Feasibility study on short radius sidetracking technology of old well in Hoja Gulluk Gas Field - A case study of H-1 well

The Hojaguruk block is located in the central and eastern region of the Amu Darya River. The reservoir is a hard carbonate reservoir with abnormally high pressure formation, which makes well control difficult. The reservoir is covered with a thick salt paste layer, and it is difficult to drill directional well in the salt paste layer. As the central and eastern parts of the right bank of the Amu Darya River enter the late stage of development, the production capacity of old lying Wells is insufficient and the reservoir energy power is reduced. Therefore, it is urgent to transform old Wells with short radius sidetracking drilling technology and revive lying Wells to improve the production of single well and achieve the goal of stable production and increase production. Based on a comprehensive analysis of the geological characteristics, development situation and technical difficulties of short radius sidetracking in the area where well H-1 is located, this paper analyzes and demonstrates the determination of the cutting point of sidetracking technology, the optimization of the well trajectory design scheme and the analysis and demonstration of the ultra-short radius sidetracking technology, and considers all factors comprehensively to study and determine that the ultra-short radius sidetracking technology is suitable for this well condition. It can meet the requirements of reservoir development design. It is suggested to carry out the key test of the ultra-short radius sidetracking technology, and promote the application in the developed blocks in the central and eastern part of the Amu Darya Project, so as to provide research ideas and directions for tapping the potential operations in other blocks.


Preface
With the continuous improvement of short-radius sidetracking drilling horizontal well window cutting technology, drilling tool assembly and completion methods, short-radius sidetracking drilling horizontal well technology is increasingly widely applied in the field of exploiting the remaining oil and gas potential in old areas.It has become one of the most effective means to increase potential efficiency, improve development effect and increase economic benefits in the middle and late stage of the development of old oil and gas fields at home and abroad [1], [2].Window sidetracking drilling technology is a special process well technology that uses special tools to run casing in a new window on one side of casing pipe for secondary development.According to the new hole construction slope, it can be divided into medium and short radius (0.5-2°/m), short radius (2-6°/m) and ultra-short radius (16-30°/m).Short-radius sidetracking technology is used to test and drill a certain horizontal distance in low-producing and inefficient Wells, abandoned Wells or Wells with damaged casing, which can fully excavate the remaining reserves between Wells and greatly improve the oil and gas production of a single well and the overall collection rate of the block [3].
Ultra-short radius sidetracking drilling technology can transform old Wells with low input cost and quick effect.It can increase production of low-producing Wells, resume production of non-production Wells and regenerate abandoned Wells, and has been applied in various oil fields at home and abroad [4].It can sidetracking drill horizontal Wells under the condition of small curvature radius, expand exposed reservoir area and improve oil recovery [5].After more than 30 years of research and development, the ultra-short radius sidetracking drilling technology has formed a series of technologies including MWD measurement while drilling, multi-angle mud motor, magneticless titanium alloy drill pipe, remote control function power taps, etc., which can monitor and adjust the wellbore inclination while drilling in real time, and truly realize the targeted drilling of reserves with a fixed orientation, depth and distance.Ultra-short radius sidetracking technology uses flexible drilling tools and other special drilling tools to conduct horizontal sidetracking in the inner casing of developed Wells, and to explore the potential of shallow reservoirs, depleted reservoirs and interlayer reservoirs.The advantages and successful application of ultra-short radius horizontal well drilling technology in exploring remaining oil and increasing oil drainage area provide a new way for efficient development of low permeability stratum, heavy oil reservoir, old oil fields and marginal oil fields [6].
After more than a decade of development and production in the central and eastern part of the project on the right bank of the Amu Darya River, the formation energy is insufficient and the reservoir production is declining.It is urgent to develop developed Wells or strata with poor reservoir properties to achieve "increased production and higher production".The treatment of low-yielding and inefficient Wells must be accelerated, and sidetracking drilling has become the most effective means for adjustment and implementation [7] [8].

