(Urumqi 8300 1 1, Planning and Design Institute of Northwest Petroleum Bureau)
Based on the obvious difference between the drilling geological design of vertical wells and horizontal wells, this paper studies the influence of several most important design parameters in the geological design of horizontal wells: vertical depth of horizontal section, effective length of horizontal section and reservoir heterogeneity on the geological design and design optimization of horizontal wells. According to the geological design optimization process of 7 horizontal wells in Triassic bottom water reservoir in Tahe Oilfield, the following understandings can be obtained: ① There are argillaceous or calcareous interlayers and interlayers with different thicknesses and shapes in continental strata, especially when the reservoir heterogeneity is serious. At this point, the reservoir description must meet the requirements of horizontal well development for the reservoir; ② For small reservoirs with limited reserves, do not pass the "Large Horizontal Well Development Plan" in the morning; ③ The relative lag of technical equipment, personnel quality and soft and hard facilities will affect the "development advantage" of horizontal wells over vertical wells.
Optimization of drilling geological design for horizontal wells in bottom water sandstone reservoir: Vertical depth and horizontal section length
The quality of drilling geological design is the basis for each well (whether exploration well or development well) to successfully achieve the drilling purpose and achieve the expected results. Because there are many differences between developing horizontal wells and vertical wells, in order to achieve good results, the geological design of horizontal wells is more critical.
Blocks 1 and 2 in Tahe Oilfield are Triassic sandstone reservoirs with thin oil layers and bottom water, with strong heterogeneity and high development risk of drilling horizontal wells. Therefore, it is necessary to carefully design the drilling geology of each developed horizontal well to ensure that the horizontal section of the horizontal well is located in the well section with good reservoir properties, and has a large enough water avoidance height and the best effective length of the horizontal section to achieve the best development effect.
1 The most significant difference between the drilling geological design of horizontal wells and vertical wells.
Horizontal well is a special directional well with a large angle of inclination (generally greater than 83) and horizontal or nearly horizontal intervals in the target interval. Generally speaking, the borehole trajectory is divided into three sections: vertical section, inclined section and horizontal section. Strictly speaking, there is no essential difference between horizontal wells, vertical wells and inclined wells, and there is no clear boundary: horizontal wells and vertical wells can be regarded as the two extremes of inclined wells. But even so, their differences in borehole trajectory make them different in drilling geological design. The following are the five most prominent points in the geological design of horizontal wells, which are different from vertical wells:
(1) Horizontal section orientation and well location optimization of horizontal wells;
(2) Drilling horizon, vertical depth and hydrophobic thickness of horizontal section;
(3) Length of horizontal segment and effective horizontal segment;
(4) Reservoir heterogeneity near horizontal section;
(5) The relative positions of targets A and B in the horizontal segment.
1. 1 Horizontal section orientation and well location optimization of horizontal wells
The projection of a horizontal well on the plane is not a point or line segment with a very limited length like a vertical well, so there is a problem of orientation when arranging wells. In general, the following factors should be considered in the selection of this orientation: ① the direction of tectonic stress; ② direction of maximum horizontal permeability; ③ Reasonable configuration with other existing vertical wells or horizontal wells; ④ The position with the best reservoir physical properties on profile and plane (meeting the requirements of hydrophobic thickness at the same time); (5) On the premise of meeting other conditions, the isobath should be constructed as parallel as possible (or point A is higher than the structure).
1.2 Horizontal drilling horizon, vertical depth of horizontal section and water-repellent thickness
For multi-layer sandstone reservoirs with bottom water, there is a problem of drilling horizon. Vertical wells (development wells) generally drill the upper part of the deepest oil layer (exploration wells or evaluation wells may drill through all oil layers); It is required to drill horizontal wells (the target layer may also be the deepest oil layer) according to the "Development Plan for Horizontal Wells in Target Layer".
Horizontal wells are required to be in the "drilling geological design stage", and the completion vertical depth of horizontal wells must be studied and designed according to the data of adjacent wells before drilling. Once the vertical depth is determined, the water avoidance height of the horizontal section of the horizontal well is also determined; The completion depth of a vertical well (generally, the inclined depth is used as the vertical depth because of the small deviation) does not represent the perforation completion depth, so the final water-avoiding thickness of a vertical well can be comprehensively determined by various new information interpreted by the electrical logging curve after drilling.
Moreover, there are obvious differences between them in the determination of water avoidance height and the utilization of interlayer (table 1).
