D. ANTONIADI
director Institute of oil, gas and energy
at Kuban State Technological University

S. FURSIN
associate professor Institute of oil, gas and energy
at Kuban State Technological University

Generally accepted method of field development is drilling horizontal wells with consequent multistage fracturing (MSF). However in some cases another economically and environmentally feasible approach can be applied. This is a multilateral deep reservoir penetration with small-diameter perforation channels from the cased wellbore. The suggestion for this technology is using a hydroabrasive sonde perforator in coiled tubing system. This perforator based on downhole abrasive container-separator improves the process of creating multiple drainage radial channels with one tripping operation. Injection of abrasive material in sonde nozzle in jet pump mode provides destruction of casing, cement and rock during one continuous operation in the optimum hydrodynamic mode – lower pressure and higher fluid rate. Circulation of the abrasive material inside the wellbore with no lifting to the surface mitigates damage to equipment. Coiled-tubing-conveyed method improves the circulation mode, provides easy control of the perforator through the electric cable inside the coiled tubing, enhances reliability of creating perforation channels in complex geology and technology conditions. The application of the abrasive material in sonde jet equipment assists in underbalanced perforation. Perforator cable connection with corresponding signal transducers provides full control of the downhole equipment and geology conditions throughout all stages of perforation. The application of box-type seismic-acoustic receivers and thermomechanical sonde deflector along with the opportunity to measure downhole parameters in real time provide active geosteering and improve perforator navigation process during multilateral drilling at high depths.

Proportion of hard-to-recover reserves grows constantly reaching 70–75% in many oil producing regions. There is a steady decrease in average well rates, increase in proportion of low-rate wells; development of some fields in Krasnodar, Sakhalin, Volga-Ural and other regions is unprofitable [1]. One of the ways of hydrocarbon production stimulation is increasing the area of reservoir penetration and permeability of rock around the wellbore and expanding the wellbore drainage area. Last years, researchers have been looking for simple and environmentally feasible technologies of reservoir perforation with small-diameter channels with the ability to cut off low-permeable mud damaged zone with a radius up to 3÷7 m and the ability to reach remote perspective reservoirs at distance of 100 m from the main wellbore or more.
The majority of perforation operations (up to 90–98%) in cased-hole wells are conducted using blasting-explosive method with jet perforators due to its usability, short operation time and relatively low cost of the operation and consumables. In spite of the constant modernization of jet perforation technology, in some cases even modern cutting-edge perforators, for example perforators from Schlumberger, lead to damage to casing and cement and fast water breakthrough. This method of perforation doesn`t always provide a reliable hydraulic connection between the reservoir and the main wellbore and doesn`t reach beyond the mud damaged zone that may exceed several meters. Generally, in order to increase reservoir permeability after jet perforation additional costly operations are conducted (acid treatments, clay cake removal, hydraulic fracturing, geodynamic reservoir degassing, vibrowave impact, etc.) [2].
Alternative sand jet, slot and milling perforators are developed in order to increase oil recovery and well service life. These perforators provide hydromechanical reservoir perforation with less damage to casing, cement and rock. Particular method of reservoir perforation is a milling perforator technology for creating channels with 1÷3 m depth using electric drill or hydraulic motor with a cable or string-cable suspension. Application of the cable suspension has its indisputable advantages; specifically, it provides effective control and management of the whole process in real time. A disadvantage of milling perforators is a relatively low perforation depth due to operation capability of the driving shaft that is rotated by a hydraulic motor or electric drill in complex mechanical conditions [3].
As it is well known, the deepest reservoir perforation is achieved by drilling horizontal wells; along with hydraulic fracturing these technologies are widely used at different stages of development of oil and gas fields. Many years of experience in field development with horizontal wells and lateral horizontal wellbores completed with modern equipment including directional coiled tubing drilling systems show that productivity of such wells is 1.5÷5 times higher than that of vertical wells. However, experience has also shown that designed effectiveness is achieved only in 50% of cases while 35÷50% of all wells are unprofitable since the rate maintains at the previous level or lower than that in vertical wells [4]. Field development with the system of horizontal wells is not always economically feasible. This is proved right by a weak correlation (correlation coefficient is 0.23) between the well rate and the overall length of horizontal wellbore when fluid flows only from high-permeability zones rather than flowing simultaneously from all perforated zones. A horizontal well costs 1,5÷2 times as much as a vertical well. It is a tough process to drill, operate and repair multihole wells, especially multilayer wells with long horizontal wellbores [5].
