(1. China Institute of Petrochemical Engineering and Technology, Beijing10010/; 2. School of Petroleum Engineering, Youshi University, China,
Beijing102249; 3. Engineering Technology Research Institute of China Petrochemical Northwest Oilfield Company, Urumqi, Xinjiang 8300 1 1)
Aiming at the problem that the conventional acid fracturing effect of horizontal wells in Ordovician carbonate reservoir of an oilfield in northwest China is poor and it is difficult to select the method and tools for staged reconstruction, the staged acid fracturing optimization design of horizontal wells is carried out. According to the actual drilling, logging and logging data of test wells, the in-situ stress analysis and the optimization results of staged acid fracturing series, combined with the characteristics of reservoir fractures and dissolved holes, the completion tools are optimized, and the staged acid fracturing completion scheme is determined based on the principle of being conducive to the effective reconstruction of reservoir intervals. Optimize parameters such as fracture length, conductivity, construction scale, proportion of pre-fluid, and displacement of each section, optimize fracturing fluid and acid fluid, and form a differentiated design scheme for staged acid fracturing optimization. The results of field construction and fracturing show that the optimized design method of segmented acid fracturing is correct and can be further popularized and applied, but the quality of tools such as throwing ball and sliding sleeve and the quality control during acid fracturing of completion need to be further improved and improved.
Optimal design of acid fracturing for horizontal wells in carbonate reservoirs: grading
Optimal design and application of multistage acid fracturing
Carbonate horizontal well
Zhang Bo 1, 2, Xue Chengjin 1, Zhou Linbo 1, Zhang Ye 2, 3
(1. China Petrochemical Petroleum Engineering Research Institute, Beijing10010/;
2. School of Petroleum Engineering, China Shiyou University, Beijing 102249,
China; 3. Northwest Oilfield Engineering Technology Research Institute
China Petrochemical Company, Urumqi 8300 1 1)
Aiming at the problems of poor horizontal conventional acid fracturing effect and difficult selection of multi-stage stimulation methods and tools in Ordovician carbonate reservoirs in an oilfield in northwest China, multi-stage acid fracturing design was carried out. According to the results of actual drilling, logging, stress analysis and multi-stage acid fracturing optimization, combined with the characteristics of reservoir fractures and dissolved holes, and the requirements of optimizing completion tools and completion tools for borehole diameter and trajectory, the multi-stage acid fracturing completion scheme is determined, and the etching fracture length, conductivity, construction scale, pre-fluid ratio and displacement parameters at all levels are optimized, and fracturing fluid and acid solution are optimized, forming a differentiated multi-stage acid fracturing design. The field test results and effects show that the optimized multi-stage acid fracturing design is correct and can be further popularized, but the quality of tools such as ball-throwing sliding sleeve and the quality control of acid fracturing completion need to be improved.
Fund Project: National Major Special Project "Development of Large Oil and Gas Fields and Coalbed Methane" (201kloc-0/zx05014).
Keywords carbonate reservoir; Horizontal well; Multistage acid fracturing; optomize design
The Ordovician carbonate reservoir in an oilfield in northwest China is a volatile paleokarst fractured-vuggy massive reservoir with a condensate gas cap driven by dissolved gas and bottom water, with a buried depth of 5400 ~ 6900 m, a formation pressure of 57.0 ~ 75.9 MPa and a temperature of118.0 ~158.7℃. At present, production mainly depends on conventional acid fracturing in vertical wells and sidetracked horizontal wells. However, there are some problems in acid fracturing reconstruction of general horizontal wells, such as long horizontal well section, large acid consumption and low acid fracturing efficiency. It is difficult to control the orientation, extension and spatial distribution of acid fracturing fractures, to effectively transform various target reservoirs with different physical properties, and to select the methods and tools for subsection transformation, which leads to poor acid fracturing effect. In order to improve the acid fracturing effect of horizontal wells, staged acid fracturing optimization design and pilot test were carried out.