Block background
The drilling block on the right bank of the Amu Darya River is located in the border region of eastern Turkmenistan and Uzbekistan, and the development target layer is limestone stratum under the huge thick salt paste layer [9].The Hojagurluk gas field is located in the eastern region of the Amu Darya River, and the structure spreads from northeast to southwest.Well H-1 is located in the Hojagurluk block, which is located in the Kisar Mountain front thrust zone.The reservoir is located in the Karov-Oxford stage (with abnormal high-pressure gas formations), and the stratigraphy is of carbonate rock with great longitudinal changes.The Gemmeri stage is 250 meters thick salt-paste layer (as shown in Figure 1), which is a "three salt-paste and two salt" layer, containing high-pressure brine layer, and it is a rock prone to collapse and plastic flow deformation.
The H-1 well is a vertical exploration well (as shown in Figure 2) with a complete drilling depth of 3,836 meters and a boerhole temperature of 135℃.The purpose of drilling is to investigate the development of Karlof-Oxford stage and gas content of the Hoja Gulluk structure in detail.The reservoirs of H-1 well are located in the Karlov-Oxford stage, which are respectively XVhp, XVa1 and XVa2, and mainly developed into XVhp and XVa1.The reservoir type is fracture-vuggy carbonate rock with low porosity, strong heterogeneity and abnormal high-pressure formation.The H-1 well is designed with a four spuds depth structure and a casing perforation completion.The well depth is 3,836 meters and the lower Middle Jurassic Batt stage has been drilled.The well depth structure design is shown in Figure 3.

Design and feasibility analysis of lateral drilling technology for ultra-short radius horizontal Wells
According to the geological data of H-1 well, well structure design and development needs, the ultrashort radius sidetracking technology is analyzed to study the feasibility of the window cutting sidetracking technology in the Hojaguruk gas field.

Window sidetracking drilling point is preferred
Similar to conventional sidetracking drilling, lateral drilling of ultra-short radius horizontal Wells in casing involves the optimization of sidetracking points.Due to the short curvature radius of borehole, the selection range of sidetracking points of ultra-short radius horizontal Wells is very small [10].Therefore, the perforation of thin reservoir should be fully considered in the selection of sidetracking points.Two targets B and C are deployed.The original borehole azimuths are 306.82°,assuming that the window cutting direction rotates 180° along the borehole direction, the following two schemes are designed according to the different positions of the sidetracking points.Scheme 1: Sidetracking point B is located 35 meters below the casing shoe of 7 "casing, and the vertical depth of sidetracking point is 3421 meters; Option 2: Sidetracking point B is located above the 9 5/8 "casing hanger at a vertical depth of 3100 m.The design of sidetracking points in the above two schemes is shown in Figure 4.
First of all, the window cutting point of the 7 "casing is located in the carbonate stratum, but the 9 5/8" casing is located in the very thick salt-paste layer, and it is difficult to make directional deviation with window sidetracking drilling technology in the very thick salt-paste layer.Secondly, 7 "casing window only needs to drill through one layer of casing, 9 5/8" casing window needs to drill through two layers of casing, the construction process is complicated, the construction cycle is long; Finally, the 7 "casing does not require technical casing to be cemented and re-entered, while the 9 5/8" casing requires technical casing to be re-run for cementing after the window is cutting, which is economically costly.In summary, the above two window-cutting sidetracking drilling schemes are selected as scheme 1, and window sidetracking cutting drilling operations are carried out on the inner wall of the 7 "casing.

Well trajectory design
Target C was selected in the XVa1 formation (vertical depth 3528.7 m) to increase lateral wellbore drainage area and interreservoir connectivity.According to the designed vertical depth of target C, the design parameters of the ultra-short radius sidetracking are shown in Table 1, and the borehole azimuth diagram is shown in Figure 5.

