By the end of 2018, more than 80% of the gas reservoirs in China that had been put into development were water-bearing ones, and their gas reserves accounted for 75% of the total gas reserves. They constitute a corner stone for stable or higher gas production. The number of water-yielding wells in such gas reservoirs is rapidly increasing as the development proceeds. On the one hand, the formation water entering the wellbore increases the flow energy loss of both gas and water phases, and as a result, the gas wells have lower natural flow ability and their productions are dramatically reduced. According to statistics on pre- and post-water yielding single-well gas productions of eight typical gasfields of CNPC, the productions of the gas wells decreased by 20%-85% after water yielding occurs, and even some gas wells stop producing gas due to water flooding^[1,2,3]. On the other hand, large quantities of gas reserves cannot be recovered because invading waters block gas reservoirs, thus reducing their recovery ratios. Domestic and international development practices in dozens of years show that deliquification technologies for gas production are main technologies to ensure stable gas produc-tion and increase recovery ratio. Among these technologies, the foam deliquification for gas production is most widely used due to ease of operation, quick effect, and low cost^[4,5]. CNPC's annual number of gas wells with foam deliquification is nearly 70% of its total annual number of gas wells with unloading measures. It has carried out more than 90 000 foam deliquification operations. The technology plays an important role in stabilizing gas productions of old gas fields.

Dozens of foaming agents have been developed across the world. Most of them are combinations of several surfactants and contain additives used for foam stabilization^[6]. Typical foam unloading surfactant categories include ampholytic surfactant^{[7,8,9,10,11]}, polymer surfactant^[12,13], anionic surfactant^[14,15], and hyamine cationic surfactant^[16], etc. In recent years, the development degrees of China's old gasfields have become deeper and deeper, and the number of complex gas reservoirs such as those containing high-temperature and/or high-salinity formation water, high-acidity gas, and/or high condensate has been increasing. Thus, the foaming agent technology as the core of the foam deliquification for gas production faces the problems of poor adaptability and high cost and its efficiency and benefits have been seriously affected. In addition, there are various water-yielding gas fields in China, and there are different controlling factors affecting the performance of the foaming agent. The foaming agents available now cannot meet the needs of these gas fields in terms of high-efficiency and low-cost unloading for gas production. So, it is significant for China's water-yielding gas fields having stable productions and higher recovery ratios to develop and apply nano-particle foaming agent series applicable for China's main gas fields.

In order to solve the problems, we established a development idea featuring the use of the Gemini surfactant as the main agent to improve the foaming performance and the foam stabilizing performance of the foaming agent, the graft trimming of nano-particles as the foam stabilizing agent to further improve the stability of the created foam, and the optimization of the characteristic additives for it to be applicable for various gas reservoirs. The Gemini surfactant, with a special molecular structure, can effectively enhance the inter-molecule cohesive force as well as the viscous elasticity of the created foam's film^[17,18]; the graft-trimmed nano-particle foam stabilizing agent is dispersively adsorbed on the liquid film to form tight film and prevent bubble agglomeration and disproportionation during the foam creation^[19]; optimal characteristic additives for different gas fields are selected to bring about the synergetic effect between different characteristic additives and those between the main agent and foam stabilizing agent, and thus, nano-particle foaming agent series suitable for China's main gas fields are developed. Through in-house tests and field applications for performance and cost comparisons with the foaming agents frequently used in China, its performance advantages and field adaptability were examined.

During the development of the foaming agent series, it is necessary to systematically evaluate the performance of the created foam. Methods characterizing foam properties are numerous, and the commonly used methods include vibration method^[20], stirring method^[21], and Ross-Miles method^[22], etc. The Ross-Miles method is the foaming agent evaluation method recommended in the related industry standard currently^[23]. However, this method requires no pressure loading and maximum allowable temperature of 90 °C during the evaluation. Thus, the evaluation based on this method is considerably different from the actual situation of the gas well and cannot truly reflect the key performance parameters of the foaming agent under the wellbore conditions. In the field applications, the lab evaluation conclusions are often inconsistent with the field effects. Therefore, the foaming agent evaluation apparatus^[24] developed by us before was used for performance testing and evaluation during the development of the foaming agent series (Fig. 1). With the maximum evaluation temperature and pressure of 200 °C and 25 MPa respec-tively, the apparatus can truly reflect the wellbore condition and provide test foundation for foaming agent synthesis and evaluation.