1 Optimal design of staged acid fracturing for horizontal wells
Sectional acid fracturing technology for horizontal wells is a new technology which is closely combined and coordinated by reservoir geology, drilling, logging, completion and reservoir reconstruction, and its main purpose is to maximize the productivity of a single well [1 ~ 3]. The optimal design process is shown in Figure 1: firstly, target reservoir identification, current geostress field analysis, wellbore imaging, other logging data analysis and fracture parameter design are carried out. On this basis, the completion scheme optimization and field implementation are carried out in turn, and the optimization of sectional acid fracturing construction parameters and field implementation are coordinated with the quality control of the whole process of drilling and completion acid fracturing to ensure the maximum productivity of horizontal wells. Through fracture monitoring and post-fracturing evaluation, the fracture morphology is analyzed, the reservoir model is revised, the construction experience and lessons are summarized, and the optimized design of staged acid fracturing for horizontal wells is improved.
Figure 1 Optimization Design Process of Sectional Acid Fracturing for Horizontal Wells
2 Basic situation of test well
This well is a development well deployed in an oilfield in the northwest of China. The drilling horizon is Ordovician Yingshan Formation, with a drilling depth of 6190m (oblique depth) and 5440.29m (vertical depth). Lithology is light gray micrite limestone and microcrystalline limestone; There is no exhaust and leakage during drilling; The logging shows gas logging anomaly 10 layer /38m (inclined) and oil trace layer 2/17m (inclined); According to logging interpretation, the secondary reservoir is 1 1 layer /8 1m (inclined), and the tertiary reservoir is 22 layers /270m (inclined). Imaging logging interpretation shows that there are 5 fractures and 5 caves between 5460.00 ~ 6129.70 m, respectively. Seismic reflection data show that faults are very developed and are favorable parts for reservoir development. The angle between the maximum horizontal principal stress direction and the borehole trajectory is 83, which is beneficial to the formation of transverse fractures perpendicular to the borehole direction during acid fracturing. Except for 1 well, all adjacent wells are completed by acid fracturing, and high water cut occurs during production. It is predicted that the fracture pressure gradient of this well is 0.0 18MPa/m, the formation pressure is 57.6MPa and the temperature is 1. 18.6℃. It is considered that this well has the conditions for staged acid fracturing reconstruction of horizontal wells, but there are some risks in the completion process, such as difficulty in selecting packer position, failure to open sliding sleeve, fracture channeling and high water cut after fracturing.
Three-stage acid fracturing completion scheme
According to the actual drilling, logging and mud logging data and the optimization results of staged acid fracturing series, combined with the characteristics of reservoir fractures and dissolved holes, and considering the requirements of completion tools for borehole diameter and trajectory, the staged acid fracturing completion scheme is determined in line with the principle of being conducive to the effective transformation of reservoir intervals.
3. 1 Selection of completion tools
Due to the development of fractures in carbonate reservoirs, in order to prevent the fractures from channeling, 4- 1/2 "water-absorbing expansion packer and sliding sleeve staged completion acid fracturing tool are adopted, with the pressure resistance of 70MPa and the temperature resistance of 170℃ [4 ~ 7]. Key tools include open hole packer, ball sliding sleeve, differential sliding sleeve, wellbore isolation valve, hanging packer, tieback plug-in seal, etc. Multi-stage packer is designed to mechanically seal horizontal open hole interval, and the sliding sleeve is placed according to the favorable fracture initiation position. Before fracturing, the packer is stably placed in clean water. During construction, the sliding sleeve is opened by throwing the ball, and the pressure is suppressed from small to large, and the acid enters the formation from the sliding sleeve to complete acid fracturing, and the composite layer flows back after acid fracturing. This technology has the advantages of saving completion cost, shortening acid fracturing construction period, flexible sealing, large acid fracturing scale and high safety performance.
3.2 Optimization of staged acid fracturing series
Fig. 2 Relationship between the number of fractures and cumulative production
Firstly, the historical production performance of three adjacent wells is fitted by reservoir numerical simulation method, and the average effective permeability of the formation in this well area is11.6×10-3 μ m2. Then, the staged cumulative production is taken as the objective function, and the staged acid fracturing series of test wells is optimized through production optimization simulation. Figure 2 shows that when the number of cracks is less than 4, the cumulative output increases linearly with the increase of the number of cracks, and the increase rate decreases after reaching the optimal number. Based on the three-year cumulative output, the optimal number of cracks is 5 ~ 6.
