Research on well selection and layer selection methods for tapping the remaining oil at the top of thick oil layers using ultra-short radius lateral drilling horizontal wells

: The flexible drilling method of ultra-short radius sidetrack horizontal wells is an effective method to utilize the remaining oil at the top of thick oil reservoirs. By analyzing the current status of the SZ oilfield well network, development stage and sand body development characteristics, combined with the characteristics of ultra-short radius lateral drilling horizontal well technology, the areas and formations suitable for ultra-short radius lateral drilling horizontal well tapping are identified, and the criteria for selecting wells and formations for ultra-short radius lateral drilling horizontal wells are established, which is a guide for the application of this technology.


Introduction
The Daqing oilfield has entered the late stage of highwater content [1] and after many years of exploitation in the SZ oilfield, there is still difficult-to-use residual oil at the top of thick oil reservoirs, which requires a new process of directional tapping.The flexible drilling method of ultra-short radius lateral drilling horizontal wells is an effective method to use such difficult-to-use reserves.

Introduction to ultra-short radius lateral drilling horizontal well technology
The ultra-short radius lateral drilling horizontal well is a process technology that uses the oil casing in the old well to drill horizontally after making a window slant at any position within the upper 30m of the target layer [2] (Figure 1), using special drill bit, guiding power motor, titanium alloy drill pipe, MWD follow-through monitoring and other tools to carry out the construction, and the radius of curvature is between 7.62m-17.4mto complete 0°-90° turning and slanting to achieve horizontal state, drilling more than 100m in the design direction with a hole diameter greater than 95mm.This technology can be used to achieve directional dredging and reconstruction, expand the drainage area, improve the production conditions of the well and increase the recovery rate.The following advantages exist for ultra-short radius lateral drilled horizontal wells.
(1) Short displacement in front of the target and small requirements for the size of the sand body.
(2) Locatable and orientable, capable of drilling along the top of thick oil formations.
(3) Multiple horizontal branches can be drilled in one straight well, resulting in a high degree of reserve recovery.
The following disadvantages exist.
(1) The sloping section can only be completed with bare borehole or screen tubing, and there is a high risk of water flooding in the sloping section.
(2) High requirements for well conditions, requiring a well diameter of not less than 120mm and good quality cementing.
(3) Inclined shafts can only be designed with horizontal sections in the direction of the shaft inclination.

Analysis of factors for well selection and formation selection for ultra-short radius lateral drilling horizontal wells
3.1 Analysis of potential strata and potential areas The ultra-short radius lateral drilled horizontal well has a 15m slant section, and in the slant section only bare-hole completions or screen pipe completions are allowed, so there cannot be high water-bearing formations within the upper 15m of the adjustment level.
SZ oil field compartment thickness is small (Table 1), Ⅱ oil group and Ⅰ oil group compartment average thickness of 12.8m, and the upper Ⅰ oil group is not welldeveloped, high-water flooding risk is small, the main potential layers in the top thick oil layer of Ⅱ oil group are unit Ⅱ-1 and unit Ⅱ-2.The thickness of the Ⅳ oil group and the Ⅲ oil group is smaller, only 5.0m, but the oil layer at the bottom of the Ⅲ oil group is poorly developed and has potential in the local area, and the main potential layers of the thick oil layer at the top of the Ⅳ oil group are unit Ⅳ-1 and Ⅳ-2.
The oil group III and V compartments are small and the upper oil layer is well developed, with little potential for ultra-short radius horizontal wells.Oil Group II is currently not completed chemical drive blocks to avoid affecting the effect of agglomeration drive, chemical drive blocks are not suitable for ultra-short radius horizontal well tapping at this stage.No chemical drive is carried out in the marginal II oil group, and the upper thick oil layer can be used as a target for adjustment.
The thickness of the upper part of the Ⅳ oil group is small, and the slope making section needs to pass through part of the Ⅲ oil group, only the block where the chemical drive of the Ⅲ oil group has been completed can be used as the area for tapping the thick oil layer in the upper part of the Ⅳ oil group.

Analysis of measure well selection
By old well side drilling ultra-short radius horizontal wells to tap the remaining oil has a better advantage [3].The ultra-short radius side-drilled horizontal wells are drilled using oil well casing, and the network of Class II oil formation wells will need to be returned in the future and cannot be used as measure wells for short radius horizontal wells.The only wells retained in the chemically driven well network of the Ⅳ oil group are Block D and Block Z at present, and the ultra-short radius side-drilled horizontal wells in other blocks can only be selected as measure wells for water-driven oil wells.

