Yang, Qinghai

After three years of technological breakthrough and field practices, a complete set of patented waterless fracturing operation technology and supporting equipment with independent intellectual property rights have been invented. Obvious oil production increase effect has been observed in multiple field tests on tight sandstone oil reservoir. In this paper, the latest development of Chinese liquid CO₂ fracturing technique is introduced through a case study of typical well.

The target depth of operating well is 1730.8m-1757m, with a daily fluid production of 1.6t and oil production of 0.5t, indicating that the displacement relationship has not been effectively established. The fracturing treatment was carried out using the independently developed equipment system, which has treatment capability of available pump rates up to 12 m³/min, sand transportation of 27 m³ and CO₂ injection of 1000 m³. During this operation, 860m3 liquid CO₂ was injected at a displacement of 5-6m³/min. and 23m³ proppant was preloaded and totally pumped into reservoirs with maximum instantaneous proppant concentration of 12%.

After the fracturing, the daily fluid production increased from 1.9t to 3.9t, the daily oil production increased from 0.7t to 2.3t and the water cut decreased from 63.2% to 41.0%, achieving a significant increase in production. In addition, the oil pressure of one adjacent well increased from 0.5Mpa to 12.4Mpa and the daily oil production of four adjacent wells increased by 0.7-1.1t through the energy enhancement and miscibility of CO₂.

The field test shows that liquid CO₂ fracturing technology has a significant effects of energy storage and stimulation, adjusts the injection-production relationship effectively, and greatly enhances the single well production. It is expected to become the key technology of the development of tight sandstone oil resources.

The salinity measurement of the down-hole water is an important parameter to optimize a production scheme for oil exploitation. However, salinity is generally measured offline by physical and chemical analysis and it has been difficult to implement a real-time measurement. With the online conductance sensor, the water salinity can be obtained indirectly based on prior knowledge. In order to solve the down-hole salinity monitoring problem, this paper proposes an embedded sampling system for real-time conductance measurement. The conductance sampling system is designed with embedded technology to achieve continuously monitoring. Based on an ARM-based embedded kernel, peripheral extended circuits are constructed for the sampling system, including an excitation source, a signal conditioner, an analog-to-digital converter, a memory module, a communication port, and a power supply module. The active excitation source generator can output a sinusoid signal with an adjustable amplitude and frequency to the electrical poles. The voltage signal from measurement poles is then sampled after adaptive signal conditioning. With a simplified Fast Fourier Transform (FFT) algorithm, the effective value is measured and the quality of the sampled signal is analyzed to feedback-regulate the excitation source and conditioner. Therefore, the stability and accuracy can be improved for conductance measurement. The conductance measurement system is tested in a physical testing environment with different conditions of water salinity, oil-water mixing ratio, liquid temperature and flow rate. Testing results show that the sampled conductivity value keeps stable and accurate with the calibrated reference conductance meter. The system has a wide measurement range together with a high accuracy and resolution. When the temperature changes, the measured conductivity value basically does not vary, which shows that the temperature compensation is effective. When measuring different salinities of water, the system can adaptively adjust the output amplitude and signal conditioner gain to get suitable sampling precision. In brief, the system provides real-time conductance measurement and the measuring accuracy can satisfy the requirements of petroleum production optimization. To ensure the system may be run down-hole permanently, the electronic scheme and the power supply circuit are specially designed with low-power consumption.

The conductance sensor based water cut meter is usually used to measure content of the oil-water two phase mixed fluid for periodical well production logging. In order to solve the real-time monitoring problem of downhole water cut, this paper proposes an online water cut measurement system based on the conductance sensor technology. Through the newly developed system, the continuous and permanent water cut measuring can be realized. The system consists of two conductance sensors, one temperature sensor, sampling mechanism and control & storage unit. Due to different density of oil and water, the two conductors with cylindrical poles, equipped on the upside and downside of the fluid inlet respectively, sense the conductivity of the oil-water mixed fluid and the detached water. With the real-time sampled downhole temperature, the conductivity values are compensated to reflect the real characters of the two kinds of liquid. According to Maxwell's model of oil-water mixed fluid and the correction parameters from offline calibration, the water cut is deduced. Since all units are designed with low-power consumption and high protection level, the system can operate permanently and provide online monitoring values. The water cut measurement system is tested in a physical testing environment with different conditions of the oil-water mixing ratio, the mineralization degree of water, the liquid temperature and the flow rate. Testing results show that the water cut in oil-water mixed fluid and the sampled conductivity follow the Maxwell's model approximately, where the error between testing data and theoretical value is within 3% especially for the high water cut cases. When the temperature changes, the measured water cut value basically does not variate, although sampled conductivity of the two sensors change a lot with temperature. Different mineralization degree of water would affect the measured water cut result slightly, which should be due to the conflicts between the large conductance range and the sampling accuracy. The flow rate is another element to make the measured result fluctuation, but the water cut would be stable when using the average value within a period. In brief, the system provides real-time water cut measurement and the measuring accuracy can satisfy the requirements of petroleum production. The conductance sensor based water cut measurement system realizes real-time measuring of oil-water mixing ratio for oil production and can provide online parameters for optimizing production process rapidly. All electronic units are designed with low-power consumption, which ensure the system to run downhole permanently.

