Six Common Oilfield Fishing Tools Classified by Fishing Methods

In oil drilling and workover operations, downhole fishing is an essential step in retrieving fallen objects (known as “fish”) ,such as broken drill pipes, tubing, and tool fragments. These fish can severely disrupt operations and cause huge economic losses. Fishing tools are scientifically classified by their fishing methods, with six typical types widely applied in the industry: taper tap, die collar, overshot, fishing spear, junk basket, and fishing cup. Each tool has its unique structural features and working principle, suitable for specific downhole operation scenarios.

1. Taper Tap: Internal Thread-Forming Fishing Tool

The taper tap is an internal fishing tool designed to retrieve perforated fish by forming threads inside their inner holes. It features an integral long conical structure, consisting of a joint and fishing threads. The thread surface is hardened to HRC 60-65 for high strength and wear resistance, with common thread specifications of 55° thread angle and 8 threads per inch, which facilitates thread forming with lower torque. Its working principle involves lowering the tool into the fish's inner hole, applying appropriate drilling pressure, and rotating the drill string to force the fishing threads to squeeze into the inner wall and form threads. When the formed threads can bear sufficient tension and torque (usually 3 or more threads), the fish can be lifted or unscrewed and retrieved. It is mainly used for fishing casing, tubing, and drill pipes with inner diameters ranging from 30mm to 75mm in 140mm casings, especially effective for pipe-type fish with couplings.

2. Die Collar: External Thread-Forming Fishing Tool

As an external fishing counterpart to the taper tap, the die collar retrieves fish by forming threads on their outer walls. It has an integral long cylindrical structure composed of a joint and a body, with the same thread specifications and material strength requirements as the taper tap. Its working principle is similar to that of the taper tap: the tool is lowered to enclose the fish's outer surface, then drilling pressure is applied and the drill string is rotated to make the internal fishing threads bite into the outer wall and form a secure connection. The die collar is suitable for fishing pipes and rods with outer diameters between 67mm and 92mm in 140mm casings, especially for cylindrical fish without internal holes or with blocked inner holes. Its advantage lies in the large annular thread-forming area, which is less likely to damage the fish top or casing, though it has a smaller gap with the casing and poses a risk of sticking.

3. Overshot: External Clamping Fishing Tool

The overshot is a widely used external fishing tool primarily for retrieving smooth tubular fish. Its core structure includes an upper joint, a barrel, slips with serrated threads, and a guide shoe. The slips generate radial expansion force when the tool is lifted, ensuring a tight grip on the fish. During operation, the guide shoe helps align the overshot with the fish top, allowing the tool to enclose the fish. As the drill string is lifted, the conical fit between the mandrel and slips forces the slips to bite tightly into the fish's outer wall. A notable feature of most overshots is their retrievability: if the fish is stuck and cannot be lifted, applying a downward impact and rotating the drill string 2-3 turns can disengage the slips from the fish, enabling safe tool retrieval. Additionally, the overshot is equipped with a sealing ring, allowing well flushing circulation after entering the fish, and can even straighten slightly deformed fish tops.

4. Fishing Spear: Internal Clamping Fishing Tool

The fishing spear is an internal fishing tool that grasps fish from their inner cavities, with common types including sliding block spears and retrievable spears. Its structure typically consists of a mandrel, slips, a release ring, and a guide shoe. In the free state, the outer diameter of the slips is slightly larger than the inner diameter of the fish. When the spear enters the fish cavity, the slips are compressed to fit the inner wall, generating outward expansion force. Lifting the mandrel increases the radial force of the slips, making them bite firmly into the inner wall to achieve fishing. The retrievable design ensures operational flexibility: if the fish is stuck, applying a downward impact to the mandrel can disengage the slips from the mandrel's serrated threads, and rotating the drill string allows the slips to retract, enabling tool withdrawal. It is widely used for fishing tubing and drill pipes, and can also be used with jarring tools for jarring and stuck release.

5. Junk Basket: Debris Collection Fishing Tool

The junk basket is specialized in collecting small downhole debris, such as drill bit teeth, bearing fragments, and rock cuttings. It usually adopts a reverse circulation structure, composed of an upper joint, a barrel, a basket frame, and a milling shoe. After lowering the tool to the well bottom, a steel ball is dropped to block the central channel, forcing drilling fluid to circulate reversely through the barrel's annular space. The reverse circulation generates a vortex at the bottom, carrying debris into the basket frame. Some models are equipped with a magnetic core to adsorb ferrous debris, further improving fishing efficiency. During operation, the tool is rotated and lowered slowly to ensure full collection of debris, playing a crucial role in cleaning the wellbore and preventing secondary downhole accidents caused by debris.

6. Fishing Cup: Debris Settling Fishing Tool

The fishing cup is another tool for retrieving small debris, often used in conjunction with drilling or milling tools. Its design features a large-diameter barrel with a small-diameter mandrel at the cup mouth, creating a sudden change in drilling fluid flow velocity. When drilling fluid passes through the cup mouth, its flow velocity decreases significantly, reducing its carrying capacity and causing heavy debris to settle into the cup. Unlike the junk basket, the fishing cup requires no special operation and works synchronously with normal drilling or workover processes. It is particularly effective in catching heavy drilling cuttings and metal debris that cannot be carried out by normal mud circulation, helping to keep the well bottom clean, extend drill bit life, and reduce accidental bit damage.

These six fishing tools, classified by their distinct fishing methods, form a comprehensive downhole fishing system. Their rational selection based on fish type, size, and downhole conditions is crucial for improving fishing efficiency, ensuring operational safety, and minimizing project delays and economic losses in oilfield operations.