Table 1 Difference Table of Water Avoidance Height and Partition Utilization in Horizontal Section of Vertical Well and Horizontal Well 1 Difference of Water Avoidance Height and Partition Utilization in Horizontal Section of Vertical Well and Horizontal Well.
1.3 horizontal segment length and effective length of horizontal segment
According to the study of reservoir engineering theory of horizontal wells, the length of horizontal section of horizontal wells is generally 300 ~ 500 m, but in the case of serious reservoir heterogeneity, muddy or calcareous interlayer is often drilled in the horizontal section. In this way, the orientation or length of horizontal section can be adjusted at any time according to MWD electrical logging data to meet the productivity design requirements of horizontal wells.
Reservoir heterogeneity near 1.4 horizontal section
The reservoir heterogeneity near the horizontal section has great influence on the productivity of horizontal wells. Therefore, we should do the following work:
(1) for pre-drilling prediction.
Before drilling, microfacies division and interlayer correlation (especially interlayer division and correlation) must be done according to the data of adjacent wells, so as to have a basic concept before drilling.
(2) Do a good job of tracking while drilling.
After entering the deflecting section, it is necessary to strengthen geological tracking while drilling, and proofread and adjust the pre-drilling prediction data according to MWD data at any time to make it closer to the actual reservoir. Before hitting target A, it is necessary to distinguish whether the mudstone drilled is "mudstone in the upper mudstone section of the target layer" or "mudstone interlayer in the target layer" to avoid "false roof" (misjudging the top surface of the target layer); Conversely, the sandstone drilled may also be "sandstone lens of mudstone section above the target layer" instead of "sandstone target layer".
(3) adjust at any time
Once the actual drilling results are found to be inconsistent with the design, corresponding dynamic adjustments should be made immediately to guide production.
Relative positions of targets A and B in the horizontal section of 1.5
Generally speaking, one end of a horizontal segment:
(1) is placed in the high part of the reservoir, and the elevation is slightly greater than or equal to the elevation of the B end;
(2) Stay away from the well (in the case that the horizontal section cannot be adjusted, it is best to keep point B close to the well);
(3) On the ground, stay away from the ground obstacles and manage as close as possible to the adjacent wells.
2 Design of main design parameters of horizontal wells
2. 1 vertical depth design of horizontal well
Because the position of horizontal well in oil layer is permanent, it is very important to choose the vertical depth of horizontal section of horizontal well. According to the dimensionless critical production formula of horizontal wells in bottom-water reservoir and the relevant data of Triassic bottom-water reservoir, the calculation result shows that the optimal water avoidance degree (water avoidance thickness/reservoir thickness) of bottom-water reservoir is 0.88h (that is, the distance between horizontal section and bottom water is 88% of reservoir thickness).
The numerical simulation results of horizontal wells in this reservoir show that the optimum water avoidance degree of bottom water reservoir is 0.7 ~ 0.9 h.
It is further pointed out in the literature that for bottom water reservoirs, no matter how the specific geological conditions of reservoir development change, the water avoidance degree of horizontal sections of horizontal wells is almost 90%.
When designing the vertical depth of the horizontal section of Triassic bottom water reservoir in Tahe Oilfield, the water avoidance degree is 88% in principle (the productivity loss is about 10%).
2.2 Effective length of horizontal section of horizontal well
The productivity of horizontal well is only proportional to the effective length of horizontal section in a certain range, so the length of horizontal section must be optimized according to the specific geological characteristics of reservoir development when drilling geological design of horizontal well.
All horizontal wells in Triassic bottom water reservoir in Tahe Oilfield are completed by sieve tube. Considering the relative roughness and tubing size of the horizontal screen, the optimal length of the horizontal screen is 300 ~ 500 m.
When optimizing the horizontal section of Triassic bottom water reservoir in Tahe Oilfield, it is generally between 300 and 400 m. ..
3 Subsequent adjustment of drilling geological design for horizontal wells
3. 1 Problems in implementation
Through the geological tracking study of 7 horizontal wells in Tahe Oilfield 1 Block and Block 2, it is found that:
(1) The original structure diagram is not realized.
Due to the low seismic resolution of the ultra-deep and low-amplitude anticline, the accuracy of the original structural map is limited. According to the actual drilling data of development wells, the structural morphology of Triassic bottom water reservoirs in Tahe Oilfield 1 and 2 blocks has changed to varying degrees.