The application of hydraulic fracturing for enhancing reservoir exposing area and permeability by means of creating fractures often boosts well rate only at the initial stage of production. Moreover, the application of hydraulic fracturing requires significant expenses and usually leads to the following: loss of cement integrity, behind-the-casing flows, increase in well watercut value, deterioration of conditions for workover operations, development of bypassed oil and decrease in the ultimate oil recovery factor [6].
Innovation technology of deep low-damage perforation of pay zones with small-diameter channels during well completion and workover was developed in order to eliminate the great number of disadvantages listed above - technology of a drilling perforator [7]. This technology is based on a small-sized turbine assembly at the end of coiled tubing (CT) that enables creating a number of small-diameter drainage channels along a curved path from the main cased wellbore during one tripping operation with strings.
However, it is difficult to apply this technology in some cases, for example, in thin heterogeneous reservoirs, interlaminated oil-water reservoirs and reservoirs with close proximity of fluid contacts when the requirements for cement integrity are raised and selective treatment for separate low-permeability zones is required. In order to solve this problem of deep perforation in complex conditions in the mid-1980s Siberian department of the Academy of Sciences of the USSR successfully started research on development of technology of radial drilling with high-rate fluid jet using coiled sonde [8]. At present time the technology of radial drilling with high-rate fluid jet using coiled tubing is presented by several companies and improved constantly [9, 10]. This technology suggests lowering of a radial deflector on a coiled tubing into the cased wellbore, creating of several holes in casing by a mill or a boring bit and penetration into the pay zone through these holes with a high-rate fluid jet from the nozzle of the coiled sonde suspended on a coiled tubing. It is possible to create the system of drainage channels with length up to 100 m from the main cased wellbore during one tripping operation with strings in a required time period.
Disadvantages of the technology of radial drilling with a high-rate fluid jet on a coiled tubing include the following. This technology assumes relatively high working pressure (around 90÷150 MPa) that may cause complications at the wellhead and require the application of special equipment (suitable pumps, junctions, valves, secondary filters, etc.). On the other hand, low rate of the process fluid (around 0.2 l/s) constrain the length (up to 100 m) of the created channels due to the problem of carrying cuttings to the surface. Apart from that, the necessity for using separate different operations - mechanical operation for casing milling and hydraulic operation for reservoir penetration - all this complicates the technology and decreases its efficiency and reliability. The disadvantage of the technology under study is the inability to predict the trajectory of the created channels due to low bending stiffness of tubular sonde and different rock strength. Perforation channels may enter water-bearing zones and concentrate near one direction that is usually around the casing due to the absence of the downhole control of tubular sonde.
The suggested coiled tubing technology of deep hydroabrasive reservoir penetration using sonde perforator has no disadvantages mentioned above. This technology uses the abrasive material (sand, corundum, cuttings, etc.) in sonde perforation with a high-rate fluid jet. Injection of the abrasive material in the nozzle from the wellhead of downhole is carried out under cover of a sealing toe pressed to the casing. This prevents the abrasive material from entering annulus and eliminates sticking of the downhole equipment. The abrasive material, in turn, breaks casing, cement and rock in one continuous operation mode without milling of casing at lower working pressure (up to 25 MPa) and higher fluid rate (5 l/s and higher). The used fluid flows back after nozzle through a sealing toe. This enables a jet mode with a local downhole circulation of the abrasive material with no need to inject the abrasive material at the wellhead and circulate it within the whole wellbore. Coiled tubing suspension improves circulation mode and provides a simple control of the tool. The application of seismic-acoustic receivers and thermomechanical deflector along with the opportunity to measure downhole parameters in real time improves drilling of drainage channels, especially in the process of multihole drilling.