Sidetrackinh drilling technology of ultra-short radius horizontal Wells
The sidetracking drilling technology of ultra-short radius horizontal well is a horizontal well technology with the shape of "T", in which the formation slope can be controlled within 16-30°/m and the window cutting, inclination and horizontal drilling can be completed within the reservoir and drilled along the strike of the reservoir [11].For tight reservoirs such as carbonate rocks and dolomites, open hole completion or flexible drilling pipe is mainly used to support the well wall.For loose reservoirs and heavy oil reservoirs with poor formation cement, sand control is carried out in branch boreholes based on flexible fine sieve tube and supplemented by chemical sand consolidation or gravel packing, or sand control is carried out in main boreholes.The technology can be used in a variety of completions and is suitable for many formation situations.The core tools of this technology is that the drilling assembly is mainly composed of a single drilling pipe with a length of 0.15m + flexible sub.The fixed Angle of the movable joint of a single flexible drilling tool is 4.5°.The arc length is determined by the length of the drill pipe, and the arc length determines the Angle.In vertical Wells, "three-dimensional drilling" can be achieved, which means any well type, adjustable dip Angle and any azimuth.Horizontal well technologies with different formation slopes are shown in Figure 6.This technology can increase the discharge area of a single well, excavate the remaining oil between the submersible Wells, improve the degree of reserves control and recovery; Compared with conventional sidetracking horizontal Wells, the reservoir has a higher drilling rate, shorter operating cycle and higher economic benefits [12].To realize zone location and directional water injection, improve water flooding efficiency to achieve the purpose of increasing production; It can control the direction of fracture formation, solve the problem of reservoir pollution, provide superior flow channels, and become an effective fracturing stimulation method in addition to water injection, fracturing and acidification.The technology was applied in the Enping oilfield, where daily production increased from 105 barrels to 583 barrels, an increase of 455%.
Geological conditions for the application of ultra-short radius sidetracking drilling technology: 1.The thickness of the reservoir is more than 1m; 2. Window sidetracking cutting points depth within 5800m; 3. Borehole temperature below 200℃; 4. Arbitrary well Angle; 5.The main applicable strata are medium high pressure, medium permeability sandstone, mudstone and carbonate rocks.

Ultra-short radius sidetracking technology construction technology and drilling assembly
(1) Through the casing well: flat shoe + tube scraper + drift diameter; (2) Run the whipstock to the set position: whipstock + feed into the drilling tool; (3) Calibration depth toolface orientation: the whipstock anchors to determine the toolface; (4) Casing window cutting and repair: compound wash cone + drill collar + drill pipe; (5) Run flexible drilling tools for directional deviation operation: bit +MWD+ flexible drilling assembly + guide pipe + centralizer + drill pipe; (6) Run the horizontal section drilling assembly (BHA) to carry out horizontal section operations: bit + guide screen + flexible BHA + multi-connector + centralizer + drill pipe; (7) Drop screen pipe, pull out flexible drilling tools and whipstock; (8) Select the appropriate completion method to complete the well and Recommendations.

Feasibility analysis of ultra-short radius sidetracking technology
The H-1 well was completed to a depth of 3,836 meters, and the ultra-short radius sidetracking technology enabled casing windows to be run at this depth.The borehole temperature is 135℃, at which the MWD tool can operate stably and normally.The lithology of the reservoir is carbonate rock, and the ultra-short radius sidetracking technique can drill the horizontal section in this lithology.Small MWD measurement-while-drilling (MWD) can be used to more accurately control the wellbore trajectory in the reservoir and ensure the penetration rate of the reservoir.In conclusion, the ultra-short radius horizontal well sidetracking drilling technology is suitable for the stimulation of H-1 well, which is expected to produce good stimulation results and meet the development design requirements.

Conclusions and Suggestions
(1) H-1 is a vertical exploration well with a completion depth of 3,836 meters, and the completion zone is the lower Middle Jurassic Batt order.The purpose of drilling is to explore the pure development and gas content.It has been proved that the Karov-Oxford order reservoir is rich in natural gas, so it is necessary to reconstruct and develop the original vertical hole in order to improve production; (2) The ultra-short radius sidetracking drilling technology can be used in formations with a depth of less than 6000 meters and a borehole temperature of less than 150℃, and the horizontal section can be extended to meet the development design requirements.It can be used to revive the original borehole of H-1 and improve the production of the well; (3) It is suggested to carry out research and test on the sidetracking drilling technology of ultra-short radius horizontal Wells, promote the technology in other blocks entering the middle and late stage of development, and increase the production of old wells and lying wells whose production decreases year by year, so as to achieve the goal of increasing production.

Figure 1 .
Figure 1.Structure map of carbonate top boundary in Hoja Guluk tectonic belt.

Figure 3 .
Figure 3. Well body design of H-1 well.

Figure 5 .
Figure 5. Drilling hole trajectory of the ultra-short radius window side.

Table 1 .
Design parameters of borehole trajectory.