Analysis of well selection factors at the top of thick oil formations
The remaining oil at the top of thick reservoirs suitable for dredging in ultra-short radius sidetrack horizontal wells are mainly in 3 categories: thin injection and thick recovery, river margins and diversion line areas.
(1) In the thin injection and thick recovery injection and recovery relationship, the well end sand body is clearly developed, and the lateral drilling horizontal well is easy to enter the target and track the sand body, which is the main type of tapping (Figure 2 and Figure 3).(2) The sand body at the edge of the river channel is susceptible to residual oil formation due to the existence of phase change and large physical variation, and its top interlayer is developed to prevent bottom water cone entry, which can be used as a potential site for lateral drilling of horizontal wells if the injection and extraction relationship is suitable (Figure 4 and Figure 5).The situations that are not suitable for dredging of ultra-short radius lateral drilling horizontal wells are mainly 7 categories: thick injection and thin extraction, small compartment, uncertain sand body, poorly developed sandwich layer, continuous layer at the bottom of thick oil layer, unsuitable well spacing and high-water flooding level of new wells.
(1) In the injection and extraction relationship of thick injection and thin extraction, it is difficult to determine whether the sand body at the end of the well is connected in the upper low flooded layer of the river or the lower high flooded layer, and is not used as a potential for ultrashort radius lateral drilling horizontal wells (Figure 8 and Figure 9).(2) The compartment is small and the upper oil layer is well developed, and there is a risk of water flooding when the slant-making section crosses the upper oil layer (Figure 10 and Figure 11).(3) Sand body uncertainty and the presence of wells with sand body variability on the horizontal well trajectory make it difficult to determine the scale of sand body development and there is a risk that the sand body will not be controlled (Figure 12).(4) The sand body is poorly interbedded and water from the highly flooded layer at the bottom will enter the upper horizontal well (Figures 13 and 14).(5) The continuous layer at the bottom of the thick oil layer will enter the highly flooded layer at the bottom of the river sand during the drilling of the horizontal well, and there is a risk of water flooding (Figure 15 and Figure 16).Based on the above analysis, the formation selection criteria for ultra-short radius lateral drilling horizontal wells at the top of thick oil formations are established (Table 2).According to the criteria for well selection and formation selection, two wells in the water-driven well network in Block Z were selected for the ultra-short radius lateral drilling horizontal well experiment.

Forecast of indicators and evaluation of benefits
According to the reservoir characteristics, production situation and utilization status, it is predicted that the initial daily fluid increase of the ultra-short radius lateral drilling horizontal well will be 15.0t and the daily oil increase will be 3.0t, the evaluation period will be 5 years and the cumulative oil increase during the evaluation period will be about 1650t.The technical service fee for the ultra-short radius (the length of the horizontal section of the lateral drilling ≤ 110m) is RMB 1.15 million, and the cost of the horizontal section of the lateral drilling exceeding 110m is calculated at RMB 3,500/m; the overhaul operation fee is RMB 250,000/well; the cost of sieve pipe is calculated at RMB 170,000/well; the cost of logging is RMB 380,000/well, and the crude oil price is calculated at RMB 50/bbl, with a predicted input-output ratio of 1.40.The economic benefits are good.

Conclusion
(1) Residual oil at the top of thick oil reservoirs can be tapped using ultra-short radius lateral drilling horizontal wells.
(2) The ultra-short radius lateral drilling horizontal wells are more restricted in tapping the remaining oil at the top of thick oil reservoirs and need to continue to make technical improvements.

Figure 1
Figure 1 Schematic diagram of an ultra-short radius lateral drilled horizontal well

Figure 2 Figure 3
Figure 2 Thin injection and thick extraction type sedimentary phase zone map

Figure 4 Figure 5
Figure 4 River margin type sedimentary phase zone map

Figure. 6 Figure. 7
Figure.6 Phase zone diagram of the divergent linear deposition

Figure 8 Figure. 9
Figure 8 Depositional phase zone map for thick injection and thin extraction type

Figure 12
Figure 12 Map of the indeterminate depositional phase zone of the sand body

Figure 13 Figure 14
Figure 13 Map of the poorly developed sedimentary zone of the sand body intercalation

Figure. 15 Figure 16
Figure.15 Phase zone diagram of a continuous layer type deposit at the base of a thick oil layer

Figure 17
Figure 17Well spacing unsuitable sedimentary phase zone map (7) The new wells have high flooding levels and are at high risk of flooding (Figure18and Figure19).

Figure 18 Figure 19
Figure 18 Sedimentary phase zone map of new wells with high water flooding level type

Table 1 .
Statistical table of compartment thickness in the SZ oil field