Adaptability Evaluation of High-Density Kill Fluid for Ultra-Deep and Ultra-High Temperature Well Testing in Tarim Oilfield

Abstract

To address the insufficient long-term stability of kill fluids in ultra-deep, ultra-high-temperature wells in the Tarim Oilfield, this study systematically evaluates the adaptability of high-density kill fluids under high-temperature and prolonged static aging conditions, with a focus on identifying dominant settling mechanisms. The correlation between the microstructure and macroscopic properties of kill fluids was elucidated through particle size distribution analysis, scanning electron microscopy (SEM), Fourier-transform infrared spectroscopy (FTIR), and rheological characterization. A quantitative grading criterion for settling stability was established using settlement values and the falling rod method. Key findings demonstrate that low-density kill fluids (1.4–1.6 g/cm³) retained rheological stability after 20 days of aging at 220 °C, fulfilling the ≥20-day operational requirements for ultra-deep well testing. In contrast, high-density systems (1.9 g/cm³) exhibited severe particle aggregation after 15 days under identical conditions, with the yield stress-to-plastic viscosity ratio dropping below 0.10 and suspension capacity deteriorating. The apparent viscosity of ultrafine barite-weighted kill fluid increases with temperature, and its settling value is positively correlated with aging time and temperature. The settling mechanism of ultrafine barite-based kill fluids was attributed to reduced surface charge density caused by the decarboxylation of polyacrylate dispersants, which diminished interparticle electrostatic repulsion. The developed “settlement value vs. falling rod time” correlation model and grading criteria lay a theoretical foundation for optimizing kill fluid formulations and evaluating field performance in ultra-high-temperature wells, offering critical engineering insights to ensure safe deep hydrocarbon testing operations.

1. Introduction

The efficient development of deep and ultra-deep oil and gas resources is an important requirement for realizing China’s energy succession strategy. It is also the focus and a key area of interest for current and future oil and gas exploration and development [1]. However, deep and ultra-deep well testing and completion technology still face significant challenges. Complex geological conditions such as ultra-high temperatures, high pressures, high stresses, and fracture development at the bottom of wells [2] have brought many difficulties to the well test and completion work in deep wells and ultra/ultra-deep wells: The high solids content of the kill fluid leads to increased friction and torque, challenges in wellbore cleaning, and difficulty in maintaining the performance of the completion fluid. Furthermore, the high wellbore temperature and prolonged testing periods increase the risk of potential formation damage. The extended exposure of kill fluids to high temperatures often causes aggravator settlement and system performance deterioration, which can obstruct the lower tubular column, pump initiation, or other complex issues [3,4,5]. Therefore, higher performance standards have been proposed for high-density kill fluids suitable for ultra-deep wells.

With the continuous progress of the deep-earth project, the TK1 well was completed on 20 February 2025, and the final depth of the well reached 10,910 m. This achievement ranks first in Asia and second globally among vertical deep wells. It also represents the fastest onshore drilling breakthrough beyond 10,000 m worldwide, and the deepest straight well drilled in Asia. The TK1 well was predicted to have a bottomhole temperature of 213 °C and pressure of 165 MPa [6], and in the oil test operation of the ultra-deep wells, after a series of complicated processes such as perforation injection, which re-lowers the completion–modification of the integrated tubing column, the kill fluid remains in the well for a long time; so, it is required that the settling stability of the kill fluid at high temperatures reaches more than 15 days or even 20 days.

The commonly utilized high-density kill fluid systems can be primarily categorized into two types: solid-free kill fluid systems and solid-phase kill fluid systems [7]. Within the realm of solid-free kill fluid systems, inorganic and organic soluble salts are typically employed for weighting purposes. Currently, the main types of solid-free kill fluid systems include formate-based, phosphate-based [8], and bromide-based fluids. Notable examples include a phosphate completion fluid with a maximum density of 1.74 g/cm³ developed by Collins et al. [9], a bromide kill fluid system achieving densities up to 1.845 g/cm³ composed of calcium bromide brine and submicron particles as reported by Mathieu Champeau et al. [10], as well as a low-solid-phase formate completion fluid system resistant to temperatures up to 170 °C designed by Hu Wenge et al. [11] for the Shunbei 1 block in Xinjiang. Additionally, Yue Chaoxian [12] constructed a composite brine completion fluid system with densities exceeding 1.9 g/cm³ for the Dongfang 13 block. However, due to the reliance on soluble salts for weighting in these solid-free kill fluids, saturation levels of salt concentration impose limitations on achievable density maxima. Furthermore, there is an absence of suitable viscosity-increasing and shear-thickening agents that can effectively withstand prolonged exposure to harsh downhole conditions characterized by high temperature and salinity. This situation results in considerable risks related to well control and corrosion, thereby constraining their applicability in ultra-deep well testing operations to some extent.

