Abstract: There are mainly two kinds of sticking accidents: one is caused by the expansion of a certain lithology itself; Second, because a certain rock stratum has good permeability and strong water absorption, the mud performance changes and the mud skin thickens rapidly. Correctly understand and analyze two different causes of drilling sticking, and take corresponding measures to prevent accidents.
Key words: performance of swelling mud in case of sticking, shrinkage, water loss and water absorption.
Step 0 introduce
Whether in oil field exploration, coal exploration or other exploration work, there are different degrees of sticking accidents. Because of the different understanding of drilling sticking, it is often attributed to the sticking caused by the expansion of rock when it meets water. In fact, after years of practice and referring to relevant information at home and abroad, the author believes that there are two main types of drilling sticking accidents: one is caused by the expansion of a certain lithology itself; Second, due to the good permeability and strong water absorption of a certain rock layer, the mud performance changes and the mud skin thickens rapidly. As far as the two causes of sticking are concerned, the latter is mostly and the former is less; As far as its harmfulness is concerned, the latter is more serious and the former is less serious. Therefore, we must correctly understand and analyze the two different causes of sticking, and take corresponding preventive measures to avoid sticking accidents.
1, sticking accident caused by rock expansion.
The so-called sticking accident is theoretically due to the emergence of free surface in the rock stratum after drilling, the rock itself loses its pressure balance, the flushing fluid (mud) penetrates into the rock stratum to make the rock swell, and the rock protrudes in the low pressure direction, which reduces the diameter of the borehole, thus leading to sticking; Theoretically, there seems to be the possibility of this kind of sticking, but in all kinds of rock formations encountered by drilling, the sticking caused by rock expansion is very rare. The common rocks in coalfield geological exploration are diatomite, bentonite, mudstone, sandstone, conglomerate, limestone and igneous rock. Among them, diatomite, bentonite and mudstone have water absorption and swelling properties. Under the same conditions, the expansion rate of bentonite is more than 10 times, and that of other rocks is 3~5 times. During normal drilling, sudden expansion is not allowed to reduce the hole diameter and get stuck. Expandable strata will not expand into the hole endlessly, but only slightly expand within the range of 2~3m around the borehole. Due to the control of surrounding rock pressure, rocks outside this range can't burst into the borehole endlessly or suddenly in a certain period of time, which obviously reduces the aperture. Moreover, rocks that swell when exposed to water are relatively soft, with a hardness of only two to three, and are easily damaged by external forces. The swelling and protruding part of this rock in a certain period of time is smaller than the part cut off by drilling tool friction. In addition, soft rock is easy to be destroyed by washing liquid (mud), and washing liquid can only enlarge the aperture but not reduce it. In the whole drilling project, the pore size of the pore section of expansive rock is larger than that of hard rock. It can be seen that the causes of expansion and contraction of some expansive rock strata that endanger drilling or cause accidents in holes are not sufficient, and it is also unfounded to analyze and judge some soil squeezing accidents as caused by drilling shrinkage and attribute them to natural phenomena.
2. permeability of rocks is good, the rock has strong water absorption, and the mud skin is thickened, resulting in a sticking accident.
In all kinds of rock formations encountered by drilling, the properties are different, some are water seepage, some are water seepage and some are water gushing. In permeable rock stratum, permeability is also different, some are good, some are poor. The rock stratum with good permeability, combined with the pressure of drilling mud column, is easy to make the mud lose water quickly. The mud that loses moisture becomes colloidal clay and is adsorbed on the borehole wall, forming a mud skin that gradually or rapidly thickens, leading to necking until a section of borehole is blocked. This kind of necking and sticking accident can block part or all of a section of borehole in dozens of hours, and it can cause sticking accident in the hole in ten minutes.
In seepage rock, this shrinkage is continuous, and water-absorbing rock absorbs water in mud all the time. During normal drilling and mud circulation, the mud skin thickens in a certain proportion with time, and at the same time, it is thinned by the rotation or up-and-down grinding of the drilling tool. The cut mud skin is taken away by the washing liquid (mud) and added to the washing liquid so as not to block the drilling hole. However, the water molecules in the mud are decreasing and the viscosity of the mud is increasing, forcing people to adjust the viscosity of the mud. Therefore, in normal drilling, it is rarely found that mud skin thickens and squeezes drilling tools. If the drilling is stopped for some reason and the mud circulation is stopped for a long time, the water molecules in the mud will be quickly absorbed by the permeable rock layer, which will reduce or block the hole diameter of this interval and prevent the drilling tool from running in. This phenomenon occurs not only in a certain area, but also in other areas.
