Trial Production Equipment Research Room, Drilling and Production Equipment Research Institute, Jianghan Petroleum Administration
With the development of oil-production technology, reservoir fracturing has become a major technical measure for long-term stable and high production in oilfields. In recent years, fracturing technology has developed rapidly, creating an urgent need for complete fracturing equipment capable of meeting high-pressure, high-displacement, high-sand-ratio, large-sand-size, large-sand-volume and high-viscosity-fluid operating conditions.
To meet these needs, experimental research on a 700 kg/cm2 fracturing manifold began in 1973. In March 1975, the first manifold set was trial-manufactured at the drilling and production machinery repair plant. After passing a 1000 kg/cm2 static-pressure test and a 700 kg/cm2 dynamic-pressure test, it was put into field testing in May. It was used successively in acidizing and fracturing operations at Jiannan Gas Mine and Jianghan Oilfield for seven well operations. The maximum pump pressure reached 605 kg/cm2 and the maximum displacement reached 1925 L/min.
1. Structural Description
The 700 kg/cm2 fracturing manifold, shown in Figure 1, is mainly composed of check valves, plug valves, bleed-off valves, unions, tees, crosses and pup joints. It has 81 connections in total. The 41 connections equipped with check valves can connect to 41 fracturing trucks. Two connections equipped with bleed-off valves can connect to the bleed-off line, and another two connections connect to the fracturing wellhead.
Figure 1. 700 kg/cm2 fracturing manifold
1- 2 in union; 2- check valve; 3- 2 1/2 in union; 4- plug valve; 5- bleed-off valve; 6- 3 in union.
The center distance between the two main lines in the manifold is 780 mm. They can be fixed on the frame of a CA-01 or CA-03 Jiefang truck to form a manifold truck. The main-line bore is 54 mm and the branch-line bore is 46 mm. The ends of the branch lines are equipped with 2 in unions and can connect directly to SYC-700 and YLC-500 fracturing trucks.
The check valve used on this manifold is shown in Figure 2. It is simple to manufacture and convenient to repair. Its valve assembly is similar to that of the ACF-700 fracturing truck, with reliable operation, low risk of sand sticking and relatively long service life.
Figure 2. Check valve
1- gland; 2- spring; 3- valve assembly; 4- valve seat; 5- valve body.
The plug-valve structure is shown in Figure 3. A pair of valve plates is arranged between the plug and the valve body. Gland 5 adjusts the clearance among the valve plates, plug and valve body, allowing the valve to seal while still opening and closing with relatively low effort. The valve cavity should be filled with high-pressure sealing grease to improve sealing and lubrication.
Figure 3. Plug valve
1- handwheel; 2- plug; 3- valve body; 4- valve plate; 5- gland.
This plug valve is easy to disassemble and assemble and opens and closes quickly: rotate 90 degrees to open and rotate 90 degrees in the reverse direction to close. Compared with similar tapered plug valves and ball valves, it requires lower operating torque and is very suitable as the main valve of a fracturing manifold.
The bleed-off valve, shown in Figure 4, has a simple structure, reliable operation and easy manufacture. It opens and closes with low effort under high pressure and is especially suitable as a pressure-relief valve.
Figure 4. Bleed-off valve
1- gland; 2- valve body; 3- valve element; 4- valve seat; 5- pressure cap; 6- valve stem.
2. Test Results
1. Static-pressure test
The check valves, plug valves, bleed-off valves and movable elbows in the manifold were each tested separately as individual parts. The seal test pressure was 800 kg/cm2 and the strength test pressure was 1000 kg/cm2. After the individual parts passed testing, the manifold was assembled according to Figure 1 and tested as a complete assembly at 1000 kg/cm2. There was no seepage, leakage or pressure drop within 10 minutes.
2. Dynamic-pressure test
Five SYC-700 fracturing trucks were connected to the check-valve connections of the manifold. One main-line connection was plugged, one was fitted with a nozzle, and the remaining connections were plugged. A 700 kg/cm2 dynamic-pressure test was carried out, and there was no seepage or leakage within 30 minutes.