The solid phase in the solid-phase kill fluid system usually refers to the weighted materials, such as limestone powder, barite powder, micro-ferrite powder, micro-manganese powder, and so on [13]. The primary high-density solid-phase kill fluid systems currently in use domestically and internationally are sulfonation-modified drilling fluid systems [14], polysulfide kill fluid systems, seafoam kill fluid systems, ultrafine barite-weighted kill fluid systems, oil-based (barite as weighting agent) kill fluid systems [15], and clay-free phase kill fluids [7]. For example, Abdullah Saleh Al-Yami et al. [16] developed a Mn₃O₄/KCl water-based completion fluid system with good thermal stability, M. J. Al-Saeedi et al. [17] developed a Mn₄O₄/saturated potassium formate water-based drilling completion fluid system with a maximum application of 1.94 g/cm³, which did not suffer from barite settlement after 3 d of standing, and Zhang Tao et al. [18] used a Mn₂O₄/saturated potassium formate kill fluid system for a difficult well completion in the Dongfang B gas field, which remained stable at 200 °C for 10–15 days without settling. However, due to the large specific surface area of micromanganese, it readily adsorbs the filter loss depressant, resulting in an increase in FL_HTHP of about 30% compared to the barite system, and micromanganese alone has a high cost. It is not widely used in China. Compared with the non-solid phase kill fluid system, the solid phase kill fluid system has the advantages of strong resistance to heat aging, excellent anti-pollution ability, simple processing, low cost, and wide application. At present, the UDM-T1 ultrafine barite-weighted kill fluid is the most widely used ultrafine barite-weighted kill fluid in the Tarim Oilfield, serving as a representative example.

The ultrafine barite-weighted kill fluid system [19] consists of ultrafine barite powder, dispersant, stabilizer, and supporting viscosity control agents. The system can significantly improve the suspension performance of the particles by modifying ordinary barite into nanoscale and submicron-scale ultrafine powders; the small-sized particles are less affected by gravity due to their light mass, and the vigorous Brownian motion helps to resist settling. At the same time, the polymer dispersant adsorbed on the surface of the ultrafine barite particles acts synergistically through spatial reluctance and electrostatic repulsion to generate a repulsive force between the particles, thus maintaining the suspension stability of the system. This design cleverly converts the settling stability of the system into the dispersion stability of the barite weighting agent, thus forming a kill fluid system with long-term high-temperature settling stability. The density of the system ranges from 1.30 to 2.30 g/cm³, particle size D50 ≤ 1 µm, D90 ≤ 3 µm, plastic viscosity 7~35 mPa·s, settling value 0.5~2.5 N, with good settling stability, high-temperature resistance, anti-pollution ability, etc. At present, the system has been applied in more than 80 wells in the Tarim Oilfield for testing and completion, with a summary of its applications in different blocks provided in

Table 1. Brief application of ultrafine barite-weighted kill fluid system in Tarim Oilfield.

Blocs	Temperature	Density	Underground Operating Hours
BOZI	112.85~160.15	1.45~2.08	12~43
KESHEN	143~190	1.3~1.9	11~40
DABEI	85.11~149.74	1.48~2.32	14~27
Others	101.84~171.00	1.55~1.98	7~22

. Among them, the density of Dabei 32 wells is as high as 2.32 g/cm³, while the temperature resistance of Keshen 132 wells reaches 190 °C.

In well-completion operations, it is extremely important to keep the completion fluid with excellent static settling stability [20]. At present, there is no unified method and standard for evaluating the settling stability of kill fluids under static conditions. Nie Qiangyong et al. [21] used the SSSI method [22] to achieve the static stability performance evaluation of a polysulfone water-based completion fluid at 180 °C × 7 d; Wenqiang Zeng and Mario Bouguetta et al. [23] calculated the SSI by layering density according to the density change in the kill fluid after static stabilization, and the smaller the value is, the more stable the performance of the system is. Xie Jianhui et al. [15] used the falling rod method for a high-temperature static aging experiment of an oil-based completion fluid, 180 °C, 15 d, a glass rod free fall to the bottom, and 1 cm soft sinking, to meet the requirements of the field design and construction. Hanson P. M. et al. [24,25] evaluated the settling stability of the kill fluid after aging by calculating the settlement factor (SF). At this stage, for the static settling stability evaluation of commonly used high-density oil test kill fluids, an indoor study with temperature < 200 °C and settling time within 15 d is the main focus, and it is difficult to evaluate the static settling stability of kill fluids with an ultra-high temperature (≥200 °C) and ultra-long settling time (≥20 d) for ultra-deep wells.

To address the above challenges, this paper presents an experimental study of the settling stability of ultrafine barite-weighted kill fluid using a combination of the falling rod method and a pin-in tensiometer test set-up. By studying the changes in the rheological properties and settling stability of the ultrafine barite-weighted kill fluid system at ultra-high temperatures and ultra-long settling times, a graded range for evaluating the settling stability of piezoelectric fluids was formed in collaboration with the settlement values from the falling rod method and the pin-in tensile device. The adaptability of the ultrafine barite-weighted kill fluid to ultra-high temperatures in ultra-deep formations was evaluated to assist with the well-testing kill operation of ultra-deep wells.

2. Materials and Methods

2.1. Experimental Sample

In this study, based on the patent CN 110028938 A [26], a standard sample of an ultrafine barite-weighted kill fluid was developed based on the following formula: water + ultrafine barite particles + dispersant + stabilizer + bilayer activator + viscous cut control agent. The specific preparation procedure was as follows; 250 mL of water was added to the preparation tank, and under high-speed mixing at 10,000–15,000 rpm, a mixture of 4–6% polyacrylate and vinyl acetate-maleic anhydride copolymer, 3–5% sodium dodecylbenzene sulfonate, a mixture of Spectra-80 and organosilicone resin, a mixture of 3–5% sodium silicate and betaine, and the amount of barite addition was sequentially added according to the density of the kill fluid, and after even mixing, it was placed in a pulverizer for grinding, and the slurry obtained after grinding was then sequentially added to a 4–6% brine mixture (potassium chloride, organic salt weight and potassium formate are mixed according to the mass ratio of 1:1:1) for high-speed mixing to produce an ultrafine barite-weighted kill fluid that meets the density requirements.