Hydroabrasive sonde perforator (fig. 1) includes: sealing sidewall toe 1 with container-separator 2, abrasive material and deflecting arm 3, coiled tubing suspension 4 with the electric cable inside and a tubular sonde 5 with the main nozzle 6. Toe 1 that is lowered in the well on the tubing 7 directs sonde 5 radially against the casing wall 8 and presses itself to the wall by means of an arm 3 through the sealing bushing 9. Container-separator 2 of the abrasive material is designed as two concentric tubes, a cover with the central nozzle 10, nozzle 11, dosing channel 12 and inlet spiral groove (turbulizer) 13, that is connected with the return channel 14.
For the jetting mode nozzle 6 is equipped with suction channels 15 (fig. 2) in its narrow part and a resin guide ring 16. When moving through the reservoir sonde 5 is controlled from the surface by thermomechanical deflector designed as two-four rods 17 or two-four plugs 18 and jet nozzles 19. Rods 17 (plugs 18) made from titanium-based alloy with shape memory effect can be deformed separately by means of selective warming with electric current through cable 20 thus changing the trajectory of sonde movement. At the end of the main nozzle 6 there is an inductance coil 21 that provides measurement of the apparent resistivity of the downhole environment in a pulse mode. For this purpose, coil 21 is fed with current pulses. Voltage drop that is proportional to eddy current and apparent resistivity of the downhole environment is measured at interpulse time. Measurement is carried out at different delay times - investigation radiuses. This provides evaluation of fluid content (oil, water) in real time by the change in apparent resistivity.
Toe 1 also contains a logging tool 22 with seismic-acoustic receivers 23 mounted symmetrically in a circle at two levels at different depths - four receivers at the upper and lower levels. As sonde 5 moves inside the reservoir receivers 23 take in elastic waves from working nozzles 6, 19 and locate the sonde along the channel and azimuth with account of data from inclinometer tool 22. The tool also measures the difference between the amplitude and the time of elastic waves coming from nozzles 6, 19 to receivers 23 mounted in different positions on a toe 1. Nozzle coil 21 and box-type coil 24 are used for establishing a temporary cable connection with tool 22. These coils are put into one another at determined equipment position (fig. 1) thus forming wireless transformer.
The procedures of coiled tubing technology of deep reservoir penetration with a hydroabrasive sonde perforator are conducted according to the following sequence.
The wellhead is equipped with the pump with working pressure of flush fluid injection up to 25 MPa. Toe 1 with a container separator 2 that is preliminary filled with the abrasive material (sand, corundum) is lowered in the casing 8 on the tubing 7 and then placed at designed well depth. Then, the sonde 5 is run inside the tubing 7 on a coiled tubing 4 and electric cable and directed radially against the casing wall 8. When the sonde is moved, coils 21 and 24 are put together with the control of data from corresponding transducers thus forming a cable connection with a tool 22 through the wireless transformer. Toe 1 is located at the designed direction by rotating the tubing 7 at the surface according to the data from the tool inclinometer 22. Then, toe 1 is pressed to the casing 8 by means of an arm 3 (connected with the electric motor, not shown) through the bushing 9 thus eliminating leakage of used fluid after nozzle 6. Then, well cleanout through coiled tubing 4 is started in an operating mode of fluid injection. Nozzle 6 (jet pump) with suction inlets 15 is located in optimum position against the outlet 11 of the container-separator 2 and the casing 8. During well cleanout it is possible to use a lighter flush fluid, for example, oil that assists in creating underbalanced mode with less damage to reservoir pay zone.
Casing 8 is destructed by a high-rate abrasive fluid jet. Fluid without abrasive material (water, oil) that is pumped at the surface into the coiled tubing 4 comes out from a narrow part of nozzle 6 (jet pump) at high velocity and sucks in the abrasive mixture through the suction inlets 15 located near the outlet 11 in a container-separator 2. The abrasive mixture that is sucked in by a jet pump accelerates at nozzle 6 to a high velocity and breaks casing 8 in a hydroabrasive perforation mode. The abrasive mixture is injected at the outlet 11 in a container-separator 2 by means of the dosed volume of the returned used fluid through the small-diameter dosing channel 12. The main volume of the used fluid is disposed through a channel 14 with a larger diameter and spiral groove 13 and injected along the spiral path into the container-separator 2. Here, as fluid flows in a spiral, central flow volume disposes of the abrasive material and returns to the wellhead through the outlet 10. The abrasive material is pressed to the outer wall by a centrifugal force, separated and accumulated in the container-separator 2. It can be used again through the dosing channel 12. Thus, a local downhole circulation of the abrasive material from the container-separator 2 without reaching wellhead and damage to the equipment creates a hole of the designed diameter in casing 8 and cement matrix.
