorrect detection of the pressure gradient of rocks;
Fluid loss sometimes occurs due to incorrect determination of the pressure gradient of rocks. If the weight of the mud is higher than the pressure of the formation, the formation is broken and loss occurs. Such incidents can be minimized by accurate and proper well design.