Since water-based drilling fluids have always occupied a dominant position in practical applications, they are divided into several types according to the composition of the system. The following are the various drilling fluid types recognized in China based on the foreign drilling fluid classification standards.
(1) Dispersed drilling fluid
Drilling Fluids)
Dispersed drilling fluid refers to a water-based drilling fluid formulated with fresh water, bentonite and various treatment agents that disperse clay and drilling cuttings (referred to as dispersants). It is a type of commonly used drilling fluid with a long history of use, simple preparation methods and low preparation costs. Its main features are:
1. It can accommodate more solid phases and is more suitable for preparing high-density drilling fluid.
2. It is easy to form a denser mud cake on the well wall, so its filter loss is generally low.
3. Certain dispersed drilling fluids, such as trisulfonic drilling fluids that use sulfonated tannins, sulfonated lignite and sulfonated phenolic resin as the main treatment agents, have strong temperature resistance and are suitable for use in deep wells and ultra-deep wells. But it also has some disadvantages compared to other drilling fluid types. In addition to poor inhibitory and anti-pollution capabilities, the high solid content in the system has adverse effects on increasing drilling speed and protecting oil and gas layers.
(2) Calcium-Treated Drilling Fluids
The composition characteristic of calcium-treated drilling fluid is that the system contains both a certain concentration (mass concentration) of Ca2+ and a dispersant. Ca2+ undergoes ion exchange with sodium bentonite, which has a strong hydration effect, and converts part of the sodium bentonite into calcium bentonite, thereby weakening the degree of hydration. The function of the dispersant is to prevent Ca2+ from causing excessive flocculation of the clay particles in the system and keep it in a moderate flocculation state to ensure that the drilling fluid has good and stable performance. The characteristics of this type of drilling fluid are that it has strong resistance to salt and calcium pollution; it also inhibits the hydration and dispersion of clay in the formation being drilled, so it can control shale collapse and well diameter expansion to a certain extent. At the same time, it can reduce damage to oil and gas layers.
(3) Saltwater Drilling Fluids
Brine drilling fluid is formulated with brine (or seawater). This type exists over the entire range of salt content from 1% (Cl – mass concentration 6000 mg/l) up to saturation (Cl – mass concentration 189000 mg/l). Salt water drilling fluid is also a type of drilling fluid that has a strong inhibitory effect on clay hydration.
(4) Saturated Saltwater Drilling Fluids
It refers to the salt water drilling fluid system when the NaCl content in the drilling fluid reaches saturation. It can be made with saturated brine, or it can be made into drilling fluid first and then added with salt until it is saturated. Saturated salt water drilling fluid is mainly used to drill large rock salt layers and complex salt gypsum layers that are difficult to deal with other water-based drilling fluids. It can also be used as completion fluid and workover fluid.
(5) Polymer Drilling Fluids
Polymer drilling fluid is a water-based drilling fluid that uses certain high molecular weight polymers with flocculation and coating functions as the main treatment agent. Due to the presence of these polymers, the various solid particles contained in the system can be kept within a coarser particle size range. At the same time, the drilled cuttings are protected by coating in time and are not easily dispersed into fine particles. Its main advantages are:
1. The density and solid content of the drilling fluid are low, so the drilling speed can be significantly increased and the damage to the oil and gas layers is also small.
2. Strong shear thinning properties. Under a certain pump displacement, the viscosity and shear force of the annulus fluid are high, so it has a strong ability to carry cuttings; while at the high shear rate at the drill bit nozzle, the flow resistance of the fluid is small, which is beneficial to Increase drilling speed.
3. The polymer treatment agent has strong coating and dispersion inhibiting effects, so it is helpful to maintain the stability of the well wall. Therefore, since the 1970s, this type of drilling fluid has been widely used at home and abroad, and its process technology has been continuously improved and developed.