3. A good example of permeability of rocks causing sticking.
3. 1 Changbai No.5-1 The hole is drilled normally before the hole depth is 350m, and it leaks at the depth of 350-400 m, and then it is drilled after the mud viscosity is increased to stop the leakage. When there is no surplus, break the vertical shaft and add a single drilling tool. After stopping the pump 15 minutes, the drilling tool cannot be lifted. When the drilling tool with reverse thread is lowered to 350m, it can't be lowered any more. The reverse thread drilling tool has been screwed several times, but the positive thread drilling tool still can't come back. Later, one was covered with a large-diameter drilling tool, and the other was reversed. On the upside-down drill pipe, the dehydrated cylindrical solid clay is consolidated, which is difficult to remove by hand. This shows that the stratum with good water permeability has a strong absorption effect on the water in the mud. This interval has been stuck for many times, and sometimes the drilling tool is pushed down to 350 meters and swept to the bottom of the hole like a core. At this time, if the pump is stopped for more than ten minutes, the drilling tool may get stuck. The main reasons are: the mud has a large solid content and a large water loss. After the mud lost water, the drilling tool was quickly stuck, which led to the termination of drilling before reaching the design depth. 3.2 Changbai No.8-2 hole, with a hole depth of 700m and a hole section of 300-400m, has the characteristics of good permeability, strong water absorption, rapid increase of mud viscosity, etc., and there have been two sticking accidents. Referring to the treatment method of Well Changbai 8-2, after the sticking accident is relieved, the mud management is strengthened and the investment of mud treatment agent is increased, so that the mud performance can meet the requirements of drilling formation and the borehole can meet the design requirements.
3.3 1992, an oil well in an oilfield, with mudstone formation and strong water absorption. When the oil well reached the design depth, due to some reasons, it failed to log in time, the isoelectric logging time was too long, and the mud circulation was not frequent, which led to the reduction of the well diameter and the failure of logging instruments to run to the required logging interval. Therefore, mud treatment agent is added into mud to adjust mud performance and reduce mud leakage. After re-sweeping the hole, it is known that many sections of the oil well have been reduced in diameter. This shows that this type of diameter reduction proves that the rock stratum with good water absorption absorbs mud, which leads to the sticking accident.
In the analysis of the causes of the above accidents, there has been controversy. Some people think that the drilling tool extrusion accident is caused by the expansion and contraction of the rock itself, and give examples of drilling and sticking in previous construction. They think that the expansion and contraction of rock formations have caused many sticking accidents, and some of them can't be handled, so the drilling holes are scrapped. Some people think that it is because the drilling hole passes through the seepage stratum, the rock has strong water absorption, and mud with high solid content and large water loss is used. In addition, the pressure of the mud column makes the mud in some boreholes change from fluid to solid, thus making the drilling tool firmly consolidated. The corresponding basis is put forward. Finally, * * * realized that poor quality mud was affected by some kind of rock with strong water absorption, which led to the change of mud performance.
4. Conclusion
For a long time, there have been different views on hole shrinkage and hole plugging. Some people think that it is mainly caused by rock swelling, while others think that it is caused by mud peeling. After years of practice, no accurate basis has been found. However, in many accidents, it is found that the change of mud performance is closely related to the permeable layer (or water-sensitive layer). Therefore, the accident of hole shrinkage and drilling tool sticking is analyzed as rock swelling, which is attributed to natural accidents and lacks scientific basis.
When analyzing the physical properties of rocks, we should admit that some rocks have certain expansibility, and more importantly, we should realize that their softness and brittleness are easily destroyed by the grinding of drilling tools and the scouring of fluids, so we can only enlarge the aperture and avoid sticking accidents caused by rock swelling.
During the decades of drilling work, the author has never found a sticking accident caused by rock swelling, and can't simply analyze it as a sticking accident caused by rock swelling in water. This misjudgment of the nature of the accident will affect the formulation of the correct treatment plan or the wrong conclusion, mislead the implementation of the correct construction plan, and its losses are also great.
The root cause of drilling sticking is that the drilling hole passes through the rock with strong water permeability, and the water in the mud is absorbed by the rock, which changes the performance of the mud, which is not conducive to wall protection and plugging. Mud that changes from liquid to colloid. Its adhesion to the hole wall and clamping force to the drilling tool are increasing. Until the drilling tool is firmly consolidated in the borehole, that is, the drilling sticking accident occurs.
When we know the root cause of sticking, we can correctly judge the nature of the accident, take effective preventive measures and treatment methods, and reduce the occurrence rate and undue losses of sticking accidents.
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