A fracturing oil displacement agent and its preparation process

Technical Field

This application relates to the field of oilfield chemical technology, and more specifically, to a fracturing displacement agent and its preparation process.

Background Technology

Against the backdrop of continuously growing global energy demand, low-permeability reservoirs, tight reservoirs, and shale reservoirs, which are difficult to extract, have become the core targets of oil and gas development. Due to their complex pore structure, extremely low permeability, and poor crude oil flow, conventional extraction technologies achieve recovery rates of less than 15% for these reservoirs , necessitating the use of efficient stimulation and oil displacement technologies to improve development efficiency.

Hydraulic fracturing technology, a key method for developing difficult-to-extract oil reservoirs, creates artificial fractures in the formation by injecting fracturing fluid under high pressure and using proppant to maintain the fractures, providing a channel for oil and gas flow. However, fracturing only solves the “channel establishment” problem, resulting in limited improvement in recovery rate after fracturing. Therefore, simultaneously achieving “fracture creation” and “oil displacement” during the fracturing process has become the core direction for improving the recovery rate of difficult-to-extract oil reservoirs, leading to the development of fracturing-displacement agents.

Fracturing displacement agents are a class of chemical reagents that integrate the dual functions of fracturing assistance and oil displacement enhancement. Their core function is to enhance the flow and production of crude oil from the matrix to the fractures by reducing oil-water interfacial tension, improving rock wettability, emulsifying crude oil, or expanding the sweep range, based on the fracturing fluid’s role in creating fractures and carrying proppant. Currently, existing fracturing displacement agents are mainly classified into the following categories: surfactant-based, polymer-based, gas-based, and composite system-based. For example, patent application CN117186863A discloses an oil displacement agent used in a fracturing fluid system and its preparation method. This oil displacement agent includes the following components: a surfactant and microencapsulated ammonium bicarbonate. The surfactant includes alkyl betaine, alkyl glucoside, and alkyl sulfonate. During the oil displacement process, the polystyrene capsule walls of the microencapsulated ammonium bicarbonate gradually swell and break down, releasing ammonium bicarbonate, which then precipitates dioxide. Carbon and ammonia can, on the one hand, create gas resistance, increase the sweep efficiency, and improve oil displacement efficiency; on the other hand, they can cause crude oil to form an emulsion, reduce interfacial tension, and improve oil displacement efficiency. Although microencapsulated ammonium bicarbonate can improve oil displacement efficiency, the carbon dioxide produced by the decomposition of ammonium bicarbonate combines with formation water to form carbonic acid, which lowers the pH value of the system and aggravates acid corrosion of the wellbore, leading to thinning of the well wall, perforation, or scaling. If the wellbore is blocked or damaged, it will hinder the normal pumping of fracturing oil displacement agents, causing the oil displacement agents to not reach the target reservoir area evenly, reducing the sweep range, and thus affecting the oil displacement efficiency of the oil displacement agents.

Summary of the Invention

To improve the oil displacement efficiency of the oil displacement agent in the reservoir environment, this application provides a fracturing oil displacement agent and its preparation process.

In a first aspect, this application provides a preparation process for a fracturing displacement agent, employing the following technical solution:

A process for preparing a fracturing oil displacement agent involves microencapsulation of a polyurea-coated surfactant composite microcapsule using a surfactant as the core material and a polyurea as the wall material.

By adopting the above technical solutions, polyurea wall materials, with their temperature- and salt-resistant rigid structure, can protect the core material surfactants from degradation or premature adsorption loss in high-temperature and high-salt reservoir environments. At the same time, by controlling the pore size of the capsule wall, the surfactants can be slowly released, ensuring their continuous effect in the deep reservoir. The released surfactants enhance the ability of crude oil to peel off and flow from the rock surface by reducing the oil-water interfacial tension, emulsifying crude oil, and improving rock wettability. The polyurea microcapsules themselves can adjust the mobility of the displacing phase by sealing the pores, expanding the coverage area, and improving the oil displacement efficiency of the oil displacement agent.

Detailed Implementation

The present application will be further described in detail below with reference to the embodiments. All raw materials involved in the present application can be obtained commercially.