Based on the predicted formation temperature of 213 °C and pressure of 165 MPa in 10,000 m deep wells, an ultrafine barite-weighted kill fluid with a density of 1.4~1. 9 g/cm³ was formulated in the laboratory, and the prepared samples are shown in

Table 2. Experimental sample table.

Serial Number	Density (g/cm3)	Temperature (°C)	Aging Time (d)
1	1.4	220	0, 5, 10, 15, 20
2	1.5	210	0, 5, 10, 15, 20
3		220	0, 5, 10, 15, 20
4		230	0, 5, 10, 15, 20
5	1.6	220	0, 5, 10, 15, 20
6	1.9	220	0, 5, 10, 15, 20
7	1.53	180	0, 10, 20, 30, 40
8	1.77	180	0, 10, 20, 30, 40
9	1.85	170	0, 5, 15, 20, 30
10	1.91	180	0, 10, 20, 30

, 1#–4#, to evaluate the settling stability under an ultra-high temperature (≥200 °C) and ultra-long settling time (≥20 d), as well as the adaptability of the ultrafine barite-weighted kill fluid in ultra-deep formations.

To evaluate the settling stability of ultrafine barite-weighted kill fluid more intuitively and efficiently, the author established a quantitative grading scale for the settling stability of ultrafine barite-weighted kill fluid by testing the settling value of ultrafine barite-weighted kill fluid samples under different densities, temperatures, and aging times. The prepared samples are shown in Table 2, 1#–10#.

2.2. Basic Performance Testing of Kill Fluids

2.2.1. Particle Size Testing

The particle size of the ultrafine barite-weighted kill fluid was tested using a Mastersizer 2000 (Juchuang Group Co., Ltd., Qingdao, China) laser particle sizer. The refractive index of the particle sizer was set to 1.64, and the kill fluid was added drop by drop until the shading reached 5–10%, at which point the sample addition was stopped, and then finally clicked on the test to read the particle size value. The particle sizes of 1#, 2#, 3#, 4#, 5#, and 6# ultrafine barite-weighted kill fluids with different densities were tested prior to aging, and the particle sizes of ultrafine barite-weighted kill fluids aged at high temperatures for 5 d, 10 d, 15 d, and 20 d were tested after high stirring, and the particle sizes of the kill fluids before and after high-temperature aging were compared with the changes in the particle size of the kill fluids (D50 and D90) to determine whether barite-weighted kill fluids underwent agglomeration and to assess their suspension stability.

In addition, barite deposits at the bottom of the aging tank after 0 d, 5 d, 10 d, 15 d, and 20 d of high-temperature aging of 6# ultrafine barite-weighted kill fluid with a density of 1.9 g/cm³ were collected to analyze their particle size distribution.

2.2.2. Electron Microscope Scan

The barite deposit samples at the bottom of the aging tank after 5 d and 20 d of high-temperature aging of the 6# ultrafine barite-weighted kill fluid with a density of 1.9 g/cm³ were collected, dried, and processed. The internal porosity of the deposit was observed using scanning electron microscopy (SEM) to analyze the barite agglomeration and sedimentation behavior.

2.2.3. Infrared Spectroscopic Testing

Barite deposits taken from the bottom of the aging tank after 0 d and 20 d of high-temperature aging of the 6# ultrafine barite-weighted kill fluid with a density of 1.9 g/cm³ were dried and processed for infrared spectroscopic analysis. This test aimed to examine changes in the adsorption of the dispersants on the surface of the barite after high-temperature static storage, providing further insights into the settling stability mechanism of the ultrafine barite-weighted kill fluid.

2.2.4. Zeta Potential Test

Slurry sediments from the 6# ultrafine barite-weighted kill fluid aged at high temperatures for 0 d, 15 d, and 20 d with a density of 1.9 g/cm³ were collected, dissolved, and dispersed, with pH adjustments made as needed. A Zetasizer potential (Colloidal Dynamics, Ponte Vedra Beach, FL 32082, USA) meter was used to measure the zeta potential of the ultrafine barite-weighted kill fluid, reflecting the negative charge density on the particle surfaces, thereby further analyzing the settling stability mechanism of the ultrafine barite-weighted kill fluid.

2.3. Rheology Test

The rheological properties of 1#, 3#, 5#, and 6# ultrafine barite-weighted kill fluid systems were tested at room temperature and after different static aging durations using a six-speed rotational viscometer. Their AV, PV, and YP were calculated, the rheological curves were plotted, and the effects of temperature and density on the rheological properties of ultrafine barite-weighted kill fluid were analyzed.

2.4. Settling Stability Test

2.4.1. Establishment of Settling Stability Evaluation Method

The in-house quantitative static settling stability testing of the ultrafine barite-weighted kill fluid was conducted using two methods: the fully sealed high-temperature aging tank static drop rod method and the pin-in tensiometer test method.

The falling bar method, based on the “Notice on Strengthening the Quality Control of Oil Trial and Completion Mud” of the Tarim Oilfield, involved selecting a round-headed glass bar with a diameter of 7 mm × 300 mm, which was freely dropped from the flush level of the tank opening, and the falling height as well as the thickness of the bottom precipitation were recorded to characterize the settlement state of the ultrafine barite-weighted kill fluid after high-temperature aging.