3.3 completion plan
According to the data of actual drilling, logging and mud logging, in-situ stress analysis and the optimization results of segmented acid fracturing series, the well is completed in five stages. The packer setting position is selected in the interval with poor physical properties and regular well diameter; The position of sliding sleeve is selected in the interval with developed fractures and weak in-situ stress, and it is located in the middle of the interval as far as possible to increase the effect of uniform acid distribution. See table 1 for the position of packer and sliding sleeve.
Table 1 staged completion scheme
(1) 1 inch = 25.4 mm.
Optimal design scheme of four-stage acid fracturing
According to the reservoir characteristics, aiming at the output after acid fracturing, the half-length of acid-etched fractures in each section is optimized, and the construction scale of each section is determined on this basis; According to the pressure requirements of downhole string and wellhead, the fracturing fluid and acid solution are optimized, and the injection mode, required equipment power and surface pumping pressure are determined; Optimize the pumping parameters, determine the pumping procedure, and form a differentiated segmented acid fracturing optimization design scheme.
4. 1 construction technology optimization
According to the length of the reconstructed interval, reservoir development degree and tool safety performance requirements, gel fracturing+surface cross-linking acid fracturing construction technology was adopted in the 1 4 interval, and the 2, 3 and 5 intervals were longer. The imaging logging interpretation results showed that many groups of natural fractures developed. In order to realize the transformation of uniform acid distribution, gel fracturing+turning acid+surface cross-linked acid fracturing technology is adopted.
4.2 Optimization of liquid system
In order to improve the effective length and conductivity of acid-etched fractures and realize uniform acid distribution in long open hole intervals, the formulations of fracturing fluid, surface cross-linked acid and steering acid system were optimized through experiments. 120℃, 170s- 1, shear 1h, the viscosity of fracturing fluid should be controlled within 200 MPa·s, and the viscosity of surface cross-linking acid and steering acid should be kept above 50m pa·s and 400 MPa·s respectively.
4.3 acid fracturing length optimization
Figures 3 and 4 show that the initial productivity and cumulative production in each stage after fracturing increase with the increase of fracture length. When the fracture length reaches a certain value, the increase of fracture length has little contribution to the average daily production and cumulative production, so the optimal acid fracturing length is100 ~120 m.
Fig. 3 Relationship between fracture length and daily output
Fig. 4 Relationship between fracture length and cumulative production
4.4 Optimization of fracture conductivity
As can be seen from Figure 5 and Figure 6, with the increase of conductivity, the initial productivity after fracturing and the cumulative production in each stage increase obviously, and the optimal fracture conductivity is 300×10-3 μ m2 m.
4.5 Construction scale and proportion of prefluid
According to the regional geological conditions, combined with the geostress and seismic profile interpretation results of adjacent wells, the calculation and analysis show that the mudstone section of Bachu Formation in the upper part of the target layer is well shielded, while the shielding in the lower part of the target layer is poor. In order to avoid communication with water layer, the acid fracturing height should be controlled at about 80m, and at the same time, in order to communicate with favorable reservoirs above the horizontal section below T47 interface, the fracturing height should not be too small. The simulation results show that the scale of single-stage acid fracturing should be controlled at a total liquid volume of about 500m3. On the premise of the same scale, the fracture conductivity decreases with the increase of the proportion of pre-fluid. In order to achieve the goal of fracture conductivity of 300×10-3 μ m2 m, the optimum proportion of preflush is about 40%. See Table 2 for the optimization results of the construction scale of each bid section.
Fig. 5 Relationship between fracture conductivity and daily production
Fig. 6 Relationship between fracture conductivity and cumulative production
Table 2 Summary of acid fracturing construction parameters and fracturing parameters
4.6 Construction displacement
The crack height is positively correlated with the construction displacement. When the construction scale is the same, the construction displacement increases and the crack height also increases. Because the construction friction of 1 interval is the largest, and the wellhead pressure is the highest under the same displacement, through calculation and analysis, the wellhead pressure is lower than 90MPa when the construction displacement of fracturing fluid is less than 5.0m3/min and the acid displacement is less than 6.0m3/min, and the wellhead and truck group of 105MPa can meet the construction requirements. According to the requirements of fracture height control and wellhead pressure limitation, the construction displacement of fracturing fluid is determined to be 4.0 ~ 5.0m3/min, and the acid displacement is 5.0 ~ 6.0m3/min, which can be adjusted according to the field pressure.
