The present invention relates to the technical field of petroleum engineering, in particular to a three-dimensional acid fracturing method for carbonate reservoirs in long intervals.
Deep carbonate oil and gas are usually stored in millimeter-centimeter-level pore reservoirs, and the oil and gas reservoirs are usually not connected with the borehole, so oil and gas cannot be produced naturally. Acid fracturing technology is the key technology to build and increase production of carbonate oil and gas wells. Acid fracturing is to break the rock to form artificial cracks, and then inject acid to unevenly dissolve the wall of the cracks, forming uneven grooves; After the construction, under the action of closing pressure, the acid-undissolved area is used as the support point, forming acid-etched cracks with certain geometric size and conductivity, and realizing the construction of “oil and gas expressway” underground.
With the increasing demand for efficient and economic development of carbonate reservoirs, in recent years, carbonate reservoirs have adopted the development mode of “thin wells and high production”, and the application of ultra-long horizontal wells has become more and more. The reconstructed interval of super-long horizontal well (the length from target A at the root of horizontal well to target B at the end) is generally long (usually ≥1000 m), and the reservoir heterogeneity is extremely strong. In order to achieve high-efficiency stimulation, the three-dimensional acid fracturing technology is an innovative stimulation technology to fully transform the reservoir in the long interval horizontally and vertically (Guo Jianchun, Gou Bo, Qin Nan, Zhao Junsheng, Wu Lin, Wang Kunjie, Ren Jichuan. Innovation of deep carbonate reservoir reconstruction concept-three-dimensional acid fracturing technology [J]. Natural gas industry, 2020, 40 (02):61-74). Two key problems in the three-dimensional acid fracturing reconstruction of long-interval reservoir: first, how to choose the segmentation technology to separate the long-interval reservoir and realize the full utilization of the long-interval reservoir; the second is how to choose the acid fracturing technology in the interval to realize the full transformation of the reservoir in the interval. Unreasonable selection of subsection technology will not only lead to poor reservoir reconstruction effect, failure to fully release reservoir oil and gas production potential and low economic benefit, but also may lead to complex wellbore engineering accidents; the transformation in the interval is not strong enough, and it can not maximize the efficient production increase of oil and gas wells. However, at present, there is still a lack of three-dimensional acid fracturing methods that comprehensively consider the reservoir stimulation potential, the engineering conditions and economic benefits of long intervals, which affects the economic and efficient stimulation of oil and gas wells.
An object of the present invention is to provide a three-dimensional acid fracturing method for carbonate reservoirs in long intervals. According to the strong heterogeneity of carbonate reservoirs in long intervals, the three-dimensional acid fracturing methods for carbonate reservoirs in long intervals under different completion modes are determined, so that the full utilization of the reservoirs in long intervals and the full transformation of the reservoirs in the intervals are realized, and technical means are provided for efficient production increase of oil and gas wells. The invention has reliable principle, simple operation and broad market application prospect.
In order to achieve the above technical objectives, the present invention adopts the following technical solutions.
A three-dimensional acid fracturing method for carbonate reservoirs in long intervals, sequentially comprising the following steps:
Preferably, the step S1 specifically comprises the following sub-steps:
Wherein: ρi is the acid fracturing fracture density of Class I reservoir, bar/m; Ni is the number and number of acid fracturing fractures in Class I reservoir; Li is the interval length of type i reservoir, m; i is the type i reservoir, dimensionless;
Wherein: rρi is the production growth rate in the fifth year when the acid fracturing fracture density of type i reservoir is ρi, %; qρi is the production of type i reservoir when the acid fracturing fracture density is ρi, m 3; qρmini is the output when the acid fracturing fracture density of type i reservoir is the minimum ρmini, m3;
Change the acid fracturing fracture density of different types of reservoirs, and calculate the output qρi corresponding to different acid fracturing fracture densities ρi. When the output growth range corresponding to adjacent fracture densities is ≤2%, the low value of these two adjacent fracture densities is the upper limit of the required acid fracturing fracture density range, and the adjacent low density value of the low value is the lower limit of the required acid fracturing fracture density range, thus determining the required acid fracturing fracture density range ρir for different types of reservoirs.
Preferably, the step S2 specifically comprises the following sub-steps:
Wherein: Ncp is the number range of acid fracturing fractures required by candidate wells, bar; i is the type i reservoir, dimensionless; Ns is the number of fractures that need to be locally increased or decreased according to the distribution of reservoir types of candidate wells, bar.
Further, the determination of Ns follows three principles:
Wherein: NPV is the economic net present value of the fifth year when the number of acid fracturing fractures in candidate wells is N, ten thousand yuan; Fj is the difference between the cash inflow and outflow in the j year, RMB 10,000; r is the discount rate of reservation, %; Cj is the cost generated in the production process of oil and gas wells in the j year, RMB 10,000; qNj is the oil and gas production in the j year when the number of acid fracturing fractures is N, m3; sj is the commodity rate of oil and gas in the j year, %; ej is the oil and gas price in the j year, RMB 10,000/m3; C0 is the initial investment cost, RMB 10,000;
When the economic net present value NPV reaches the maximum, the corresponding number of acid fracturing cracks is the most economical number Ne.