The needle penetration tensiometer test device [27] is used to carry out the needle penetration detection of the degree of sedimentation after the high-temperature static aging of high-density test oil-based working fluids under simulated oil testing conditions. The test device (needle penetration tensiometer V200, Shenzhen Mycen Technology Co., Ltd., Shenzhen, China) is shown in Figure 1a, where the bottom pressure value is measured by the pressure transducer, and the pressure value is used to reflect the degree of sedimentation, to achieve the purpose of the quantitative assessment of the stability of sedimentation. After aging, four points are taken from four points: the center point of the aging tank and three positions along the wall of the tank, as shown in Figure 1b. The settlement values of these four measurement points are read, and then the average value is taken as the settlement value of the measurement.

To evaluate the static settling stability of kill fluids in the laboratory, both the falling rod method and the pin-in tensile testing method were employed. These methods provided both qualitative and quantitative insights into the settling stability of ultrafine barite-weighted kill fluids under various densities and high temperatures. A correlation between the falling rod method measurements and the kill fluid settlement values was established, enabling the development of a quantitative grading range for the settling stability of ultrafine barite-weighted kill fluids.

2.4.2. Settling Stability Test

Given the extreme conditions of ultra-deep wells, an indoor evaluation of the settling stability of 1–10# ultrafine barite-weighted kill fluids with a density of 1.4~1.9 g/cm³ was carried out at temperatures ranging from 170 to 230 °C for various settling durations. By utilizing the quantitative grading range for the settling stability of the ultrafine barite-weighted kill fluid, the study aimed to determine whether the ultrafine barite-weighted kill fluid meets the requirements for oil testing operations in ultra-deep wells and to further explore the mechanism of the settling stability.

3. Results and Discussion

3.1. Ultrafine Barite-Weighted Kill Fluid Basic Performance Results

3.1.1. Ultrafine Barite-Weighted Kill Fluid Particle Size Test Results

Comparison of Particle Size Before and After High-Temperature Aging for Different Densities of Ultrafine Barite-Weighted Kill Fluid

The particle size changes in the 1.4, 1.5, 1.6, and 1.9 g/cm³ ultrafine barite-weighted kill fluids 1#, 3#, 5#, and 6# after static aging at 220 °C for 0–20 d (20 d in the table) are presented in

Table 3. Changes in particle size before and after static aging of ultrafine barite-weighted kill fluid of different densities.

Experimental Condition	1.4 g/cm3		1.5 g/cm3		1.6 g/cm3		1.9 g/cm3
	D50	D90	D50	D90	D50	D90	D50	D90
0 d	0.98	2.357	0.84	2.267	1.037	2.344	1.125	2.641
5 d	1.101	2.431	1.002	2.345	1.115	2.463	1.205	2.753
10 d	1.042	2.561	1.107	2.682	1.173	2.649	1.348	3.105
15 d	1.113	2.597	1.127	2.705	1.206	2.776	1.382	3.326
20 d	1.116	2.683	1.145	2.778	1.264	2.874	1.421	3.562
Average Value	1.0704	2.5258	1.0442	2.5554	1.159	2.6212	1.2962	3.0774
Standard Deviation	0.0588	0.1309	0.1269	0.2320	0.0869	0.2182	0.1257	0.3856

. The increase in D90 after the static aging of 1.4, 1.5, and 1.6 g/cm³ ultrafine barite-weighted kill fluids for 20 d was 13.8%, 22.5%, and 22.6%, respectively, and the particle size increased gradually over time but at a slow rate, as shown in Figure 2. These three densities of ultrafine barite-weighted kill fluids maintained D50 < 1.3 µm and D90 < 3 µm after aging at a high temperature for 20 d, with the standard deviation of D90 less than 0.25. These results indicate minimal particle size variation post-aging, no significant barite agglomeration, and a stable suspension performance for the ultrafine barite-weighted kill fluid system.

In contrast, for the high-density 1.9 g/cm³ ultrafine barite-weighted kill fluid, the D90 value increased by 34.9% after 20 d of aging—a more substantial increase in barite particle size compared to the lower-density samples. The total standard deviation of the particle size under different aging periods reaches 0.3865, indicating greater fluctuation in particle size before and after aging. Notably, D90 exceeds 3 µm after aging for 10 d, and the D90 reaches 3.562 µm after aging for 20 d. These findings suggest that in long-term high-temperature static storage, the ultrafine barite-weighted killing fluid of 1.9 g/cm³ undergoes significant agglomeration, and the suspension performance of the killing fluid deteriorates.

Comparison of Particle Size Before and After Aging of Ultrafine Barite-Weighted Kill Fluid at Different Temperatures

The results of particle size tests of ultrafine barite-weighted kill fluids 2#, 3#, and 4# before and after aging at 210 °C, 220 °C, and 230 °C are shown in

Table 4. Particle size test results of the static aging experiment of ultrafine barite-weighted kill fluid at different aging temperatures.

t/d	210 °C		220 °C		230 °C
	D50	D90	D50	D90	D50	D90
0	0.840	2.267	0.84	2.267	0.84	2.267
5	0.993	2.291	1.002	2.345	1.031	2.438
10	1.023	2.532	1.107	2.682	1.134	2.671
15	1.106	2.741	1.127	2.705	1.143	2.689
20	1.139	2.834	1.145	2.778	1.156	2.895
25	1.173	2.996	1.194	2.967	1.237	3.073
Average Value	1.0457	2.6102	1.0692	2.624	1.0902	2.6722
Standard Deviation	0.1218	0.2973	0.1289	0.2671	0.1391	0.2931

and Figure 3. When the aging temperature is ≤220 °C and the aging duration is less than 25 d, the increase of D90 after 25 d of high-temperature aging is 32.2%, 30.9% and 35.6% respectively. These results indicate that the particle size increases gradually with aging temperature, but at a relatively slow rate. When the aging temperature is ≤220 °C and the aging period is within 25 d, the ultrafine barite-weighted kill fluid D50 is <1.2 µm and D90 is <3 µm. The overall standard deviation of the particle size is not much less than 0.3, with minimal fluctuations in particle size before and after aging. The comprehensive analysis indicates that no barite agglomeration occurs in the kill fluid under the three kinds of aging temperatures, and the suspending performance of the system remains stable.

