During drilling operations, to prevent formation fluid intrusion into the well, keep the drilling fluid column pressure slightly higher than the formation pressure at all times. However, in the actual drilling operation, the pressure of the drilling fluid column in the well is often lower than the formation pressure because of various influences, so that the balance of the pressure in the well is destroying, which leads to the generation of blowout. As a result, human casualties, rig damage, reservoir damage and so on.
Well control equipment
Well control equipment is a comprehensive set of specialized equipment, instrumentation, and tools usin to implement well pressure control techniques. Therefore, it is an important guarantee for well pressure control to understand the performance of well control equipment, install and maintain it correctly, and make it play its due function. One of the most important pieces of equipment is the BOP. There are two main types of blowout preventers: universal blowout preventer and ram blowout preventer.
The annular preventer is naming for its sealing element, the rubber core ring. Also known as the universal blowout preventer, multi-effect blowout preventer, or ball preventer.
According to the shape of the rubber core can be dividing into: conical ring preventer, spherical ring preventer, cylindrical ring preventer;
Annular BOP function
- 1) When there is a string in the well, the annular space formed by the string and the wellhead can be closing with a rubber core;
- 2) The well head can be fully sealing when the well is empty;
- 3) The space formed by kelly pipe, cable, wire rope, accident handling tools and wellhead can be sealing in case of overflow or blowout in the process of drilling and milling, casing grinding, logging and debris fishing;
- 4) With the cooperation of decompression regulating valve or small energy storage device, it can forcibly pull down the 18° non-fine buckle butt welded pipe string joint;
- 5) In case of severe overflow or blowout, it is using to achieve soft shut-in with ram BOP and throttle manifold.
Ram blowout preventer
Ram preventers are known as ram preventers that hydraulically p ush the capping element — two RAMS with a rubber core — from the left and right to the center of the hole to seal the wellhead.
First of all,according to the number of ram, it can be divided into single ram preventer, double ram preventer and three ram preventer.
Second,according to the role of ram, it can be dividing into: full seal ram BOP, half seal ram blowout preventer, shear ram blowout preventer, variable diameter ram blowout preventer;
According to the locking method, it can be dividing into manual locking ram blowout preventer and hydraulic locking ram BOP;
According to the side door switch mode, it can be dividing into: rotating side door ram blowout preventer linear motion side door ram blowout preventer;
► Ram BOP function
- 1) When there are drilling tools in the well, the annular space of the wellhead can be closing with half sealing ram corresponding to the size of the drilling tools;
- 2) When there is no drilling tool in the well, the fully sealed ram can completely seal the wellhead;
- 3) When it is necessary to cut the drilling tool in the well and seal the wellhead completely, shear ram can be using to cut the drilling tool in the well and seal the wellhead completely;
- 4) Some ram BOP RAMS allow load bearing to be using to hang the drill;
- 5) The shell of the ram BOp has side holes, which can be using for pressure relief;
- 6) Ram preventers can be using for long-term well capping;
In the field application of the blowout preventer, multiple combinations are using according to the needs. Generally, the upper part is the universal blowout preventer, the middle part is the variable diameter ram blowout preventer, and the bottom part is the shear ram blowout preventer. There are many different combinations according to the needs of the field. The various combinations of blowout preventers are collectively known as blowout preventer packs (BOPs).
Choice of preventer combination
The main factors that should be considering in the selection of hydraulic blowout preventer combination are: well type, formation pressure, casing size, formation fluid type, personnel technical status, process technical requirements, climate impact, traffic conditions, material supply status and environmental protection requirements. In short, it should be able to achieve balanced drilling, ensure drilling safety and save drilling costs.
- 1) The selection of pressure level
The operating pressure of the hydraulic BOP assembly depends on the internal compressive strength of the casing used, the fracture pressure of the open hole formation at the casing shoe, and the maximum wellhead pressure expected to be appliing. But it is primarily determined by the maximum wellhead pressure that the BOP combination is expecting to experience. There are five pressure levels of the blowout preventer: 14MPa, 21MPa, 35MPa, 70MPa, 105MPa and 140MPa.
- 2) Path selection
The diameter of the BOp package depends on the casing size in the structural design of the well, which must be slightly larger than the outer diameter of the casing to which it is attaching. There are nine diameters of the blowout preventer: 180mm, 230mm, 280mm, 346mm, 426mm, 476mm, 528mm, 540mm and 680mm. Among them, 230mm, 280mm, 346mm and 540mm are commonly using on site.
- 3) Selection of combination form
The selection of combination form is mainly based on formation pressure, drilling process requirements, drilling tool structure and equipment supporting conditions.