For example, the upper Triassic oil formation in Area 2: the contour line in the east of the structure shrinks to a high point, the high point where TK203 well is located disappears, the oil-bearing boundary shrinks, and the oil-bearing area is reduced from the original 3.7km2 to 3.1km2; The southeast of Well An 1-S56 and the northwest of Well AN2 are low-lying. The structural high point between TK202H-AN 1 well moved to AN 1 well, and the oil-bearing area map changed from ellipse to horseshoe shape.
Another example is the Triassic middle oil formation: the western high point of the structure in the south of Well TK202H and Well AN 1 moves to the southeast of Well TK20 1H and the southeast line of Well S56, and the structural top gradually widens, and the northwest isoline slightly extends outward. TK202H is a relatively low-lying place in the anticline. The amplitude of the east high point of the northeast structure of Well AN2 decreases, and the east isoline slightly contracts to the high point. The oil-bearing area increased slightly (from 4.3km2 to 4.9km2).
(2) Reservoir heterogeneity becomes more complicated.
There are serious reservoir heterogeneity in all three oil groups, and there are different numbers of argillaceous, calcareous or argillaceous-calcareous interlayers in the layers, and the interlayer distribution in the horizontal direction is quite complicated.
3.2 Main steps and methods of design adjustment
(1) Modify the structure diagram first.
For the newly drilled horizontal development well, if there are pilot holes, the original structural map is revised according to the top and bottom depth data of each line of the oil group actually drilled with pilot holes, and compared with the adjacent wells; If there is no pilot hole, after the drilling enters the critical stage (after deflecting, before and after the target), the top and bottom depth of the actually drilled sandstone mudstone section should be collected in time, and the structural map should be revised in time for the data of adjacent wells again, so as to provide a high-precision basic map for the adjustment and optimization of drilling geological design.
The structural map of the lower oil group in Tahe Oilfield 1 block has undergone four major changes; The structural map of the upper and middle oil groups in Area 2 of Tahe Oilfield has changed twice.
(2) On the basis of small layer correlation, make the distribution map of muddy interlayer between wells.
According to the mud-calcareous interlayer and layer thickness revealed by the pilot hole or (and) deflecting section of the newly drilled horizontal development well, combined with the existing data of adjacent vertical wells, the distribution and connectivity of interbeds between wells are further explained by using sedimentary microfacies constraints. Guide the drilling process and wellbore trajectory of horizontal wells while drilling, especially the adjustment of wellbore trajectory during horizontal drilling.
3.3 Well location adjustment
After the geological design of six horizontal wells was submitted, the * * * meter was adjusted eight times during the tracking while drilling (Table 2). The reasons for well location adjustment are as follows:
Table 2 Tracking adjustment times of horizontal wells in Triassic bottom water reservoir in Tahe Oilfield 1 and Block 2 Table 2 Tracking adjustment times of horizon wells in Triassic bottom water reservoir in Tahe Oilfield (1, No.2)
(1) There are "follow-up data of adjacent wells" and "data of this well while drilling" to prove that the structural shape has changed, and the well has moved to a thicker oil layer under the condition that the design principle remains unchanged.
(2) Follow-up numerical simulation results: When the design well has a good horizontal section orientation, adjust the horizontal section orientation of the horizontal well in time.
The above two situations each account for almost 50% of the total adjustment times.
3.4 vertical depth adjustment
During the tracking of 6 horizontal wells while drilling, 10 times was adjusted (Table 2). The reasons for adjusting the vertical depth of the horizontal section are as follows:
(1) There are "subsequent data of adjacent wells" and "data while drilling of this well" to prove that the structural shape has changed, resulting in changes in the vertical depth of the top surface of the target layer and changes in the effective thickness of the oil layer. Under the condition that the design principle of vertical depth remains unchanged, the vertical depth should be adjusted in proportion.
(2) The data while drilling shows that the original designed horizontal section is located in argillaceous interlayer, and the horizontal section is forced to move up (when the hydrophobic height cannot be reduced) or down (when there is enough hydrophobic thickness, or the physical properties of the upper reservoir of the horizontal section are too poor) (at this time, the design parameters may not be optimal).
3.5 Adjustment of horizontal section length
During the tracking while drilling of 6 horizontal wells, the instrument was adjusted twice (Table 2). The reasons for adjusting the length of the horizontal segment are as follows:
The data while drilling shows that the profile position of the horizontal section in the original design is muddy interlayer. When the horizontal section can't move up and down, we can only consider lengthening the design length of the horizontal section to meet the reservoir engineering design with the best effective length of the horizontal section.