5 field acid fracturing construction
5. 1 Introduction to Construction
On April 26th, 2002, 2065438, two-stage acid fracturing operation was carried out continuously, and the total liquid volume pressed into the formation was 1208. 1m3, of which 4 13. 1m3 was slippery, 365m3 was gel, 380m3 was diverting acid and 380 was surface cross-linking acid.
5.2 Drainage production situation
2065438+On April 26th, 2002, the well was controlled by a 5mm choke. As of April 28th, oil pressure19.1MPa, oil content is 30%, gas production is 3079m3, and cumulative liquid discharge is 240.7m3; as of May 20th12, the oil pressure of 3mm choke is 25.5MPa, and casing pressure/kloc-.
Table 3 Sectional acid fracturing construction and fitting parameters
6 problems that need to be improved
1) Strengthen quality control during completion. After the pump is turned on by acid pressure, it shows that the oil sleeve pressure is connected. The reasons are as follows: First, the sealing of the sealing sleeve failed. After the insertion of the rear sleeve string, there was no lifting action to verify the ratchet anchoring. When the pressing torque reached 4 1.02MPa when the differential pressure sliding sleeve was opened, the displacement of the rear plug joint was 90. 14cm under this pressure, and the packing inserted into the sealing sleeve was 20cm long, which may lead to the failure of the insertion seal when the differential pressure sliding sleeve was opened; Second, the hanging packer and the water swelling packer are not completely set. In the future, the quality control of the whole completion process should be strengthened to ensure the smooth implementation of staged acid fracturing.
2) Quality control of sliding sleeve. 1 and 2-stage pitch sliding sleeve openings are not obvious. After the completion of the first stage construction, the sliding sleeve was used many times, and the displacement was 1.5 ~ 4.9m3/min, with no obvious opening display. According to analysis, the sliding sleeve may have been opened and transferred to the second phase of construction. After the completion of the second phase construction, the sliding sleeve with the displacement of 1.5 ~ 3.0m3/min was used many times, and the display was not obviously turned on. Throw the ball with the density of 2.8g/cm3 again, and use the sliding sleeve with the displacement of 1.5 ~ 4.5m3/min for many times, which shows that it is not obviously opened. The reason is that the ball delivery displacement is large and the quality of the sliding sleeve is problematic. In the future, on the basis of ensuring the quality of sliding sleeve, the ball delivery displacement should be optimized to ensure the smooth implementation of staged acid fracturing.
7 conclusion and understanding
1) staged acid fracturing technology for horizontal wells is a multidisciplinary technology, and its key points are the wellbore quality of horizontal wells, the temperature resistance of water-swellable packers, effective sealing, sliding sleeve ball seat sealing and pressure relief opening performance. It is suggested to further strengthen key research tests, improve and improve tool performance, and at the same time do a good job in quality control of the whole process of completion and acid fracturing, so as to provide effective technical support for improving the development effect of carbonate reservoirs.
2) The staged acid fracturing completion scheme for horizontal wells should be based on the principle of realizing effective transformation of favorable reservoir sections. According to the actual drilling, logging and mud logging data, on the basis of in-situ stress analysis and optimization of staged acid fracturing series, combined with the development of reservoir fractures and holes and the requirements of completion tools for borehole diameter and trajectory, the positions of packers and sliding sleeves are determined.
3) Optimize the parameters such as the length of acid-etched fracture, conductivity, construction scale, proportion of pre-fluid, displacement, etc., optimize fracturing fluid and acid solution, and form a differentiated staged acid fracturing optimization design scheme. The results of field construction and post-fracturing show that the construction parameters and measures for controlling joint height are reasonable and effective, and the optimization design method of sectional acid fracturing is correct, which can be further popularized and applied.
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