Water is essential for energy exploitation, and moreover the contradiction between water resources and energy recovery seen in China is more severe than those in other countries. Given this, CO₂ waterless fracturing, which improves the production and recovery factor of an individual well and meanwhile serves for water preservation and CO₂ underground storage, can contribute to the sustainable development of China's oil industry.

The continuity and reliability of equipment is a key technical aspect for the successful waterless fracturing, in which the operation is required to be done in a sealed, pressurized environment during the whole workflow, and the proppant-carrying capability of fluids is low. Therefore, strict requirements are raised up upon the equipment. On the basis of the dynamic fluid balance combined with the fluid phase evolution during the whole construction workflow and its effects on stimulation treatments, this paper optimized the design of key construction equipment, such as CO₂ storage tanks, booster pumps, sealed blender trucks and fracturing pump trucks.

Major improvements can be concluded as: 1) the vertical tank is used for the sealed blender, which enhances the control stability of sand supply process jointly by the pressure difference regulation and auger; 2) booster pump unit with high pump-rate capability are included in the system for liquid supply and fluid phase control; 3) the liquid supply combines the mobile transport tanks and fixed storage tanks to increase the liquid supply capability; 4) the fracturing system is equipped with eight special fracturing pumps for waterless fracturing, fulfilling the construction requirement of 20,000 hydraulic horse power. The whole equipment system has treatment capability of available pump rates up to 12 m³/min, sand volume of 27 m³ and CO₂ injection of 1500 m³. In 2017, this equipment system was used in waterless fracturing for six times, with a maximum proppant input of 23 m³. Both the liquid and sand supply processes are found stable, and the production gain after stimulation is considerable.

It is estimated that in tight reservoir, oil production brought by 1 unit volume of CO₂ equals to that of 2.4 unit volume of water-based fracturing fluid. Providing that the average CO₂ injection of waterless fracturing wells is 630 m³, a single well can save 1512 m³ water resource. This equipment system fully meets the requirement of fracturing in vertical and horizontal wells of unconventional resources, and can effectively support the further development of the waterless fracturing technology.

During liquid CO₂ fracturing treatment in PetroChina Jilin Oilfield, CO₂ flows through storage tank, booster pump, blender, fracturing pump, and eventually into wellbore and production zone, generating a very complex phase evolution behavior. morphology control of CO₂ is a key in liquid CO₂ fracturing, as it greatly affects the stability of fluid and sand supply.

Temperature and pressure sensors are positioned at 12 critical nodes in a liquid CO₂ fracturing field test for tight oil to provide insight on CO₂ fluid phase evolution behavior. Then an analysis of phase behavior at each process node is done, to investigate its influencing mechanism on operation stability, as well as its major control factors. Based on this, the fracturing technological process is optimized and restructured, aiming at the optimization of fluid performance and construction stability.

Results are as follows. 1) Storage conditions of CO₂ are inconsistent between different storage tanks, which may cause vaporization of CO₂ during low-pressure fluid-feeding stage, thus greatly affect the construction stability. A buffer vessel should be introduced into the fracturing system, between the tanks and boosters, to improve the fluid-feeding stability. 2) The storage temperature and pressurein Jilin Oilfield is much higher than thatin North America, so the boosters contribute little to phase control here. A heat exchanger is a better alternative. 3) When the pressure difference between in-tank and outlet of blender is less than 0.05 MPa, the sand-feeding process is stable. This can be achieved by adjusting phase behavior of CO₂, liquid supplement rate to blender and opening degree of butterfly valve.