1.2.1. Synthesis and test of Gemini surfactant

The Gemini surfactant was developed through using a connecting radical to connect two or more surfactant molecules at or near a hydrophilic group. Its special comb-like structure is favorable for forming an adsorption film on which molecules^[17], and its double-tail chain can effectively enhance the cohesive force between adsorbed molecules and dramatically increase the viscous elasticity of the adsorption film as well as the stability of the created foam. Thus, the basic properties of the foaming agent can be effectively enhanced if the Gemini surfactant is used as the main agent. A Gemini surfactant was synthesized by changing the linker length, connecting a hydroxyl radical to the linker, and increasing the length of the tail chain, etc.^[25,26], and it was tested and evaluated. In the test, the temperature, formation water salinity, and H₂S content were set at 160 °C, 250 000 mg/L, and 100 mg/L respectively to reflect the formation and wellbore flow conditions of most gas fields in China; in addition, the test pressure was set to 15 MPa because the bottom-hole pressures in most of the gasfields with unloading for gas production are below 15 MPa and the impact of the pressure on the performance of the foaming agent is not significant when the pressure is higher than 8 MPa^[24] (for gas wells with the bottom-hole pressure higher than 15 MPa, it is unnecessary to further increase the test pressure). The test results are as shown in Figs. 2 and 3. The foaming property, foam stabilizing property, and liquid carrying ability are characterized respectively by initial foaming volume, half-life period, and liquid carrying rate.

The test result curves show that with the increase of its concentration, the foaming property, foam stabilizing property, and liquid carrying ability of the Gemini surfactant were strengthened and then weakened, and they reached the maximum values at the volume fraction of the foaming agent of 0.3%, i.e. critical micellar concentration. Therefore, the optimal volume fraction of the main agent was determined to be 0.3%. To make test results consistent and comparable, all the following tests were done at this concentration.

1.2.2. Nano-particle foam stabilizing agent and its preparation procedure

Foam system is a thermodynamically unstable system^[27]. It takes several to dozens of minutes for the foam to be carried along with the gas flow to the wellhead after its creation at the bottom of the well. If the created foam is not stable and broken midway, the liquid carried by it would drop to the bottom to form a liquid accumulation and unloading for gas production fails. So, it is very crucial for developing a high-performance foaming agent to have a high-performance foam stabilizing agent. Graft-trimmed nano-particles are very effective in improving the stability of the foam system^[28]. Its working mechanism is that the nano-particles adsorb on the gas-liquid interface at a certain contact angle during the creation of the foam to form a tight nano-particle film that can prevent bubble agglomeration and disproportionation and dramatically improve the stability of the foam. The preparation of a nano-particle foam stabilizing agent typically consists of two steps^[19]: (1) the Stöber hydrolysis method is used to prepare <100 nm spherical nano silicon dioxide particles; (2) graft modification of the silicon ball solution rich in hydroxyl radicals could be carried out by silane coupling agent, obtaining nano-particle solid foam stabilizing agent with contact angle of 65°-85°.

1.2.3. Optimization of ratio of the main agent to nano-particle foam stabilizing agent

In order to work out the optimal ratio of the Gemini surfactant to the nano-particle foam stabilizing agent, the main agent was prepared into solution with the concentration of 0.3%, and then, different quantities of nano-particle foam stabilizing agent were added to examine the properties of the system at different nano-particle contents. The test temperature, formation water salinity, H₂S content, and test pressure were set at 160 °C, 250 000 mg/L, 100 mg/L, and 15 MPa respectively. The test results are shown in Table 1. The test results show that the foaming property, foam stabilizing property, and liquid carrying ability of the Gemini surfactant get better and then worse as the nano-particle mass concentration increases, and they reach the maximum values at the mass concentration of 60 mg/L. Thus, the basic system was selected as a solution with the mass proportion of the main agent and the nano-particle foam stabilizing agent of 50:1. Under the test conditions, the basic system has the initial foam volume, foam half-life period, and liquid carrying rate of 2 204 mL, 936 s, and 12.1 mL/min respectively. It can work well at high temperature, high salinity, and high acidic gas content.