4 Optimization of drilling geological design for horizontal wells
Well (1) is located in a thick oil layer, and the horizontal section is about 88% of the thickness of the oil layer above the oil-water interface. If it is found that the thickness of the oil layer becomes thinner during drilling (structural reasons), the horizontal section of the horizontal well should be moved up as far as possible to increase the hydrophobic thickness on the premise of ensuring that the horizontal section does not come out of the top surface of the oil layer.
(2) Try to arrange the well in the high part of the structure. The horizontal wells in Tahe 1 and No.2 oilfields are evenly distributed in the high part where the oil layer thickness is greater than 15m. After drilling a horizontal well, the structural map should be revised in time, and then the drilling trajectory of the next horizontal well should be designed according to the new map to avoid unnecessary losses.
(3) The design length of the horizontal section of the early horizontal well is 360m. After the horizontal well was put into production, according to the comparison of development effect and development performance of the vertical well and the new understanding of reservoir heterogeneity, it was found that there were muddy interlayers with different lengths in the horizontal section, and the effective length of the actual horizontal section often failed to reach the design length, so the subsequent horizontal section length of the horizontal well was designed to be 400m, which was flexibly mastered in actual production.
For example, in Well TK20 1H, the original designed horizontal section length was 360m, and 259m argillaceous interlayer was drilled in the horizontal section during drilling, so the horizontal section was lengthened to 443m (the effective length of oil layer was only 184m).
5 Conclusions and suggestions
Some thoughts on 5. 1
(1) Reservoir Description To meet the needs of horizontal well development in the reservoir,
Incomplete structure and insufficient understanding of reservoir heterogeneity will affect the accuracy of horizontal well drilling geological design: it can neither guide the optimization of horizontal well location and horizontal section orientation, nor ensure the horizontal section drilling to achieve the purpose of reservoir engineering (or meet the design requirements), and may also lead to the extension of well construction period, the decline of completion quality and the unsatisfactory development effect.
(2) The "Large Horizontal Well Development Scheme" is not applicable to small reservoirs.
If the ultra-deep bottom water reservoir with small reserves (thin oil layer and small oil-bearing area) is developed by "large-scale horizontal well development scheme", it is often necessary to go to the horizontal well before the detailed description of the structure and reservoir is implemented. On the premise of uncertain resources, it may lead to one-time over-investment, and at the same time, it is necessary to rely on horizontal well information to improve or further evaluate the reservoir. If this structure is implemented, the well pattern density has met the development requirements (the dynamic adjustment method currently adopted in Tahe Oilfield 1 and 2 blocks has minimized the harm caused by this possibility).
(3) The lag of production, research and management weakens the advantages of horizontal well development over vertical well development.
The technical equipment, personnel quality and soft and hard settings are not in place, which makes the development cost of horizontal wells in this enterprise (technology outsourcing, equipment leasing, design adjustment while drilling and solving new problems that may arise at any time, etc.). ) higher than mature regions or enterprises under the same conditions. This will affect the development benefits of horizontal wells, and even eat up all the benefits of horizontal well development compared with vertical well development, especially in the case that structural and reservoir heterogeneity is not implemented.
(4) When the reservoir heterogeneity is particularly serious, it is not advisable to drill horizontal wells parallel to the reservoir.
Oil reservoirs in China are mostly continental deposits, and there are more or less interlayers and interlayers with different thicknesses in the oil layers, especially between two wells, which are often difficult to identify. The interlayer and interlayer in oil layer have no obvious influence on the productivity and recovery of vertical wells, but have significant influence on the productivity and recovery of horizontal wells. This kind of reservoir is not suitable for drilling horizontal wells parallel to the reservoir (unless the reservoir is too thin and the horizontal trajectory is difficult to adjust).
5.2 Some suggestions
5.2. 1 Optimal design of vertical depth and position of points A and B of horizontal well
In the following cases, it is suggested to increase the vertical depth of point A of a horizontal well (horizontal section inclines from A to B):
The reservoir thickness of (1) is large enough to prevent premature water breakthrough at point B due to the inclination of section AB (because under normal circumstances, because the production pressure difference at point A is the largest, water breakthrough at point A usually occurs first; The purpose of improving point A is to prolong the breakthrough time of bottom water at point A on the premise that the vertical depth of point B remains unchanged, thus prolonging the waterless period of horizontal wells.