Phase evolution during liquid CO₂ fracturing has been identified, and the fracturing technological process is optimized and restructured based on this. This helps improve the fluid performance and construction stability, and enhances the success ratio and stimulation result of fracturing.

Liquid CO₂ fracturing is a novel stimulation technology, which helps realize multiple objectives such as conservation of water, sequestration of greenhouse gases and enhancement of single-well productivity and ultimate recovery. The blender used in PetroChina Jilin Oilfield is a huge vertical pressure vessel with a short auger conveyer at its bottom, which has poor sand controllability on sand feeding process. Making matters worse, the viscosity of liquid CO₂ is very low, resulting poor sand-carrying capacity, which further enhances the difficulty on sand feeding.

Stability of sand feeding process is improved via coordination of multi-disciplinary systems. 1) Develop new additive system for liquid CO₂, which can enhance the viscosity efficiently and has little damage to reservoir; 2)Adjust the auger conveyer structure to increase its control ability; 3)Ensure the pressure difference between in-tank and outlet of blender less than 0.05 MPa, by adjusting phase behavior of CO₂ during treatment, liquid supplement rate to blender and opening degree of butterfly valve; 4) Apply casing fracturing instead of tubing fracturing, to enhance the pump rate.

Results are as follows. 1) Two new additive systems are developed, both can enhance the viscosity to 5cp with a dosage of 1wt%. 2) Tilted auger conveyer is applied, thus increases its controllability on sand feeding and accuracy on sand ratio measurement. 3) Small positive differential pressure between in-tank and outlet of blender forms easily in winter, but difficultly in summer. The reason is that the temperature is high and large amount of CO₂ is gasified, resulting in the increase of in-tank pressure. In this case a heat exchanger is needed to sub-cooling CO₂. 4) By apply casing fracturing technique, the pump rate can rise from 3m³/min to 8m³/min, make the sand feeding process much easier.

The sand feeding process of liquid CO₂ fracturing is optimized via coordination of multi-disciplinary systems, which increases the sand ratio and total sand volume during fracturing, improves the construction stability, and eventually enhances the stimulation effect.

Liquid CO₂ fracturing is an outstanding stimulation treatment for unconventional reservoirs. It has several advantages including high flowback, small damage in reservoir and outstanding stimulation effect. However, as liquid CO₂ suffers from its low viscosity and high friction, many problems arise in liquid CO₂ fracturing operations, such as high operating pressure, poor proppant-carrying capacity and high filtration loss.

Two kinds of additive systems are tested in the liquid CO₂ fracturing operations in Jilin Oilfield. One system is a single fluoropolymer, while the other is composed by a surfactant thickener and another agent. Laboratory experiments show that both systems dissolve easily in liquid CO₂ under construction conditions, and can enhance the viscosity to the level of slickwater. Two fluids are separately applied in tubing and casing fracturing cases. All treatments are conducted in neighboring blocks. The discharge, sand ratio and operating pressure are recorded, and then the proppant-carrying capacities and friction properties of two fluids are compared.

Results show that for tubing fracturing technique, two fluids perform similarly, with a discharge of 3.8m³/min, sand ratios around 5.5%, and average operating pressures range from 46 to 48 MPa. For casing fracturing technique, with a discharge of 7.9m³/min, the sand ratio and average operating pressure of construction using the first fluid is 6.4% and 21MPa, respectively. For the second fluid with a rate of 7.4m³/min, the two values are 6.2% and 23 MPa, respectively. Obviously, the first fluid system is superior to the second one under large discharge, since the sand-adding process is more stable and its friction loss is smaller. Considering that the proppant-carrying mechanism of liquid CO₂ mainly depends on turbulence, the friction property is crucial. It is concluded that the first fluid is more suitable for liquid CO₂ fracturing.

An applicable fluid system for liquid CO₂ fracturing is selected via the comparison of field test performance. It could solve the problem of poor proppant-carrying capacity of liquid CO₂ to some degree, which can greatly enhance the stimulation effect and expand the application scope of liquid CO₂ fracturing.

In liquid CO₂ fracturing, liquid CO₂ is injected into reservoirs to crack open the formation, providing highly conductive paths for hydrocarbons. It's superior to water-based fracturing, because water is excluded during the whole process, resulting in formations free from water damage. Many field tests have been conducted in Jilin oilfield, with some problems exposed. The uncontrollability of fluid feeding due to the pressure difference among storage tanks, the liquid CO₂ vaporization in low pressure fluid feeding stage, and the slow-dissolving of additives are three main issues.