Temperature, formation water salinity, acidic gas content, and condensate content are important factors affecting the performance of the foaming agent^[29]. Gas fields in China developed are of various kinds, they vary widely in temperature and fluid composition, so a specific foaming agent needs to be developed according to the wellbore environments and fluid characteristics of the specific gas field. One kind of foaming agent is suitable for only one or several kinds of formation fluids. A foaming agent that is applicable for all the gas fields and of good quality hasn’t been developed. In order to develop nano-particle foaming agent series suitable for different kinds of gas fields, the gas fields need to be classified firstly according to the wellbore temperature and fluid characteristics; accordingly, foaming agents can be classified. For this reason, the related data (formation temperature, and wellbore fluid, etc.) of 11 typical gas fields in six main gas-producing basins of China were collected (Table 2).

Table 2 shows that the formation temperature and salinity values of the 11 typical gas fields are high except those of Sebei gas field; some of them are rich in acidic gas and some are rich in condensate. The basic system has high resistance to high temperature, high salinity, and high acid gas, but is not resistant to condensate. The presence of condensate allows the wellbore fluid to be in the three-phase state rather than in the two-phase state. The foaming agent that should be accumulated at the gas-water interface is mostly present at the oil-water interface, and therefore, the foam creating ability decreases dramatically or even no bubbles are created. For the gas field containing condensate, characteristic auxiliaries specially designed to resist condensate needs to be selected. In general, such additives are costly. In addition, the gas fields containing no condensate do not need them. So according to the produced fluid containing condensate or not, the nano- particle foaming agent series are classified into two main categories considering property and cost, the one resistant to condensate and the one not resistant to condensate.

After the initial classification, the two main categories are classified into six foaming agent series according to the reservoir temperature and wellbore fluid properties of the specific gas fields and the needs in terms of cost reduction and production increase, and a set of high-standard performance indexes (Table 3) has been set to meet the drainage gas recovery needs of the main gas fields according to the key factors of the gas fields affecting the performance of the foaming agents.

To develop the foaming agent series, different characteristic auxiliaries need to be selected according to the main factors of the gas fields affecting their performance on the basis of the basic system, so as to further improve the performance of the system. The kinds of characteristic auxiliaries required by the six foaming agent systems are shown in Table 4.

The development processes of the six foaming agent systems are similar. The differences of them lie in kind, quantity, and usage of characteristic auxiliaries. So, the high temperature-resistant and high condensate-resistant system is taken as an example to describe the development process. The test conditions were set at the temperature of 160 °C, the formation water salinity of 50 000 mg/L, the condensate content of 40%, and the pressure of 15 MPa. Under these conditions, the initial foam volume and half-life period of the basic system are respectively 645 mL and 52 s. The previous test results show that the liquid carrying performance is positively correlated with the foaming property and foam stabilizing property. In order to reduce the number of tests, the test samples were evaluated only in terms of foaming and foam-stabilizing properties, and the final formula of system was optimized first, then the system performance was evaluated.

In order to maximize the high condensate-resistant performance of the system, four condensate-resistant auxiliaries were selected out of more than 20, and binary matching and orthogonal tests were done to work out the mixing proportions of the basic system and the four characteristic auxiliaries (A and B are fluoro-carbon additives and C and D are poly-silicon additives) to improve the overall performance of the system by giving full play to the synergetic effects between the basic system and characteristic auxiliaries and those between the characteristic auxiliaries. The test procedure included two steps: (1) A binary mixing proportion test of the basic system and the four characteristic auxiliaries to determine the optimal mixing proportions of the basic system and the four condensate-resistant characteristic auxiliaries, the test results (Table 5) show that the condensate-resistant property of the system containing condensate-resistant auxiliaries dramatically improves, and the optimal mixing proportions of the basic system and the four condensate-resistant characteristic auxiliaries are: 15:1, 5:1, 10:1, and 1:1. (2) An orthogonal test of the basic system and the four characteristic auxiliaries to determine the optimal proportion between auxiliaries and the final optimal proportion with basic system, and thus the final formula of the high temperature-resistant and high condensate-resistant system (the test results are shown in Table 6). Table 6 shows that the performance of the system in the orthogonal test is much higher than that of each binary system, and the performance of the sample 5 with the initial foam volume of 2 215 mL and the half-life period of 817 s is the best.

