Water-locking agents: a key technological support for solving the water-locking problem in oil and gas extraction.

I. Water Locking: A “Hidden Obstacle” in Oil and Gas Extraction
  During oil and gas field development, especially in the exploitation of low-permeability and ultra-low-permeability oil and gas reservoirs, as well as unconventional oil and gas resources such as shale gas and coalbed methane, water lock acts like an “invisible hand,” severely restricting the effective production of oil and gas. Essentially, water lock occurs when external fluids (such as drilling fluid, completion fluid, and fracturing fluid) invade the reservoir pores, forming a water phase blockage at the pore throat, thus obstructing the flow of oil and gas and significantly reducing permeability.
  From the perspective of formation mechanism, when external fluids enter the reservoir, on the one hand, a water film forms on the surface of rock particles, increasing the resistance to oil and gas flow; on the other hand, under the action of capillary forces, the aqueous phase is easily trapped in the small pore throats, forming a “water blockage,” making it difficult for oil and gas to overcome this barrier and achieve effective production. Relevant data show that in the development of low-permeability oil and gas reservoirs, the water-locking phenomenon can lead to a 30%-80% reduction in reservoir permeability, and in some blocks, even the predicament of oil and gas wells shutting down immediately after production, causing huge economic losses to oil and gas field development.
  As oil and gas exploration and development continue to advance into deeper, low-permeability, and unconventional areas, the problem of water lock has become increasingly prominent. Statistics show that globally proven low-permeability oil and gas reservoirs account for over 40% of total reserves, with my country’s low-permeability and ultra-low-permeability oil and gas reserves representing an even larger proportion. In the development of these reservoirs, effectively mitigating water lock damage and restoring and improving reservoir permeability has become crucial for ensuring oil and gas production and enhancing development efficiency. Water lock removers have emerged in this context, becoming a core technological means to solve the water lock problem.

II. Classification of Water-Locking Agents: A Diversified Technical System
  Based on their chemical composition, mechanism of action, and application scenarios, water-lock deactivators can be divided into several categories. Different types of water-lock deactivators have different focuses in terms of performance characteristics and applicable scope, collectively forming a diversified water-lock deactivation technology system.
(I) Surfactant-based water-lock deactivators
   Surfactant-based water-locking agents are currently the most widely used type of product. Their core function is to improve reservoir wettability and promote the removal of retained aqueous phase by reducing the interfacial tension between oil and water and the contact angle between water and the rock surface. Based on the type of ion, they can be further classified into anionic, cationic, nonionic, and amphoteric surfactant water-locking agents.
  Anionic surfactants (such as sodium dodecylbenzenesulfonate and petroleum sulfonates) exhibit good interfacial activity, effectively reducing interfacial tension, and are relatively inexpensive, making them widely used in water-lock removal in conventional sandstone reservoirs. However, these surfactants tend to precipitate in highly salinized formation water, affecting their effectiveness. Cationic surfactants (such as hexadecyltrimethylammonium bromide) can alter the wettability of rock surfaces through adsorption, changing them from water-wet to oil-wet, thus facilitating oil and gas flow. However, they suffer from the problem of excessive adsorption, which can easily damage the reservoir.
  Nonionic surfactants (such as polyoxyethylene ethers and Tween surfactants) have good salt resistance and strong compatibility with other chemical agents, making them suitable for high salinity and complex formation conditions. Amphoteric surfactants (such as betaines) combine the advantages of anionic and cationic surfactants, exhibiting high interfacial activity, good salt and temperature resistance, and minimal damage to the reservoir, making them a hot research and application area in recent years. For example, alkyl betaines can effectively reduce the interfacial tension between fracturing fluid and reservoir rock during the flowback process after fracturing in shale gas reservoirs, improving flowback efficiency, reducing water-locking damage, and increasing shale gas well production by 15%-30%.
(II) Alcohol-based water-locking agents
Alcohol-based water-locking agents mainly include small molecule alcohols such as methanol, ethanol, isopropanol, and ethylene glycol. Their mechanism of action is mainly to reduce the viscosity and surface tension of water, improve the fluidity of the aqueous phase, and at the same time, utilize the hydrophilicity and volatility of alcohols to promote the evaporation and discharge of the retained aqueous phase.