(2) If the ground conditions and horizontal well pattern requirements permit, horizontal wells should be drilled from the high part of the structure to the low part as far as possible to ensure sufficient hydrophobic thickness at point A; Secondly, point A can be as far away from the edge water as possible (to avoid the edge water bursting in the hypertonic area).
(3) When laying a horizontal well near the water breakthrough well, try to keep point A away from the water breakthrough well.
5.2.2 Optimization design of horizontal well completion mode
Generally speaking, when a horizontal well only passes through one oil layer and only one fluid passes through it, non-selective completion methods (open hole and gravel packing completion) are adopted; However, if the target layer of horizontal well exploitation is heterogeneous, or a multi-layer reservoir with bottom water and fractures, in order to facilitate the smooth progress of various measures in the later stage of reservoir development, selective completion methods (such as slotted liner with external packer or cementing/perforation completion) can be adopted. Therefore, it is suggested that in the future, under similar circumstances, newly drilled horizontal wells should use selective completion as far as possible in proportion.
5.2.3 The understanding and description of reservoir heterogeneity should be "dynamic and static combination" and "vertical and horizontal combination".
(1) Dynamic and static combination
For example, the number and thickness of interlayers are given by vertical wells, and the interlayer distribution characteristics of the whole reservoir as close as possible to the actual oil layer are obtained through the comparative study of sedimentary microfacies and small layers; At the same time, the interlayer distribution characteristics of single well and oil reservoir can be verified and corrected by numerical simulation method.
(2) the combination of flat and straight
If a vertical well and a horizontal well encounter the same interlayer, then the vertical well gives the number of layers and the layer thickness (Z axis), and the horizontal well gives the plane distribution (X and Y axis); According to the "width-to-thickness ratio" obtained from sedimentary microfacies, it can be inferred whether the horizontal well reveals the long axis or the short axis (although it is not necessarily correct, it is better than judging the distribution range of interlayer only by "well spacing"), and then it can be further judged whether it is connected with the interlayer of adjacent wells.
Moreover, vertical wells reveal reservoir heterogeneity in the vertical direction, while horizontal wells reveal reservoir heterogeneity in the horizontal direction. The organic combination of the two can objectively reflect reservoir heterogeneity (much more accurate than parameter interpolation between vertical wells).
refer to
[1] Wan Renpu. Horizontal well exploitation technology of different types of reservoirs in China. Beijing: Petroleum Industry Press, 1997.
[2] Translated by Dr. S.D. Josh and Ban Jingchang. Horizontal well technology. Beijing: Petroleum Industry Press, 1998.
[3] Luo Yingjun et al. translated Horizontal Well Mining Technology. Beijing: Petroleum Industry Press, 199 1
Fan zifei Study on determining the optimal position of horizontal section and vertical section of horizontal well. Petroleum exploration and development,1995,22 (3)
Zhang et al. Study on the influence of horizontal well trajectory on productivity and oil recovery in continental sedimentary reservoirs. Petroleum exploration and development,1999,26 (2)
Optimization of drilling geological design of horizontal wells in Triassic bottom water reservoir in Tahe Oilfield 1-2 block
Tan Chen Chengjun Wang Shumei Zhao Meiling Liqun
(Planning and Design Institute of Northwest Petroleum Geology Bureau, Urumqi 8300 1 1)
Abstract: Based on the related geological design of vertical wells and horizontal wells, some important design parameters in the study of geological design of horizontal wells are analyzed. The results show that the geological design of horizontal well and its optimization design are controlled by the vertical depth of horizontal section, the effective length of horizontal section and the influence of reservoir heterogeneity on the geological design of horizontal well.
By optimizing the geological design of seven Triassic horizontal wells with poor bottom water and oil in Tahe Oilfield, the following conclusions are drawn: (1) The continental strata have argillaceous or calcareous interlayers and isolation layers with different thicknesses and shapes, especially when the reservoir heterogeneity is serious. In this case, reservoir description will best meet the needs of developing reservoirs through horizontal wells; (2) For limited reservoirs and small-scale reservoirs, it is not appropriate to design a large-scale horizontal well development plan; (3) Backward technical equipment, personnel quality and computer processing system will determine the development of this technology-horizontal wells are better than vertical wells.
Keywords: optimization of drilling geological design of horizontal wells in bottom water sandstone reservoir, vertical depth of horizontal section and length of horizontal section