According to the same procedure, the proportions of the other five foaming agent systems were optimized and their performances were systematically evaluated. The test results are all up to the index requirements set in Table 3, forming the nano-particle foaming agent series suitable for China's main gas fields. The temperature resistance, salinity resistance, H₂S resistance, CO₂ resistance, and condensate resistance of the foaming agent series can reach 160 °C, 250 000 mg/L, 100 mg/L, 100%, and 40% respectively. For concise expression, the phrase "nano-particle foaming agents" will be used to represent the various kinds of foaming agents belonging to the different nano-particle foaming agent series in the following comparison lab tests and field application analyses with conventional foaming agents.

In order to compare the performance and cost of the nano- particle and conventional foaming agents, foaming agents widely used in China's main gas fields were collected for the systematic comparison tests. The test results show that the nano-particle foaming agents are superior to the conventional ones in terms of performance and cost. The high temperature-resistant, high salinity-resistant and high condensate-resistant system is taken as an example to elucidate.

2.3.1. Performance comparison

Four kinds of conventional foaming agents used in Sulige gas field of Changqing and Dongping gas field of Qinghai were collected to compare with the nano-particle foaming agent series. These gas fields feature high temperature, high salinity, and high condensate content. In order to reduce the number of tests, the formation water salinity and condensate content were set at the maximum values of 250 000 mg/L and 40% respectively. When the initial foam volume and foam half-life period were evaluated, the temperature was used as a variable for testing; when the liquid carrying rate was compared, the test temperature was set at 160 °C; in the tests, the pressure was set at 15 MPa, which is close to the field values. The test results are shown in Figs. 4 to 6.

The comparison results show that the foaming property, foam stabilizing property, and liquid carrying ability of the high temperature-resistant, high salinity-resistant, and high condensate-resistant nano-particle foaming agent series are obviously higher than those of the conventional foaming agents, their advantages are more obvious when the temperature is higher, and their half-life periods are more than twice those of the conventional foaming agents when the temperature reaches 130 °C. It can be seen that the nano-particle foaming agent series can meet the unloading needs of the gas fields with complex well conditions including high temperature, high salinity, and high condensate.

2.3.2. Cost comparison

Currently, the sales prices of the agents are mainly used for cost comparison. The shortcoming of the method is that it doesn’t consider the performance of the agents and cannot reflect the comprehensive cost of the foaming agents in the field. A foaming agent with a lower price may perform poorly. In the field, the usage of the foaming agent needs to be increased in order to achieve the expected production. As a result, its total cost may be higher than that of the foaming agent with higher price yet good performance and less usage. In order to solve this issue, the parameter "cost per ton of drained water", the agent cost required to drain one ton of water, was used for the comparison. This method considers both cost and performance.

The kinds of foaming agents and test conditions used in the cost tests are the same as those in the performance tests. The comparison results are shown in Table 7. The test results show that the costs per ton of drained water of the nano-particle foaming agents are over 45% lower than those of the conventional foaming agents of the same kind.

From 2015 to 2018, the nano-particle foaming agent series were applied 8685 times in low-production, low-pressure, and water-yielding wells in multiple gas fields in the Changqing, Xi’nan, Qinghai, and Daqing areas. The wells treated had an average single well production increase of 62.48%, an average decrease of tubing-casing pressure difference of 18.9%, and an average comprehensive cost reduction of over 45% than the conventional foaming agents, realizing significant cost reduction and production increase.

In order to fully verify the application effect of the nano- particle foaming agent series in the field, the high temperature-resistant and high condensate-resistant system was taken as an example, and three groups of field application comparison tests were conducted in Sulige gas field. The test process lasted two years, its field application results were systematically examined.