3. Field test
This manifold was used first for acidizing in one well at Jiannan Gas Mine and later for fracturing in six wells at Jianghan Oilfield. The operating parameters are shown below.
Operating parameters for acidizing and fracturing with the 700 kg/cm2 fracturing manifold
| No. | Operation date | Well No. | Max pump pressure | Max displacement | Sand ratio % | Total fluid | Total sand added | Remarks |
|---|---|---|---|---|---|---|---|---|
| 1 | May | Jian 29 | Acidizing, no leakage, plug valve not installed | |||||
| 2 | June | Wangxi 7-4 | 580 | 80 | 3.1 | Fracturing, no leakage, plug valve not installed | ||
| 3 | June 21 | Wangxi 12-7 | 580 | 1500 | 10.5 | 80 | 4.65 | 1. Plug valves installed 2. Truck connections increased from 12 heads to 16 heads 3. Three tubing-thread leaks |
| 4 | July 8 | Wangxi 9-6 | 650 | 1600 | 11 | 80 | 4.2 | |
| 5 | July 9 | Wangxi 7-5 | 640 | 1640 | 12 | 80 | 5.6 | |
| 6 | July 17 | Wang 51 | 350 | 1925 | 16.7 | 80 | 8.64 | |
| 7 | July 25 | Wangxi 10-7 | 530 | 10 | 80 | 5.3 | ||
| Note: after the number of connections was increased from 12 to 16, the line was disassembled and reassembled, which damaged tubing threads. As a result, three tubing-thread leakage points appeared. | ||||||||
The main components of the fracturing manifold are three types of valves. After fracturing four wells, the check-valve rubber was inspected and remained in good condition. Because the discharge line of the fracturing truck had no pressure-relief valve, the test did not directly show whether the check valve could seal after a fracturing truck stopped during operation. However, analysis indicated that as long as the check-valve rubber was not damaged, opening the relief valve on the fracturing-truck outlet line after a mid-operation shutdown would create differential pressure across the check valve, allowing it to seal.
The plug valve was also field-tested in four wells. The first two plug valves had insufficient manufacturing quality because the ports of the plug, valve plates and valve body were not aligned accurately. They were therefore heavily eroded by sand, but they could still seal. In September, four more plug valves were trial-manufactured, and each passed assembly and pressure testing on the first attempt. They were prepared for installation on the second manifold for field testing. After seven acidizing and fracturing well tests, the bleed-off valve sealed reliably and opened and closed flexibly with low effort.
3. Overall Comments
1. This fracturing manifold can basically meet current acidizing and fracturing process requirements and can be used for acidizing and fracturing operations at 700 kg/cm2. Small-batch production and expanded testing are recommended.
2. Based on the previous trial manufacture and test results, the following design improvements are recommended.
First, the original 2 1/2 in tubing-thread connections used for the main line, tees and crosses have high machining requirements. Their sealing reliability at 700 kg/cm2, especially after acidizing, is insufficient. In addition, the mounting positions of the tees and crosses are difficult to adjust and install. They should be changed to T58 x 5-3 and T70 x 4-3 trapezoidal-thread connections with O-ring sealing. The mounting positions of tees and crosses can be adjusted by adding or removing shims. The plug-valve flange connection should also be changed to a T58 x 5-3 trapezoidal-thread connection.
Second, the connection line from the manifold outlet to the wellhead originally used 3 in drill pipe. The longest pipe was 4 m and weighed more than 100 kg, making installation difficult. The movable elbow was also heavy, at more than 60 kg, and difficult to install. The connection line should be changed to 2 in drill pipe with lengths of 3 m, 2 m and 1 m, and the movable elbow should be modified accordingly to reduce weight and improve installation convenience.
Third, to meet new process requirements, the number of plug valves should be increased from two to five, and two needle valves should be added to meet ball-drop and remote-measurement requirements. The modified structure is shown in Figure 5.
Figure 5. Improvement plan for the 700 kg/cm2 fracturing manifold. The left side, not shown, is the same as Figure 1.
1- check valve; 2- plug valve; 3- bleed-off valve; 4- needle valve.