Comparison of Sediment and System Particle Size After Aging of Ultrafine Barite-Weighted Kill Fluid

A comparison of the particle size analysis of the system and the sediment after aging at 220 °C for different durations with a density of 1.9 g/cm³ for ultrafine barite-weighted kill fluid 6# is presented in Figure 4 and Figure 5. The results indicate that (1) the sedimentation value reached 3.38 N when aging to the 20th day, indicating that the system exhibited the poor stability of sedimentation during this period; (2) during the 0–15 days of high-temperature aging, the difference in particle size between the system of piezoelectric fluid and the sediment is not prominent. However, on the 20th day of aging, the D50 and D90 particle sizes of the sediment increased by more than 80% compared to the system fluid, indicating significant barite particle agglomeration within the sediment.

3.1.2. Electron Microscope Scanning

The results of the electron microscope scanning of the sediment of 1.9 g/cm³ density ultrafine barite-weighted kill fluid 6# after aging at 220 °C for different days are shown in Figure 6. From the electron microscope scanning diagram, it can be seen that with the increase in aging time, the structure of the sediment after 20 days of aging of the ultrafine barite-weighted kill fluid is denser and less porous than that after 5 days of aging, which indicates that more barite particles in the system of the ultrafine barite-weighted kill fluid undergoes the phenomenon of settling and agglomeration, and this phenomenon is consistent with the trend of the particle size change in the sediment after 20 days of aging.

3.1.3. Infrared Analysis

The infrared spectroscopic analysis of the sediment of ultrafine barite-weighted kill fluid 6# with a density of 1.9 g/cm³ aged at 220 °C for 0 d and 20 d is presented in Figure 7. At 0 d of aging, there was a double peak of carboxylate (-COO⁻), which appeared as a symmetrical and 1629 cm⁻¹ antisymmetrical vibrational telescopic vibration at 1396 cm⁻¹ and antisymmetric vibrational telescopic vibration at 1396 cm⁻¹; meanwhile, C=O vibrational telescopic vibration at 1708 cm⁻¹ was observed, accompanied by O-H vibrational telescopic vibration at 3439 cm⁻¹ and 982 cm⁻¹ bending vibration. At 20 d of aging, there is a double peak of carboxylic acid (-COO⁻), which is shown by symmetric and antisymmetric vibrational telescopic vibrations at 1415 cm⁻¹ and 1622 cm⁻¹. At the same time, an increase in the peak area of the C=O telescoping vibration at 1722 cm⁻¹ is observed, accompanied by an increase in the O; the peak area of the C=O stretching vibration at 1722 cm⁻¹ is increased, accompanied by the O-H stretching vibration at 3427 cm⁻¹ and the bending vibration at 979 cm⁻¹. After 20 d of aging, the area of the carboxylic acid (-COO⁻) double peaks decreased, while the area of the C=O stretching vibration peaks increased significantly.

3.1.4. Zeta Potential Analysis

The results of the zeta potential analysis of the ultrafine barite-weighted kill fluid #6 aged at 220 °C for 0 d, 15 d, and 20 d are shown in

Table 5. Zeta potential evaluation results of ultrafine barite-weighted kill fluid.

t (d)	Zeta Potential (mV)
0	−40.3
20	−26.4
25	−22.7

. The results of the potential analysis indicate that (1) the absolute value of the zeta potential of the ultrafine barite-weighted kill fluid decreases with the aging time; (2) the absolute value of the zeta potential at 0 d is 40.3 mV, and that of the ultrafine barite-weighted kill fluid at 20 d is only 22.7 mV. (3) The absolute value of the zeta potential at 0 d of aging is 40.3 mV and that at 20 d of aging is 22.7 mV, which represents a 43.67% decrease compared to 0 d of aging.

3.2. Ultrafine Barite-Weighted Kill Fluid Rheology Test Results

Various densities of the ultrafine barite-weighted kill fluid were statically aged at 220 °C. Given the harsh conditions of ultra-deep wells, the densities of the ultrafine barite-weighted kill fluid were selected to be 1.4 g/cm³, 1.5 g/cm³, 1.6 g/cm³, and 1.9 g/cm^3, and the rheological properties were comparatively evaluated through high-temperature in-house aging experiments.

As shown in Figure 8, the dynamic shear of different densities of the ultrafine barite-weighted kill fluid in the figure decreased slowly with the increase in aging time and gradually stabilized; the apparent viscosity of the ultrafine barite-weighted kill fluid exhibited an increasing trend, and there were still 14. 5 mPa·s, 16 mPa·s, 19 mPa·s, and 38 mPa·s after 20 d of high-temperature aging. Except for the 1.9 g/cm³ fluid, the viscosity-cutting properties of the other three densities were well-maintained. Therefore, when the low-density 1.4~1.6 g/cm³ ultrafine barite-weighted kill fluid was statically aging at 220 °C for 20 d, the rheological properties remained stable and the slurry maintained good fluidity. The high-density 1.9 g/cm³ ultrafine barite-weighted kill fluid failed to meet the requirements of oil testing and construction because the dynamic–plastic ratio of the slurry was <0.10 when it was aging at 220 °C for 15 d.