  Methanol and ethanol have high volatility and solubility, allowing them to quickly mix with stagnant water in reservoirs, reducing surface tension and viscosity, making them suitable for water-lock release in shallow wells and low-temperature formations. However, these alcohols are relatively toxic and prone to compatibility issues with formation water, requiring strict dosage control. Isopropanol is less toxic than methanol and ethanol and has better salt tolerance, making it more effective in medium- and low-temperature reservoirs with moderate salinity. Polyols such as ethylene glycol and propylene glycol have higher boiling points and stability, making them suitable for deep wells and high-temperature formations. They maintain good water-lock release effects at higher temperatures and also provide some anti-waxing and anti-scaling properties.
  In practical applications, alcohol-based water-locking agents are often used in combination with surfactants to further enhance the water-locking effect through synergistic effects. For example, when isopropanol is combined with a nonionic surfactant for water-locking in low-permeability sandstone reservoirs, the reservoir permeability recovery rate can reach over 85%, which is far higher than the effect of using alcohols or surfactants alone.
(III) Polymer-based water-locking agents
  Polymer-based water-locking agents mainly include water-soluble polymers such as polyethylene glycol, polypropylene glycol, and polyvinylpyrrolidone. Their working principle is to form a protective film by adsorbing polymer molecules on the rock surface, reducing the adsorption and retention of water phase on the rock surface. At the same time, the thickening effect of polymer molecules is used to adjust the mobility ratio of the injected fluid, thereby increasing the sweep range of the water-locking agent in the reservoir.
  Polyethylene glycol (PEG) possesses excellent water solubility and biocompatibility. The hydroxyl groups on its molecular chain can form hydrogen bonds with hydroxyl groups on the rock surface, creating a stable adsorption film that effectively inhibits the adsorption of aqueous phase on the rock surface. Simultaneously, the viscosity of PEG increases with its molecular weight. By selecting PEG with an appropriate molecular weight, the viscosity of the water-locking agent can be adjusted, ensuring both effective diffusion within the reservoir and a certain sweep volume. In the development of low-permeability coalbed methane reservoirs, treatment with PEG-based water-locking agents can restore the permeability of the coal seam to over 90% of its original permeability, significantly increasing coalbed methane production.
  Polyvinylpyrrolidone (PVP) possesses excellent surface activity and dispersibility, enabling it to interact with clay particles in the reservoir, preventing clay swelling and migration. Simultaneously, the polar groups on its molecular chain reduce the surface tension of water, promoting the removal of stagnant water. PVP-type water-locking agents show outstanding performance in low-permeability reservoirs containing clay minerals, effectively avoiding secondary damage caused by clay swelling during the water-locking process.
(IV) Other Novel Water-Locking Agents
  With the continuous development of materials science and petrochemical technology, some new water-locking agents have emerged, providing new ideas and methods for solving the water-locking problem. These mainly include nanomaterial-based water-locking agents and biosurfactant water-locking agents.
  Nanomaterial-based water-locking agents utilize the small size, surface, and interfacial effects of nanoparticles to more effectively adsorb onto oil-water interfaces and rock surfaces, reducing interfacial tension and contact angle, and exhibiting excellent temperature and salt resistance. For example, surface-modified nano-silica particles can serve as highly efficient water-locking agents, maintaining stable performance even under high-temperature and high-pressure formation conditions (temperatures above 150℃ and pressures above 30MPa), achieving a reservoir permeability recovery rate of over 90%. Furthermore, nanoparticles can fill the micropores in the reservoir, adjusting the reservoir’s pore-throat structure and reducing water retention, further enhancing the water-locking effect.
  Biosurfactants, specifically water-lock deactivators, are a class of surface-active substances produced by microbial fermentation, such as rhamnolipids and sophorolipids. These water-lock deactivators are environmentally friendly, biodegradable, and have low toxicity, meeting the requirements of green oil and gas development. Simultaneously, biosurfactants possess excellent interfacial activity and wettability regulation capabilities, effectively reducing oil-water interfacial tension, improving rock surface wettability, and promoting the removal of stagnant water. In the development of oil and gas fields in ecologically sensitive areas, biosurfactants have shown broad application prospects. For example, after applying rhamnolipid water-lock deactivators in a low-permeability onshore oil field, oil well production increased by 20%-25%, without causing any pollution to the surrounding soil and groundwater environment.