A new method of oil and water well completion involving the implosion effect

Features of colmatation zones of a productive formation in terms of rotary drilling by drilling muds are considered. A brief analysis of the most popular decolmatation methods is performed. Special attention is paid to the decolmatation method involving implosion effect. An example of one of the existing installations is represented with following consideration of its application features. A new principally different installation, free from the disadvantages found in the available one, is considered; a patent for the installation is obtained in Kazakhstan. It does not involve tubing with its packer and wellhead sealing. It also does not require a compressor and its manifold. Characteristic features of the newly developed installation for creating implosion effect are represented; its operation at producing both single and any number of repeated implosion acts is examined. A mathematical analysis is performed to calculate the empty space limits required for the implosion effect by a criterion to prevent casing string collapse by hydrostatic pressure of the drilling mud remained in the annular space. An example of dependency of the maximum permitted length of empty space in the casing string on its mechanical strength and its wall thickness, calculated by means of a computer program, is represented.

A separate area of work is the development of technologies for the construction of hydrogeological wells and well cleaning systems for the geological and technical conditions of Kazakhstan and IOP Publishing doi:10.1088/1755-1315/1348/1/012056 2 Ukraine [16,17].
Well completion involves bringing the well's production up to the maximum possible value by means of decolmatation of the productive formation near the bottom-hole area [18].Rotary drilling is usually performed with drilling muds.They help prevent problems caused by instability of the well walls.At the same time, the muds clog the formation pores and channels, blocking the formation liquid inflow.Particles of the drilling mud's hard phase, which size is greater than the formation pores, remain on the well walls, producing a mud cake.The higher the wellbore pressure is and the lower the size of particle and the fluid viscosity are, the deeper smaller particles, capable of passing through the pores, penetrate in the formation.Having reached the maximum possible distance, the drilling mud at rest turns into gel, and the formation becomes impermeable.Due to low viscosity, filtrate of the drilling fluid penetrates into the formation at the greatest distance and defines the outer colmatation boundary.Filtrate of the reagent-containing mud may react with the formation fluid and produce insoluble sediments, which can also block the formation pores.
The decolmatation methods are classified [19] by their efficiency and ability to effect upon all the above mentioned zones of colmatation.
The most available and most popular method is well flushing with water.A drilling mud in the sumps is replaced with water; then it is supplied into a well through a lowered drill string and pushes the filling mud out of the well.Since the water viscosity (0.0001 Pa•s) is much lower than the viscosity of any drilling fluid, water flushing removes the filter cake and cleans the openings of the filter.However, flushing does not influence considerably the deeper colmatation zones.
Flushing is commonly followed by pumping the formation fluid out.The liquid in the well drops to some dynamic level; the lowering characterizes depression of the formation.Liquid enters the well from the remote zones of the formation and carries the colmatation material down to the well.
While pumping, the decolmatation is achieved by constant and longtime (up to several days) static depression on the formation.Dynamic depression, realized in the form of pulses, can have a more intensive effect.The simplest method of producing such pulses is bailing.A bailer is sunk into the well from the rig's draw work until it is below the level of the liquid.In the course of sinking, the valve of the bailer is open by the resistance of the drilling fluid, and the bailer is being filled up.When the bailer is filled up, it is taken out.It should be noted that when it is being returned to the surface, the weight of the liquid keeps the valve shut and the bailer remains filled.The liquid is removed at the surface, and the operation is repeated many times.At every cycle of bailing when the bailer is brought to the surface, its level in the production string lowers as water is removed from the bailer volume.The corresponding pulse of depression brings about a pulse of inflow, continuing up to the moment when the static level is restored.
Increasing water production is possible through the development of a new, more efficient installation with an implosion action.
To eliminate disadvantages of the technologies described above, we need to: -Study problems of the development of colmatation in formations by drilling mud; -Study different methods of decolmatation; -Perform critical analysis of the inventions on installations that use implosion action for decolmatation; -Develop a new installation that has no drawbacks of the available ones; -Examine the specifics of the proposed invention.The purpose of the research is to modernize the existing method of developing oil and water wells using implosion action.