The density of the 1.5 g/cm³ ultrafine barite-weighted kill fluid, which meets the requirements of ultra-deep well pressure, was statically aged at different high temperatures (210 °C, 220 °C, 230 °C). Its rheological properties were further investigated, and the results are shown in Figure 9. Observing the overall pattern, it is found that the dynamic shear of the ultrafine barite-weighted kill fluid at different temperatures decreases slightly with the increase in aging time, and gradually stabilizes, while the apparent viscosity of the fluid at different temperatures shows an increasing trend. After 25 d of high-temperature aging, the apparent viscosities remain at 19 mPa·s, 20. 5 mPa·s, and 23 mPa·s, indicating good viscous shear performance. With increasing aging time at a high temperature, AV, PV, and Gel values all increase with the aging temperature. When aged at 230 °C for 25 d, the fluid still exhibits a final cut of 4.5 Pa, demonstrating excellent suspending performance. Thus, the ultrafine barite-weighted kill fluid with the density of 1.5 g/cm³ maintains proper rheology, thixotropy, and suspension properties at ≥200 °C for 25 d of static aging.

3.3. Results of Settling Stability Tests of Ultrafine Barite-Weighted Kill Fluid

3.3.1. Method Creation

The settling stability of ultrafine barite-weighted kill fluid with densities ranging from 1.53 to 1.91 g/cm³ was tested under high-temperature static aging conditions (170 °C to 220 °C). The settling stability was evaluated using the falling rod method and needle tensile testing method, with test results recorded at different aging times. Some of the results are presented in

Table 6. Experimental results of settlement values and bottoming state of falling rods of ultrafine barite-weighted kill fluid at high temperatures (partial).

Density (g/cm3)	Temperature (°C)	t (d)	Sedimentation Value (N)	Drop Shot
1.53	180	10	0.67	The glass rod falls freely to the bottom and rebounds against the wall.
		20	1.12	The glass rod falls freely for 17 cm and is gently inserted into the bottom.
		30	1.93	The glass rod tilted, softly sunk 2 cm, and gently inserted into the bottom.
		40	2.44	The glass rod is tilted, softly sunk 5 cm, and gently inserted into the bottom.
1.77	180	10	0.69	The glass rod falls freely to the bottom and rebounds against the wall.
		20	1.02	The glass rod falls freely to the bottom and rebounds against the wall.
		30	1.49	The glass rod falls freely for 15 cm and is gently plunged to the bottom.
		40	4.01	The glass rod is upright, soft sinking 9 cm, and forced to the bottom.
1.85	170	5	1.77	The glass rod is tilted, softly sinking 1 cm, and gently inserted into the bottom.
		15	2.34	The glass rod is tilted, softly sinking 3 cm, and gently inserted into the bottom.
		20	3.92	The glass rod is upright, softly sinking 7 cm, and forced to the bottom.
		30	7.67	The glass rod is upright, hard sinking 2 cm, not forced to the bottom, and the glass rod is suctioned.
1.91	180	10	0.96	The glass rod falls freely to the bottom and rebounds against the wall.
		20	1.63	The glass rod is tilted, softly sinking 4 cm, and gently inserted into the bottom.
		30	2.91	The glass rod is upright, soft sinking 6 cm, and forced to the bottom.

. The scatter plot illustrating the relationship between the falling rod method results and sedimentation values is shown in Figure 10, along with the classification of the settling stability performance levels of the ultrafine barite-weighted kill fluid.

According to the description of the settlement state of different densities of kill fluids at different times of high-temperature aging, the glass rod bottoming has four states: free fall to the bottom, light insertion to the bottom, soft sinking, and hard sinking. Among them, the first two states, along with soft sinking of less than 5 cm, where the glass rod is tilted, indicate that the ultrafine barite-weighted kill fluid has a good settlement state. However, if the soft sinking exceeds 6 cm or results in hard sinking, the glass rod is upright, signaling that the ultrafine barite-weighted kill fluid system is in an extremely poor settlement condition. Figure 10 presents a total of 90 sets of test results of the falling rod method and the needle tensile test method, where the orange color represents a “glass rod upright” state in the results of the falling rod method, while the blue color represents a “glass rod tilted” state. The vertical coordinate corresponds to the settlement value. A comprehensive correlation analysis of the two methods reveals that it can be roughly divided into three regions to quantify the settlement value of the partition when 0 < settlement value ≤ 1.5 N. A total of 47 experimental groups were tested, of which 100% of the glass rods were tilted, indicating an excellent settlement state; when 1.5 < settlement value ≤ 2.5 N, 19 of 25 experimental groups showed tilted glass rods, meaning that 76% of the experiments were in a good state; when the settlement value is >2.5 N, 18 experimental groups and 100% of the glass rods were upright, indicating an extremely poor settlement state. In these cases, the slurry failed.