Further, C0 is determined by the following formula:
C
0
=e
d
H
m
+NV
a(ea+ec)+Fm (5)
Where: ed is the cost price per meter of well depth, such as drilling, logging, cementing and logging, and it is RMB 10,000/m; Hm is oil and gas well sounding, m; Va is the volume of fracturing and acidizing working fluid, m3; Ea is the unit price of fracturing and acidizing working fluid, RMB 10,000/m3; ec is the cost of fracturing truck set required for pumping this amount of fracturing acidizing working fluid, RMB 10,000/m3; Fm is other maintenance costs, RMB 10,000.
Preferably, the step S3 specifically comprises the following sub-steps:
When the horizontal well is completed with open hole or casing perforation, and the downhole subsection tools are available, the sliding sleeve subsection acid fracturing technology is used to distribute the joints, so as to realize the reconstruction of multiple acid fracturing fractures in the long interval (Wu Peng, Deng Wei, Xu Gang, Guo Xiudong. Optimization Technology of subsection completion of ultra-deep horizontal wells in carbonate reservoir of Tahe Oilfield [J]. Chemical Management, 2013, (04): 10-1);
When the horizontal well is completed in open hole or liner, when there is no downhole sectional tool, the temporary plugging of the seam is generally turned to the sewing by craft, which forces the formation of multiple fractures in the long interval (Li Xinyong, Li Chunyue, Shen Xin, Zhao Bing, Zhang Xiong, Wang Shibin, Guo Jianchun. Acid fracturing technology design of three-layer temporary plugging of horizontal wells in Tahe Oilfield [J]. Drilling and production technology, 2021, 44 (03):52-55).
When the sliding sleeves are used for staged acid fracturing, the maximum number of sliding sleeves Mmax is determined by the following formula according to the minimum demand of acid fracturing construction displacement at the end of horizontal wells and the wellhead construction pressure limit:
Wherein: M is the number of sliding sleeves, sleeves; Mmax is the maximum number of sliding sleeves that can be deployed under the minimum displacement requirement of target B at the end of horizontal well. η is the safety factor of wellhead construction pressure, dimensionless; ρhl is the maximum limiting value of wellhead construction pressure, MPa; f1 is the friction gradient produced when fracturing acidizing working fluid flows through wellbore tubing, MPa/m; α is the fracture extension pressure gradient, MPa/m; ρa is the acid density, kg/m3; H is the vertical depth of the candidate well, m; L1 is the tubing length, m; L2 is the length of sliding sleeve, m; D1 is the inner diameter of tubing, m; D2 is the inner diameter of the sliding sleeve, m; Q is the injection displacement, m3/s; g is the acceleration of gravity, m/s2, d is the diameter tolerance of sliding sleeve, m; D2max is the maximum inner diameter of sliding sleeve, m;
The derivation process of equations (6)˜(7) is as follows.
Wellhead pressure during acid fracturing construction is:
p
h
=p
s
+p
f
−p
ah (8)
Wherein: ph is the wellhead construction pressure, MPa; ps is the extension pressure of bottom hole fracture, MPa; pf is the friction produced when fracturing acidizing working fluid flows through wellbore string, MPa; pah is the wellbore fluid injection pressure, MPa.
In Formula (8), the bottom hole fracture extension pressure ps and wellbore fluid injection pressure are calculated by the following formulas respectively:
p
s
=αH (9)
p
ah=10−6ρsgH (10)
When the sliding sleeve is used for staged fracturing, the friction of fracturing acidizing working fluid in the wellbore consists of two parts, one is the friction pft generated by flowing through the tubing, and the other is the friction pfs; generated by flowing through the sliding sleeve. When sliding sleeve staged fracturing is not used, the friction of fracturing acidizing working fluid in wellbore is mainly the friction caused by flowing through tubing. Existing:
p
f
=p
ft
+p
fs (11)
In Formula (11), the friction pft of fracturing and acidizing working fluid in tubing is calculated according to the following formula:
p
ft
=f
1
L
1 (12)
Wherein: pft is the friction produced by fracturing and acidizing working fluid in tubing, MPa; pfs is the friction produced by fracturing and acidizing working fluid in the sliding sleeve, MPa; f1 is the friction gradient of fracturing and acidizing working fluid in tubing, MPa/m; L1 is the tubing length, m.
The inner diameter of the sliding sleeve is usually smaller than the inner diameter of the tubing (as shown in
Wherein: M is the number of sliding sleeves, one; f2 is the friction gradient of fracturing and acidizing working fluid in the sliding sleeve, MPa/m; L2 is the length m of the sliding sleeve; ξ1 is the local sudden reduction of head loss coefficient, dimensionless; ξ2 is the local sudden expansion of head loss coefficient, dimensionless; v2 is the velocity of fracturing and acidizing working fluid in the sliding sleeve, m/s.
