Sticky and suction sticking, also known as differential sticking, is the most common sticking accident during drilling. The most likely to jam are drill collars, which may gradually move up as the drill string loses its freedom of movement
一， reason:
The existence of filter cake on the borehole wall is the cause of sticking, because most drilling fluids are solid and liquid, in which the solid particles adsorb on the borehole wall to form filter cake. Some people think that the filter cake is caused by the filtration loss of drilling fluid. Without the filtration loss, there would be no filter cake. Based on this thinking, they always think that the filtration loss of drilling fluid in sandstone is large, and the filter cake will be formed. However, in the open hole section, mud shale also has a filter cake, which is much thicker than that in the sandstone section.
That's because filter cakes form for three reasons:
(1) Adsorption: Solid particles in the attached drilling fluid adsorb on the rock surface, regardless of sandstone and mudstone have this property.
(2) Deposition: During the flow of drilling fluid, the flow rate near the borehole wall is almost equal to zero, and the solid particles in drilling fluid are deposited on the borehole wall. The diameter of mudstone well section is much larger than that of sandstone well section, and the deposition is more significant, so the mudstone well section is easy to form thick filter cake.
(3) Filtration: It accelerates the deposition of particles in drilling fluid on the surface of permeable rock. At the same time, we also note that mudstone also has filtration property, and is a hydrophilic material, can be infiltrated by water, as long as the water-based drilling fluid, even if the filtration loss is equal to zero, this infiltration process cannot be stopped.
Due to the micro crack mudstone contains a lot of, some of these micro crack is caused by strata stress, some of them are caused when bit broken rock, once the shale surface by water infiltration, in the fine cracks form a layer of adsorption film, can effectively split function, decrease the strong degree of the mudstone and make it broken off, most caliper mudstone interval is greater than the bit diameter. However, after the mudstone is relatively stable, due to the infiltration of water, the molecules, atoms or ions on the surface of the mudstone show polarity, and there is an unbalanced and free part of the force field. The direction of this part of the force field points to the drilling fluid, which can absorb a large number of particles with different charges in the drilling fluid. Before the adsorption equilibrium is established, the concentration of the adsorption decreases gradually in the drilling fluid and increases gradually on the mudstone surface. If you increase the concentration of certain particles in the drilling fluid, you increase the number of them adsorbed to the mudstone surface per unit time, which is a cumulative process. In addition, under the action of the fluid pressure of the drilling fluid column and the rotary force of the drill string, part of the moisture of the adsorption layer is squeezed back into the drilling fluid, forming a thick and complex filter cake on the borehole wall, and the performance of these filter cakes is worse than that of the sandstone section.
We can draw the conclusion that as long as the filter cake exists, it is possible to stick and suck stuck drilling. The sandstone well section can be stuck drilling, and the mudstone well section can also be stuck drilling. However, the diameter of the mudstone well section is often irregular, and the contact area with the drill string is less, so the chance of stuck drilling is less.
The pressure difference between formation pore pressure and drilling fluid column pressure is the external cause of sticking. In the same open hole section, the formation pore pressure gradient is not uniform, and the fluid column pressure always balances the highest formation pore pressure in the section, resulting in a positive pressure difference for those formations with relatively low formation pressure gradients. When the drill string is wall filter cake stick after absorption, close to the wall side of one side of the drill string is coming through the filter cake formation pore pressure, on the other side of drilling fluid column pressure, if the latter is greater than the former, namely the positive pressure difference exists, can be pressed to the drill string and borehole wall further reducing the gap between the adsorption surface, enhances the adsorption capacity, The contact area between drill string and borehole wall is further expanded.
Drill string in resting, because any well has a certain slope, the drill string for horizontal component of the pressure produced by its own weight to the under side of the borehole wall, drive away the middle isolation layer, the distance between the drill string and the filter cake, when reduced to between polar molecules each other within the scope of work, the adsorption effect of occurs, it is the main cause of glue sticking absorption.
（二）Prevention of stick sticking
（1）Use neutral drilling fluid, or cation system drilling fluid;
(2) Most of the water-based drilling fluids used are anionic system drilling fluids. With the increase of the well deviation or the increase of the density of drilling fluids, the possibility of sticking is increasing, and the best way is not to let the drill string static;
(3) For anionic drilling fluid system, it is required to have good lubricity, small filtration loss, appropriate viscosity and shear force, if necessary to add lubrication to reduce the friction coefficient of filter cake;
(4) Solid control
(5) Drill as close to balanced pressure as possible
(6) Using reasonable drill string structure, the general idea is to increase the support point, reduce the contact surface
(7) In order to prevent the stuck point from moving up, it is best to press down two-thirds of the total weight of the drill string to reduce the contact area between the drill string and the filter cake on the wall. The inclined well cannot be pressed, because the bottom drill depends on the lower limit of the wall, the more the pressure is dead.
(8) The weight gauge must be sensitive and reliable to prevent wrong judgments
(9) To maintain good wellbore quality
(10) The drill-while jar is included in the drill string, as it is effective to shake the stick out during the initial stages of sticking.
三． Drill stick treatment
(1). Strenuous activity: Stick will become more and more serious with the extension of time, so in the initial stage of stick, it should be in the equipment and drill string safe load with the maximum force activities.
(2) Shock solution: If the drill string is equipped with a jar while drilling, immediately start the jar up or down to solve the jam, which is more concentrated than the simple up and down force, and much faster. If there is no jarring while drilling, the position of the clamping point should be measured first, and the drill tool should be reversed from above the clamping point by blasting the release method, and then the appropriate jarring tool should be selected to drill under the clamping and then shock the clamping to solve the clamping. If the shock cannot solve the card, the use of note card agent side soak side shock, its effect will be better
(3) Depressurize solution: the difference between slurry column pressure and formation pore pressure.