Example 1

This embodiment provides a preparation process for a fracturing displacement agent, including the following steps:

Dissolve 10g of sodium heavy alkylbenzene sulfonate in 90mL of deionized water to form an aqueous solution;

100 mL of aqueous solution was added to 400 mL of cyclohexane containing 2 wt% Span -80 , and stirred at 3000 r/min to form an emulsion; wherein the 400 mL of cyclohexane containing 2 wt% Span -80 was obtained by adding 6.5 mL of Span -80 to 393.5 mL of cyclohexane and stirring until homogeneous;

7.63 g of isophorone diisocyanate was added dropwise to 100 g of emulsion at a rate of 1 mL/min. After stirring for 30 min , 8 wt% ethylenediamine aqueous solution was added, and the mixture was reacted at 50 ℃ for 3 h to obtain the product. The product was centrifuged at 3000 r/min for 15 min and filtered to obtain the precipitate. The precipitate was washed three times with the same volume of anhydrous ethanol and twice with the same volume of deionized water, and then vacuum dried at 60 ℃ for 3 h to obtain polyurea-coated surfactant composite microcapsules. The 8 wt% ethylenediamine aqueous solution was prepared by adding 2.37 g of ethylenediamine to 27.26 mL of deionized water and stirring until completely dissolved.

Performance testing

The fracturing oil displacement agents prepared in each embodiment and comparative example were diluted 200 times with distilled water to obtain the reagent.

Interfacial tension

Using crude oil as the oil phase, crude oil and the reagent were mixed at a mass ratio of 1:1. The interfacial tension of the oil displacement agent with Examples 1-16 and Comparative Example 1 was tested using a TX500D rotating drop interfacial tensiometer. The rotation speed was 5000 rpm and the temperature was 70 ℃. The test results are shown in Table 1 .

Oil displacement efficiency

Water displacement was performed using saturated crude oil from artificial cores with a gas permeability of 10 mD. The volume of water discharged was recorded. The reagent was added to the oil washing device containing the core. Immersion tests were conducted on Examples 1-16 and Comparative Example 1 at 70 °C. When the volume of discharged oil remained unchanged for 72 hours , the total volume of discharged oil was recorded. The oil displacement efficiency was calculated using the following formula: Oil displacement efficiency = (Efficiency of Displacement) × 100. The test results are shown in Table 1 .

Table 1 Performance Test Data

	Interfacial tension ( 10⁻³ mN/m)	Oil displacement efficiency ( % )
Example 1	0.93	52.1

from Example 1 and Comparative Example 1 , and referring to Table 1 , the polyurea-coated surfactant composite microcapsules prepared by microencapsulation using surfactant as the core material and polyurea as the wall material in this application exhibit lower interfacial tension and better oil displacement efficiency compared to using a mixture of surfactant and microencapsulated ammonium bicarbonate as a fracturing oil displacement agent. The polyurea wall material, with its rigid structure resistant to temperature and salt, protects the core material surfactant from degradation or premature adsorption loss in high-temperature, high-salt reservoir environments. Simultaneously, by controlling the pore size of the capsule wall, the surfactant is slowly released, ensuring its continuous effect in the deep reservoir. The released surfactant enhances the stripping and flow of crude oil from the rock surface by reducing oil-water interfacial tension, emulsifying crude oil, and improving rock wettability. Furthermore, the polyurea microcapsules themselves can adjust the mobility of the displacing phase through pore sealing, expanding the sweep range and improving the oil displacement efficiency of the oil displacement agent.

from Examples 1 , 4 and 5 and Table 1 , this application reduces the interfacial tension of the prepared fracturing oil displacement agent and improves the oil displacement efficiency by using a mixture of heavy alkylbenzene sulfonic acid and fatty alcohol polyoxyethylene ether as a surfactant compared to using a surfactant alone. When combined, the negative charge repulsion of sodium alkylbenzenesulfonate prevents excessive aggregation of nonionic molecules, while the steric hindrance of fatty alcohol polyoxyethylene ether alleviates salting out of anions in high-salt environments, jointly reducing interfacial tension and weakening the adhesion between crude oil and the rock surface. Furthermore, fatty alcohol polyoxyethylene ether can reduce metal ion precipitation by encapsulating the hydrophilic groups of sodium alkylbenzenesulfonate with polyoxyethylene chains, while simultaneously controlling the emulsion to be easily displaced and demulsifiable ( O/W) type. In addition, sodium alkylbenzenesulfonate preferentially modifies the strongly oil-wetted areas of the rock through electrostatic adsorption, converting them to water-wet regions, while fatty alcohol polyoxyethylene ether inserts into the crude oil adsorption layer on the rock surface through hydrophobic chains, gradually replacing crude oil molecules and modifying the weakly oil-wetted areas. The combination of these two can achieve comprehensive modification of rock surfaces with different wettability, increasing the sweep efficiency of the oil displacement agent in the pores. In summary, the combination of sodium alkylbenzenesulfonate and fatty alcohol polyoxyethylene ether can improve the oil displacement effect of the oil displacement agent.