Where, the relationship between f2 and f1 is calculated according to the following formula:
ξ1 and ξ2 are calculated as follows:
The flow rate of fracturing and acidizing working fluid in the sliding sleeve is:
It is easy to get the friction in the sliding sleeve from equations (12)-(16) as follows:
From equations (7)-(11) and (17), it is easy to get that the wellhead construction pressure when using sliding sleeve diversion is as follows:
According to the tolerance d, the diameter of the sliding sleeve gradually decreases from the maximum inner diameter of the target point A at the root of the horizontal well near the borehole to the target point B at the end of the horizontal well far away from the borehole, so the inner diameter of the sliding sleeve at any section is:
D
2
=D
2max−(M−1)d (21)
Therefore, the combined formula (19) and formula (20) become:
In order to ensure the safety of acid fracturing construction, wellhead construction pressure is required to meet the following requirements:
p
h
≤ηp
hl (22)
Combining the formulas (20)-(22), it is easy to obtain the maximum number Mmax of sliding sleeves that meet the minimum construction displacement of target B.
When the horizontal well is equipped with downhole subsection tools, and when Ne≤Mmax, the sliding sleeve subsection acid fracturing technology is used for joint distribution, and the number of sliding sleeves deployed is Ne; When Ne>Mmax, the sliding sleeve is segmented+the seam in the segment is temporarily blocked to turn to the sewing of composite craft cloth, and the sliding sleeve is adopted. The number is Mmax, and the number of seam temporary plugging is Ne−Mmax;
When the horizontal well is not equipped with underground sectional tools, the seam is temporarily blocked and turned to industrial sewing, and the number of seam temporary plugging is Ne.
Preferably, the step S4 specifically comprises the following sub-steps:
Where: ζi is the ratio of the length li of the well section of type i reservoir to the total length L of this section, %;
When there are three types of reservoirs in the interval, ζi≥33%, indicating that the interval is dominated by type i reservoirs; When there are two types of reservoirs in the interval, ζi≥50%, indicating that the interval is dominated by type i reservoirs;
The acid fracturing target of Class I reservoir is to remove the pollution near the well and dredge the fractures and caves in the near-well zone. Turning acid fracturing is adopted to remove the pollution near the well bore at a displacement of 2.0-3.0 m3/min, and then the maximum acid injection displacement is adopted according to the wellhead construction pressure to break through the near-well polluted zone and dredge the fractures and caves. The acid injection amount is determined at 1.0-1.5 m3/m according to the reservoir thickness;
The acid fracturing target of Class II reservoir is to make long fractures and improve the conductivity. Pre-fluid acid fracturing is adopted. First, the fractures are made with weakly reactive working fluid (such as fracturing fluid or authigenic acid), and then etched with gelled acid. The acid injection amount is 1.5-2.5 m3/m, and the weakly reactive working fluid is 50% of the acid consumption, and the acid injection is discharged.
The acid fracturing target of Class III reservoir is to make long fractures, and two-stage alternating acid fracturing is adopted, that is, weakly reactive working fluid and gelled acid are injected alternately in two stages, with the acid injection amount of 1.5-2.5 m3/m, the working fluid with weak reaction is 50% of the acid consumption, and the acid injection displacement is constructed with the maximum acid injection displacement according to the wellhead construction pressure.
Compared with the prior art, the invention has the following beneficial effects:
Aiming at the characteristic of strong heterogeneity of carbonate reservoir in long well section, based on the concept of geological engineering integration and the concept of three-dimensional reconstruction and production of long well section, the invention fully considers the personalized demand of heterogeneous reservoir for cracks, the influence of deep bad wellbore conditions on acid fracturing subsection operation, economic cost and other issues, and determines the three-dimensional acid fracturing method of long well section reservoir in different completion modes, which is beneficial to realizing the economic and efficient three-dimensional reconstruction of long well section reservoir.
The invention will be further explained with the attached drawings and field application examples. It should be noted that the embodiments in this application and the technical features in the embodiments can be combined with each other without conflict. It should be pointed out that unless otherwise specified, all technical and scientific terms used in this application have the same meaning as commonly understood by ordinary technicians in the technical field to which this application belongs.
Refer to
A Three-dimensional acid fracturing method for carbonate reservoirs in long intervals, the specific process is as follows:
Carbonate X gas reservoir can be divided into two types according to porosity and permeability characteristics, as shown in Table 1. There is a horizontal well X9 drilled in this gas reservoir. The inclination depth of target A at the root of the horizontal interval is 5148 m, and the inclination depth of target B at the end far from the borehole is 7070 m. The length of the reformed interval is 1922 m, the formation pressure coefficient is 1.1, and the formation temperature is 153° C. The reservoir porosity, permeability and gas saturation explained by the reformed interval are shown in Table 2.
Although the present invention has been disclosed in terms of preferred embodiments, it is not intended to limit the present invention. Any person familiar with this field can make some changes or modifications to equivalent embodiments by using the technical contents disclosed above without departing from the scope of the technical scheme of the present invention. However, any simple modifications, equivalent changes and modifications made to the above embodiments according to the technical essence of the present invention are still within the scope of the technical scheme of the present invention.
Number | Date | Country | Kind |
---|---|---|---|
202210887215.4 | Jul 2022 | CN | national |