3.2.1. Overview of the test area

Sulige gas field is the largest gas field in China, featuring low permeability and low abundance, etc^[30]. It has an average well depth of 3 000 m, formation temperatures of 90-120 °C, formation water salinity of 30 000-50 000 mg/L, and condensate content of 5%-20%. By the end of 2018, wells with low production and low pressure accounted for more than 60% of the total wells in the gas field, and many gas wells had serious liquid loading. Now, unloading measures have been used widely across the gas field. Foam unloading was adopted in 70% of the wells. With the development going on, both the formation pressure and production decrease constantly, the conventional foaming agents are poorer in adaptability and higher in cost.

3.2.2. Comparison of application results of nano-particle and conventional foaming agents in gas wells of the same kind

In 2016, field comparison tests (same-block and same-well type) were carried out in three blocks of Sulige gas field. Before the test, the wells were almost the same in benchmark production, tubing pressure, and casing pressure. The test duration was two months. The test results show that compared with the conventional foaming agents, the nano-particle foaming agents was 15.04% higher in effective rate. The tested wells had an average cumulative gas production increment of 125.87%, an average reduction of agent usage per 10 000 cubic meters of natural gas production of 56.05% lower, and reduction of operation times of 61.26% (Table 8).

3.2.3. Comparison of nano-particle and conventional foaming agents in the same well

In 2017, comparison tests, in which conventional foaming agents were used firstly and nano-particle foaming agents secondly, were done in 45 wells in a block of Sulige gas field. From May to July 2017, conventional foaming agents were used for unloading in the 45 wells; in July to September 2017, nano-particle foaming agents were used in them. After the application of the nano-particle foaming agents, the wells had an increase of average effective rate of 8.52%, average gas production increment of 39.76%, a reduction of average usage per 10 000 cubic meters of increased gas of 32.73%, and a reduction of operation times per 10 000 cubic meters of increased gas of 17.86%.

3.2.4. Comparison of application results of nano-particle and conventional foaming agents in the same wells in two years

From 2015 to 2016, comparison tests were done in eight wells in a block of Sulige gas field. From May to September 2015, conventional foaming agents were used in the eight wells. From May to September 2016, nano-particle foaming agents were used in the same eight wells. Through systematic comparison, on the premise that the natural production decline within the two years is not considered, when using nano-particle foaming agents, the wells had an average usage reduction of 50%, average daily gas production increase of 8.04%, and an average tubing-casing pressure difference drop of 20.83% than using conventional foaming agents (Table 9).

The nano-particle foaming agent series have been applied on a large scale in gas fields of different kinds, and the results show that they have high adaptability and high efficiency. Now, the low-production and water-producing wells in China's various gas fields have exceeded 8000 and are increasing at the rate of 3% to 5% every year. Most of them are suitable for foam unloading. According to the previous field application results, when widely used, the technology will play an important role in unloading water, increasing gas production and reducing cost, so it will have a broad application prospect.

Using the Gemini surfactant as the main agent, six series of nano-particle foaming agents in two categories suitable for China's main gas fields have been developed, with temperature resistance, salinity resistance, H₂S resistance, CO₂ resistance, and condensate resistance of 160 °C, 250 000 mg/L, 100%, 100%, and 40% respectively. The nano-particle foaming agents are superior to the conventional ones in terms of foaming property, foam stabilizing property, liquid carrying ability, and cost. The higher the temperature, the better their foam stabilizing property is. When the temperature is higher than 130°C, their half-life periods are more than twice those of the conventional ones. Their cost per ton of drained water is more than 45% lower than the conventional ones.

The nano-particle foaming agents have been applied 8685 times in China's main gas fields including Changqing and Xinan, fully verifying that they are superior to conventional foaming agents in improving gas well production and adaptability. Compared with the conventional foaming agents, they increase the average per-well gas production rate by 62.48% and reduce the tubing-casing pressure difference by 18.9%. Their usage and comprehensive cost are more than 45% lower. They are strikingly effective in cost reduction and production increase and have a good application prospect.