To date, ultrafine barite-weighted kill fluid has been used in the Tarim Oilfield in completed operations of over 80 wells. A statistical analysis of 62 wells over the past five years showed that the average downhole operating time was 21 days, with an average stationary time of 7–15 days. The settling stability of the ultrafine barite-weighted kill fluid was analyzed by the evaluation results of the integrated settlement value and the glass rod method. When the settlement value is <2.5 N after aging in the indoor evaluation, the kill fluid remains stable without hard sinking at the bottom, and the pump can be opened smoothly when the pumping pressure is less than 5 MPa during the on-site application, while when the settlement value is >5 N, hard sinking occurs at the bottom, leading to an excessively high on-site opening pumping pressure. Combined with the experimental analysis and field statistics, a settlement value of 2.5 N was established as the threshold for good kill fluid performance. Accordingly, the quantitative grading range of the settling stability of ultrafine barite-weighted kill fluid was developed, as shown in

Table 7. Range of quantitative grading for settling stability of ultrafine barite-weighted kill fluid.

Gradings	Sedimentation Value/N	Drop Shot
excellent	0~1	The glass rod falls freely to the bottom and rebounds against the wall.
	1~1.5	The glass rod falls freely 15~18 cm and is gently inserted to the bottom.
good	1.5~2.5	The glass rod is tilted, soft sinking 2~5 cm, and is gently inserted into the bottom.
poor	2.5~5	The glass rod is upright, soft sinking ≥ 6 cm, and forced to the bottom.
	5 or more	The glass rod is upright, hard sinking ≥ 2 cm, not forced to the bottom, and the glass rod is suctioned.

.

3.3.2. Settling Stability Test

Different densities of the ultrafine barite-weighted kill fluid at 220 °C in the static aging settling stability test results are shown in

Table 8. Test results of settling stability of different densities of ultrafine barite-weighted kill fluid with static aging.

Density (g·cm−3)	t (d)	Sedimentation Value (N)	Drop Shot	Gradings
1.4	5	0.31	The glass rod falls freely to the bottom and rebounds against the wall.	excellent
	10	0.42	The glass rod falls freely to the bottom and rebounds against the wall.
	15	0.76	The glass rod falls freely to the bottom and rebounds against the wall.
	20	0.83	The glass rod falls freely to the bottom and rebounds against the wall.
1.5	5	0.44	The glass rod falls freely to the bottom and rebounds against the wall.	excellent
	10	0.53	The glass rod falls freely to the bottom and rebounds against the wall.
	15	0.79	The glass rod falls freely to the bottom and rebounds against the wall.
	20	0.89	The glass rod falls freely to the bottom and rebounds against the wall.
1.6	5	0.52	The glass rod falls freely to the bottom and rebounds against the wall.	excellent
	10	0.63	The glass rod falls freely to the bottom and rebounds against the wall.
	15	0.89	The glass rod falls freely to the bottom and rebounds against the wall.
	20	1.23	The glass rod falls freely for 17 cm and is gently inserted into the bottom.
1.9	5	0.9	The glass rod falls freely to the bottom and rebounds against the wall.	excellent
	10	1.45	The glass rod falls freely for 15 cm and is gently plunged to the bottom.
	15	2.22	The glass rod is tilted, softly sinking 4 cm, and gently inserted into the bottom.	good
	20	3.38	The glass rod is upright, softly sinking 7 cm, and forced to the bottom.	poor

; with the increase in aging time, the settlement value of the ultrafine barite-weighted kill fluid shows an increasing trend. For the ultrafine barite-weighted kill fluid with a density of 1.4–1.6 g/cm³ at 200 °C, after 20 days of aging, the settlement value remains < 1.5 N, and the glass rod free falls to the bottom, indicating the performance of the slurry is stable, as shown in Figure 11. For the ultrafine barite-weighted kill fluid with a density of 1.9 g/cm³ at 220 °C, after 15 days of aging, the settlement value exceeds 2 N, the glass rod is tilted, and obvious soft sinking is observed at the bottom; after 20 days of aging, the settlement value exceeds 3 N, the glass rod stands upright, and the thickness of the soft sinking increases significantly, in which case the glass rod can only be bottomed out by force. According to the standard “Notice on Strengthening the Quality Control of Oil Trial Completion Mud” and the quantitative grading range, at this time, the slurry fails to meet the requirements of field construction.

The settling stability test results for the ultrafine barite-weighted kill fluid with a density of 1.5 g/cm³ at different temperatures in the static aging conditions are shown in

Table 9. Settling stability test results of 210 °C static aging experiment of ultrafine barite-weighted kill fluid.

Temperature (°C)	t (d)	Sedimentation Value (N)	Drop Shot	Gradings
210	5	0.25	The glass rod falls freely to the bottom and rebounds against the wall.	excellent
	10	0.34	The glass rod falls freely to the bottom and rebounds against the wall.
	15	0.54	The glass rod falls freely to the bottom and rebounds against the wall.
	20	0.69	The glass rod falls freely to the bottom and rebounds against the wall.
	25	1.25	The glass rod falls freely for 17 cm and is gently inserted into the bottom.
220	5	0.44	The glass rod falls freely to the bottom and rebounds against the wall.	excellent
	10	0.53	The glass rod falls freely to the bottom and rebounds against the wall.
	15	0.79	The glass rod falls freely to the bottom and rebounds against the wall.
	20	0.89	The glass rod falls freely to the bottom and rebounds against the wall.
	25	1.73	The glass rod falls freely to the bottom and rebounds against the wall.
230	5	0.36	The glass rod falls freely to the bottom and rebounds against the wall.	excellent
	10	0.46	The glass rod falls freely to the bottom and rebounds against the wall.
	15	0.73	The glass rod falls freely to the bottom and rebounds against the wall.
	20	0.93	The glass rod falls freely to the bottom and rebounds against the wall.
	25	1.96	The glass rod is tilted, softly sinking 3 cm, and is gently inserted into the bottom.	good

, and the settlement value increases with aging time. At static temperature ≥ 200 °C aging for 20 d, the settlement value <1 N, the glass rod free falls to the bottom, and rebounds against the wall, indicating that the system remains stable, as illustrated in Figure 12. At 220 °C and 230 °C aging temperatures for up to 25 d, the settlement value ranges between 1.5 and 2.5 N, with partial soft sinking observed at the bottom. The glass rod is lightly inserted to the bottom. According to the standard “Notice on Strengthening the Quality Control of Oil Trial Completion Mud” and the quantitative grading range, the slurry maintains a relatively good high-temperature settling stability and continues to meet the field oil test construction requirements.