To solve the above problems, a pressure-bearing inclosed tank is designed as the buffer vessel, which is placed between the liquid CO₂ storage tanks and the boost pumps. Meanwhile, the additive adding system, which used to be located at the discharge connection of blender, is moved forward to the place near the discharge connection of the buffer vessel. A heat exchanger is added to the process too, to eliminate the vaporization. At last, a static mixer is installed after the blender.

During construction, liquid CO₂ of different storage tanks is firstly driven into the buffer vessel, discharged into the boost pumps, mixed with the proppants in blenders, and at last subcooling using the heat exchanger before driven into the pumps. In this case, the pressure of low-pressure fluid feeding system become stable, the residual volume of liquid CO₂ in a single tank is reduced, and the vaporization is eliminated. Meanwhile, the one-time storage of liquid CO₂ is somewhat increased. After discharged from the buffer vessel, liquid CO₂ is mixed with the additives immediately, which greatly prolongs the mixing time. The mixer also aids to the dissolving of additives. The additives dissolve in liquid CO₂ earlier, thus the proppant-carrying capacity and friction property of fracturing fluid are greatly improved both in the surface pipeline and in the wellbore segment.

The technological process of liquid CO₂ fracturing is adjusted, which realizes the optimization of fluid feeding system and additive adding system, improves the construction stability, and greatly enhances the stimulation effect.

Liquid CO₂ fracturing is a novel stimulation technology, which helps realize multiple objectives such as conservation of water, sequestration of greenhouse gases and enhancement of single-well productivity and ultimate recovery. During operations, CO₂ flows through storage tank, booster pump, blender, fracturing pump, and eventually into wellbore and production zone, generating a changing temperature and pressure distribution. CO₂‘s phase state, density and viscosity properties change consequently, which influence significantly the reliability and stimulation effect.

In a liquid CO₂ fracturing field test for tight oil, temperature and pressure sensors are positioned at 12 critical nodes (including booster pump, blender, fracturing pump, wellhead and bottom hole) to monitor CO₂ fluid. To ensure the reliability, CO₂ is required to maintain in liquid state both on the surface and subsurface. Inlet, inside and outlet pressure of the blender should be concerned, because the blender utilizes non-mechanical pump, which requires sufficient motive flow to draw proppants into the main pipe, while the pressure difference directly impacts the flow rate of motive flow.

The field test is successfully implemented with satisfactory result, 21 m³ proppants are added into the formation. The main conclusions are as follow. (1) In low-pressure fluid feeding stage, partial CO₂ is gasified, which influences the stability of fluid feeding; In future a buffer vessel will be placed between storage tanks and booster pumps, which will provide adjustment for phase control; And a heat exchanger may help by further reduce the temperature of CO₂.(2) Pressure difference among inlet, inside and outlet pressure of blender fluctuates during the whole process, with the probable reason of two additional static mixers, which create system pressure drop. (3) The temperature of CO₂ is very low in low-pressure stage, and the pipes are frosted; When pumping pressure reaches 38MPa, the temperature gradually exceeds 0°C, and the pipes are defrosted.

Phase evolution during liquid CO₂ fracturing has been identified, and phase control method has been determined. This helps improvethe stability of fluid feeding and sand adding, and enhances the success ratio and stimulation result of fracturing.

With increasing environmental awareness, novel strategies to effectively separate oil from industrial wastewaters and polluted oceanic water are highly desired. Using special wettability to design new materials for oil/water separation is an effective and facile way. Herein, polyimide (PI) aerogel is designed and prepared by freeze-drying PI precursor poly(amic acid) ammonium salt (PAS) water solution followed by imidization. With high hydrophobicity, the PI aerogel can be used for the absorption and separation of oil and water. 30–195 times weight of organic pollutants and oils can be absorbed by PI aerogel. To demonstrate the cyclic distillation test, cyclohexane was absorbed by the PI aerogel. As the size and the porosity structures of PI aerogel stayed the same during the separation/distillation cycles, no obvious change in absorption capacity was found after five cycles, indicating the highly stable recycling performance. During separation, water quickly permeated through the PI aerogel and dropped into the beaker below, while oil was retained above it and no external force was employed. Additionally, PI aerogel is also usable under harsh conditions. This research paved the way for fabricating high efficient and recyclable oil/water separation PI aerogel which can be used in the petroleum industry in the future.