During the 11th Five-Year Plan period, the Victory Oilfield exploration area tested a total of 524 wells, and a total of 53,646,000 tons of oil geological reserves were identified, with more than half being low-permeability oil reservoirs. For low-permeability reservoirs, acid fracturing is an important measure for increasing production, which requires specific procedures for fracturing fluid return, including timely return of fracturing fluids post-acid fracturing to minimize formation contamination; as the scale of fracturing expands, the depth increases, and the intensity of fluid drainage grows, the volume of oil testing work also increases annually, leading to longer drainage cycles that affect the effectiveness of fracturing and acidification measures.
Current fracturing and backflow processes face several issues: the time required for transitioning from fracturing to pumping is lengthy, taking 5 to 7 days; as fracturing depths increase and the proportion of deep well oil tests grows, low liquid levels lead to easy degassing of crude oil, affecting pump efficiency; the phenomenon of uneven wear becomes more pronounced, increasing tubing wear. To address these issues, our institute has conducted research on the integration of fracturing and backflow technology for low-permeability oil reservoirs. This new technology allows for direct switching from fracturing to pumping and oil testing without removing the tubing string. The process can reduce the need for two lifting operations, shorten the oil testing cycle, minimize post-fracturing contamination, enhance fracturing effectiveness, protect fracturing tubes, and reduce operational costs. The integrated fracturing and backflow technology for low-permeability oil reservoirs can achieve timely and rapid fluid drainage after fracturing, shorten the drainage cycle, mitigate pollution of the formation, and simultaneously capitalize on the early high conductivity of fractures to maximize the potential of hydraulic fracturing for increased production. This technology improves the acidification and fracturing stimulation effects.
In the past, after fracturing, the process typically involved washing the well, removing the fracturing string, running a string to probe for sand, and lowering a pump, with an average turnaround time of 6.9 days. This method was prone to contaminating the oil layer, affecting fracturing results. The integrated fracturing and fluid displacement process string allows for direct transition from fracturing to production without moving the fracturing string, completing acidification, fracturing, and production extraction in a single run, thereby shortening the fluid displacement cycle.
The process string composition mainly consists of fracturing strings, rod strings, fracturing and pumping combined oil pumps, fracturing gas prevention devices, wear-reducing couplings, and packers, etc.
Casing Features: Achieves direct pumping of fluid with no pressure fracturing casing, shortening transfer and withdrawal time, and reducing the fluid discharge cycle. Effectively separates gas and liquid, minimizing the impact of gas on pump operation, and enhancing pump efficiency. Reduces casing wear, protects the fracturing oil casing, and lowers operational costs.
The conventional oil testing method using a rod-type pump requires the removal of the original well string first. To minimize operational steps and prevent environmental pollution, a fracturing-pumping combined oil pump has been developed, which allows direct pumping after fracturing and acidification without the need to move the pipe string.
The pump's outer casing is lowered into the well along with the fracturing string. After fracturing operations, the working barrel is lowered into the string with the rod string, and the fracturing string is left stationary for the pump to pump and drain the fluid. It is primarily composed of the sealing part, pump barrel, plunger, oil outlet valve, outer casing, oil inlet valve, and locking device.
Operation Principle: During the upward stroke, the pump rod drives the plunger upwards, causing the cavity volume below the plunger to increase and the pressure to drop, thereby opening the intake valve. Crude oil from the well enters the cavity below the plunger. During the downward stroke, the cavity volume below the plunger decreases, the pressure rises, and the intake valve closes. The crude oil in the cavity is then discharged through the outlet valve into the annular space between the pump rod and the oil pipe, and finally pumped to the surface.
Key features include: Increased bore diameter of the working cylinder to minimize flow resistance during acid fracturing; dual-stage sealing for long-term stability and reliability; equal-diameter sand scraping structure to prevent plunger sanding; and, due to the absence of valve seats on the outer cylinder, no issue of high-pressure fluid erosion damage, which extends the production time of the process string.