3.3.3. Mechanisms of Settling Stability

Based on the agglomeration phenomenon of barite and the deterioration of the settling performance of the system of the ultrafine barite-weighted kill fluid with a density of 1.9 g/cm³ after aging at 220 °C for 20 days, it is shown that the dispersion of nano-micron-sized barite particles in the ultrafine barite-weighted kill fluid deteriorates, and this phenomenon coincides with the trend of the settling value. By comparing the infrared spectrograms in the sediments before and after aging, the results are shown in Figure 13.

As shown in Figure 13, after 20 days of aging, the bimodal peaks of carboxylic acid (-COO⁻) were narrowed down to a symmetric vibration at 1396 cm⁻¹ and an antisymmetric telescopic vibration at 1629 cm⁻¹. Meanwhile, an increase in the peak area of the C=O telescopic vibration at 1708 cm⁻¹ was observed, accompanied by the O-H telescopic vibration at 3427 cm⁻¹ and the bending vibration at 979 cm⁻¹. The changes in the infrared spectra are attributed to the partial decarboxylation of the dispersant polyacrylate, resulting in the formation of free carboxylic acid. In the ultrafine barite-weighted kill fluid system, the polyacrylate dispersant is ionized in water to form polymer anions (-COO⁻) and sodium/ammonium ions (Na⁺, NH₄⁺), and the anions are tightly bound to the surface of the particles by adsorption, causing the particles to be negatively charged, while the counterions (Na⁺, etc.) diffuse into the surrounding liquid to form a double electrical layer structure. This type of electrostatic repulsion of the same type of charge effectively prevents the particles from approaching each other and agglomerating and maintains the stability of the dispersed system. When the decarboxylation reaction occurs, the carboxyl group (-COOH) is detached from the molecular chain, releasing carbon dioxide and water, resulting in a decrease in the number of negative charges on the molecular chain, as shown in Figure 14. This process directly reduces the charge density of the polyacrylate, and the absolute value of the zeta potential of the ultrafine barite-weighted kill fluid after 20 days of aging was 43.67% less than that at 0 d. This also illustrates that the reduction in charge density weakens the ability of the dispersant to stabilize the particles by electrostatic repulsion and reduces the dispersibility of the barite-weighted kill fluid, which results in the poorer stability of the kill fluid system to settle. Although the changes in carboxylate can be seen in the infrared spectra, the more accurate spectral changes in decarboxylation still need to be further analyzed and verified by other means such as mass spectrometry, which is also the focus of the subsequent research on the stability mechanism of ultrafine barite-weighted kill fluid settling.

4. Conclusions

(1) The viscosity of the ultrafine barite-weighted kill fluid shows an increasing trend with rising temperature, and the settling value of the ultrafine barite-weighted kill fluid also increases with an increasing aging time and resting temperature.

(2) For ultrafine barite-weighted kill fluids with a low density (1.4~1.6 g/cm³) at 220 °C and 20 d of static aging, the rheological stability of the kill fluid remains adequate, the settlement value is less than 1.5 N, and the glass rod free falls to the bottom. This indicates that the low-density ultrafine barite-weighted kill fluid exhibits good fluidity and settling stability, meeting the operational requirements for oil testing lasting over 20 days.

(3) The dynamic–plastic ratio of the ultrafine barite-weighted kill fluid with a high density of 1.9 g/cm³ is less than 0.10 Pa/mPa·s after aging at 220 °C for 15 d. The sedimentation value is more than 3 N and D90 > 3 µm after aging for 20 d, leading to slurry agglomeration and a decline in suspension performance. Under these conditions, applying the ultrafine barite-weighted kill fluid in the field may result in difficulties in starting the pump and an increased risk of clogging the tubing column and other operational complications.

(4) By evaluating the settling stability of ultrafine barite-weighted kill fluids with varying densities and temperatures at high temperatures, a correlation between the method of dropping the bar of the kill fluid and the settling value is established. Furthermore, a quantitative grading range for the settling stability of ultrafine barite-weighted kill fluids is developed. However, the static aging conditions in the laboratory cannot completely replicate the dynamic environment in the field, and the quantitative grading of settling stability still has some limitations.

(5) The settling stability of the ultrafine barite-weighted kill fluid system mainly depends on the state of the dispersant adsorbed on the surface of the nano-micron-sized barite particles. If the polyacrylate dispersant is partially decarboxylated, it will lead to a decrease in the number of negative charges on the molecular chain, which will weaken the ability to stabilize the particles by electrostatic repulsion, and this change will reduce the dispersibility of barite, which will lead to a deterioration in the settling stability of the entire ultrafine barite-weighted kill fluid system. This change diminishes the dispersibility of barite, leading to a decline in the overall settling stability of the ultrafine barite-weighted kill fluid system.