Then, the sonde 5 is moved inside the reservoir on a coiled tubing suspension at the same injection mode with rock cutting jet. Junk cuttings from the reservoir are used as an abrasive material that is circulated by means of suction inlets 15 and a guide ring 16 through the nozzle 6 (jet pump) thus performing effective work. Apart from the cuttings another abrasive material can be used. It can be injected through the additional container-separator above the nozzle (not shown) also with the local circulation. The sonde 5 movement inside the reservoir is controlled according to the data from the nozzle coil 21 and receivers 23 transferred by a constant and temporary cable connection lines. Measuring signals informing about the apparent resistivity and reservoir rock saturation from the coil 21 that is moved inside the reservoir are transferred to the surface in real time through a constant connection – cable 20 and electric coiled tubing cable 4. This data is used for active geosteering of the perforator, for example, during unplanned penetration of water-bearing deposit, when immediate control of the sonde trajectory is required. Signals from receivers 23 informing about the current sonde position are stored by the tool 22 and transferred to the surface periodically during reaming of the perforation channel when coils 21 and 24 are put together.
The change of sonde 5 direction is conducted by means of a thermomechanical deflector. For example, when sonde 5 deviates downwards to the non-productive bottom of the reservoir, there would be registered an increase in amplitude and simultaneous decrease in time of receiving elastic waves at the lower-level receivers 23 as compared to the upper-level receivers 23. In this case in order to adjust sonde 5 trajectory the voltage is applied to one of the rods 17 of the deflector, for example, to the upper rod thus deforming it to the required direction due to a warm-up with the electric current. Upon this, the nozzle section is rotated upwards and returns sonde 5 to the designed trajectory in the middle of the reservoir. Similarly, the sonde trajectory is controlled by plugs 18 by deforming them selectively for closing one of the jet nozzles 19.
After creation of the first perforation channel with the optimum trajectory and length, sonde 5 is lifted on the coiled tubing suspension into the tubing 7. Then, the toe 1 position is changed according to the azimuth and depth data. The next channel is created in a similar sequence. After multihole radial drilling is over, downhole equipment is lifted from the cased well according to the following sequence. Sonde 5 is lifted on a coiled tubing suspension first, and then toe 1 is lifted on the tubing 7.
At present time, the elements of the coiled tubing technology of deep reservoir penetration with hydroabrasive sonde perforator are at the pilot testing stage in Krasnoyarsk region fields.

CONCLUSIONS
The analysis of the problem of penetration into pay zones in cased-hole well with a deep low-damage perforation method has been conducted. It is established that radial reservoir penetration with high-rate fluid jet using coiled tubing equipment is the most promising technology.
The paper suggested the improved technology of well completion and workover with the application of hydroabrasive sonde perforator as a part of coiled tubing system. Injection of the abrasive material in the sonde nozzle provides destruction of casing, cement matrix and reservoir rock in one tripping operation in the optimum mode – lower pressure and higher fluid rate. The application of the abrasive material in the sonde jet equipment assists in underbalanced perforation.
The used fluid flows back through the sealing toe of the perforator. This enables using jet pump mode with a local downhole circulation of the abrasive material for the rock destruction. Circulation of the abrasive material inside the wellbore with no lifting to the surface mitigates the damage to equipment and enhances reliability of the technology.
The coiled tubing suspension of the perforator improves the circulation mode and provides a simple control of the tool through the electric cable inside the coiled tubing. The perforator cable connection with corresponding signal transducers provides full control of the downhole equipment and geology conditions throughout all stages of perforation.
The application of seismic-acoustic receivers and thermomechanical deflector for geosteering along with the opportunity to measure downhole parameters in real time improves controllability of perforator during multihole drilling in complex geology and technology conditions.
The suggested technology allows creating the system of drainage channels with the optimum trajectory in a simple, reliable and time-saving way. This improves profitability of field development and stimulation of hydrocarbon recovery, especially with hard-to-recover reserves, and enhances the overall production.