As the fracturing depth increases, the low liquid level leads to gas stripping in crude oil, affecting pump efficiency. During the upward stroke, the presence of gas inside the pump prevents the pressure from rapidly dropping during fluid extraction, thus unable to timely open the fixed valve, reducing the effective stroke of the pump. Additionally, as both liquid and gas enter the pump simultaneously during the intake process, the fluid discharge volume per stroke is reduced. Furthermore, due to the gas present in the pump's internal cavity, during the downward stroke, the pressure inside the pump cannot rapidly increase, leading to the late opening of the sliding valve, which decreases the oil discharge time. In extreme cases where gas interference is severe, it can cause reciprocating expansion and compression of gas within the pump cavity, rendering both the fixed and sliding valves inoperable, creating a gas lock that prevents the pump from drawing out liquid.
Additionally, the oil testing pipe string is a closed pipe string with no space for gas release. Establishing a gas exhaust passage is a technical challenge in the oil testing process. Therefore, a gas-proof device for the fracturing pipe string has been developed, which can effectively separate gas and liquid, reduce the impact of gas, and improve the pump's filling degree. The device is mainly composed of a central pipe, sliding sleeve, inner tube, and outer tube, etc.
The principle of operation is as follows: the inner and outer casings are connected at the lower section of the fracturing casing and are lowered into the well together. After fracturing operations, when pumping is required, the center pipe is lowered into the casing along with the rod string and the fracturing pumping inner cylinder, thereby opening the exhaust passage. As the fluid enters the outer casing, due to the difference in gas-liquid density, it is separated by gravity. The oil flows down the inlet channel into the center pipe, while the gas rises through the annular space between the inner and outer casings and is discharged through the exhaust passage, thereby reducing the impact of the gas and improving the pump efficiency.
The oil rod column is suspended underground, with complex and variable forces acting at each point. During the upward stroke, the resultant force at each point of the rod column is upward, straightening the rod. Below the midpoint (where the resultant force is zero), the oil pipe晃动 and experiences eccentric wear with the rod coupling. During the downward stroke, the resultant force at each part of the oil pipe is downward, straightening the pipe. The forces acting on the rod column points are complex and variable. Due to the high plasticity of the rod column, the upper weight does not quickly exert pressure on the lower section. Meanwhile, the lower rod column is still moving upward under the inertia force during the upward stroke, significantly increasing the bending degree of the middle and lower sections of the rod column. This phenomenon of bending is called instability. Rod column instability is a major cause of eccentric wear. As the pump depth increases, the eccentric wear caused by rod column instability becomes more severe, increasing the wear of the rod and pipe, thereby raising the operational costs.
To protect frac pipes, rod and tube anti-wear couplings have been developed. The anti-wear coupling for sucker rods involves熔fusing high-strength non-metallic materials on the surface of the sucker rod coupling, providing excellent anti-wear properties; while the anti-wear coupling for oil pipes involves hardening and smoothing the inner surface of the oil pipe coupling, ensuring a smooth transition at the step area to reduce friction and eliminate collisions and屑wear with the sucker rod coupling, thereby preventing uneven wear between the rod and pipe and protecting the oil pipe.
To accelerate the pumping speed, the use of continuous sucker rods is recommended. The main advantages of steel continuous oil rods include fatigue resistance, corrosion resistance, high tensile strength, and light weight. These features can significantly reduce the suspension load of the pumping unit and minimize wear on sucker rods and oil pipes. Moreover, as they do not use coupling joints, they are not affected by the coupling piston effect, eliminating the issue of joint failure, and achieving excellent practical results.
Before entering the well, a seamless continuous rod of the required length is welded without any joints. The lower end of the continuous rod is welded to the pump coupling, which is then welded to the polished rod on-site. It can be coiled onto a large drum with a diameter of approximately 5.5 meters for transportation and storage, with winding speeds up to 1000 meters per hour.