Methods
A much more powerful pulse is produced by the effect of implosion [19].An implosion method involves formation of depression by instantaneous connection of the production zone of the well, being under the formation pressure, with a zone of low pressure.At that moment, the formation liquid rushes into the well at a velocity up to 200 m/s.Such a stream tears the colmatation material from its place and carries it in the well; then it is removed out of the well by flushing.
Figure 1 [20] shows a well-known installation for producing the implosion effect.Tubing string 2 is used to deliver it to the well production zone 12. Packer 3 insulates the space below it.Compressor 13 supplies air through gates 24 and 19 into the space between casing string and tubing string, closed by the wellhead sealing.The compressed air pushes the flush fluid out of the well through valve 11 and gate 18.As a result, the level of that liquid is brought down to the planned depth, and its hydrostatic pressure becomes lower than the formation pressure.Next, the compressor is stopped, and ball 8 is dropped into the tubing string, where it falls within sit 7, covering channel 6.The compressor is started again and supplies air through gates 21 and 17 into the tubing.It acts upon the ball, spring 22, and plunger 5.They are moving down until the openings in the plunger get even with windows 4 in body 1 of the inlet valve.That brings about implosion effect.The standing under high formation pressure liquid under packer 3 rushes through the tubing string and windows into the low-pressure zone created between body 1 and the casing string.The described installation is very complicated.It involves the wellhead sealing, the tubing string, the packer, the sliding valve, and the compressor with many gates.The compressor must work throughout all the time of the operations.The installation reliability is rather doubtful as numerous colmation particles carried by the inflow can interfere with the sliding valve movement.All those facts complicate wide implementation of such installations.
In that connection a new installation, having no drawbacks mentioned above, was proposed, (figure 2).Initially it was intended for water supply wells [18].An application for invention had been submitted.

Results and discussion
While drilling at the depth close to the production zone, there is a transition to a smaller diameter.When drilling is over, a production casing string 2 is lowered, coupled with the filter column 5. Inlet valve 8 is kept closed by spring 10 and formation pressure.For that reason, the casing string remains empty, and drilling mud is displaced through the casing string-borehole annulus and wellhead.Bearing disk 3 with sealing 4 is positioned at the place, where the production casing string is replaced with the filter casing string.Being located between disk 3 and the rock bench and pressed by the weight of the casing string, it isolates a production zone of the well from the above-situated one.Bailer 11 is sunk into the well until its face rests on partition 7.In some fractions of a second before that, shank 18 of the bailer valve meets spring 10, and valve 16 starts rising to disclose openings 15.When valve 16 reaches stopper 17, full weight of the bailer begins acting on spring 10, and inlet valve 8 opens.Only when all those actions are completed, the bailer lies on the partition and the sag of the rope indicates the implosion process.
Opening of valve 8 connects immediately the empty space of the casing string, situated above partition 7, with the production zone, where liquid is under the formation pressure.The instantaneous depression creates an intense water inflow into the well, removing the colmatation plugs and even producing some new inflow channels.Water above partition 7 starts rising until it reaches its static level.
To have a repeating implosion effect, the bailer is taken out of the well, which brings about closing the inlet valve by its spring 10.While closing, the valve keeps water inside the bailer.Thereafter, by means of bailing, the empty space is restored in the production casing string.After that, the bailer is sunk again on partition 7.
After completion of the implosion processing, inlet valve 8 is removed by cutting pins 19 by a sharp impact of the bailer.
Formation of empty space in the casing string to provide implosion effect involves a risk of pipe collapsing.For that reason, it is important to determine the permissible limits of such a space., mm.
The combinations are introduced as follows: where e is pipe ovality.If pipes are up to 219 mm in the diameter, it should be not more than 0.01; for the diameter up to 324 mm, it is 0.015; and for greater diameters, the value is 0.02.
And finally: While applying practically the formula in the oil and gas industry, a safety degree must be considered.For production casing strings, they are nmin =1.3; for technical ones, it equals to 1.1.Minding that, the critical values of the pipes collapsing pressure is as follows: In terms of the well conditions: where ρР is drilling mud density; g is gravity acceleration; Нk is permissible height of the empty interval 10 (figure 3) Thus the depth, up to which the casing string can remain empty without risk of collapsing, is determined by hydrostatic pressure Pk of the drilling mud in the annular space.
The hydrostatic pressure inside the remaining part of the casing string Hi The causing implosion impact depression is: where Рfp is formation pressure.
When sinking the casing string, it must be filled with following volume of the drilling mud: Table 1 illustrates the above presented algorithm with an example.

Figure 3 .
Figure 3. Disposition of the drilling fluid in the well: 1production casing string; 2filter casing string; 3water bearing formation; 4adapter; 5sealing; 6partition; 7inlet valve; 8well wall; 9drilling mud; 10air; 11confining layer.D1outer diameter of the production column; D2the same for the filter column; σ1wall thickness of the production column; σ2the same for the filter column; H1production column length; H2length of the filter column; mits power; Hplstatic head of the formation; Hicritical length of the empty part of the column; Hkis the height of the solution above the partition.

Table 1 .
Dependency of the collapsing parameters for 168 mm casing pipe on its wall thickness.