Fund Project: Supported by the National Science and Technology Major Special Project "Large-scale Oil and Gas Field and CBM Development" Project 60 "Demonstration Project of CBM Vertical Well Development in Southern Qinshui Basin" (ProjectNo.: 2006/2007). :2009ZX05060)。

About the author: Zuo Jingluan, female, engineer, now the mailing address of Zhonglian Coalbed Methane Co., Ltd.: No.88, Andingmenwai Street, Dongcheng District, Beijing; Postal code: 1000 1. Email: zuojingluan @ hotmail.com.

(1. China United CBM Co., Ltd. Beijing100011; 2. School of Petroleum Engineering, Youshi University, China, Dongying 25706 1)

Abstract: In view of the problems that the borehole wall of coal reservoir is easy to collapse and the drilling fluid is easy to pollute coal reservoir, a hollow glass bead low-density drilling fluid system is developed. The drilling fluid has good rheology and filtration, and the mud cake is thin and dense. At the same time, it has good temperature resistance, pollution resistance, collapse prevention, settlement stability and reservoir protection performance. The field test of 1 well was successfully carried out in Qinnan demonstration area, which effectively prevented the liquid from polluting the coal reservoir.

Keywords: field test of rheological filterability of low-density drilling fluid contaminated by coal reservoir Study on light drilling fluid of coalbed methane

Study on low density drilling fluid for coalbed methane

Sakyo Luan 1, Sun Hansen 1, Lu Kaihe 2.

(1. China United CBM Co., Ltd., Beijing100011; 2. School of Petroleum Engineering, China Shiyou University, Dongying 25706 1)

Abstract: In view of borehole collapse and coal seam pollution caused by drilling fluid, a low-density drilling fluid was studied by adding hollow glass beads to reduce its density. The research shows that the low-density drilling fluid has good rheology and filtration, and the mud cake is thin and dense. In addition, the drilling fluid has excellent properties such as temperature resistance, pollution resistance, anti-sloughing, settlement stability and formation protection. This low-density drilling fluid has been successfully tested in a well in Qinbei demonstration area. Good performance of protecting coal formation from pollution was observed.

Key words: the formation of coal; Pollution; Light drilling fluid; Rheological properties; Filtration performance; field test

China coal reservoirs are generally characterized by low pore pressure, poor permeability and developed fractures. The invasion of drilling fluid is easy to cause coal seam pollution and affect the output of coalbed methane. In drilling construction, appropriate drilling circulation media should be selected according to different requirements and strata, in line with the principles of saving costs, ensuring underground safety and protecting the original structure of the target coal seam from damage.

In this paper, a low-density drilling fluid system is developed to protect wellbore stability and reduce reservoir pollution, and it has been successfully applied in the field.

Study on the cause and mechanism of 1 coal reservoir damage

The physical parameters, X-ray diffraction analysis and scanning electron microscope analysis of coal samples collected from Qinnan demonstration area were carried out. The results show that coal reservoirs are characterized by low porosity, low permeability and developed fractures. At the same time, coal reservoirs also have the characteristics of low pressure and low water saturation. These characteristics determine that if effective measures are not taken in the process of drilling and completion, the reservoir will be greatly damaged, resulting in a decline in permeability and production. The causes of reservoir damage during drilling mainly include the following aspects.

Stress sensitivity damage 1. 1

See table 1 for the influence of stress on permeability of coal and rock. It can be seen from the table 1 that when the effective stress increases, the permeability of coal and rock drops sharply, indicating that it has strong stress sensitivity.

Table 1 experimental results of stress sensitivity of coal and rock

1.2 speed-sensitive damage

The flow experiment was carried out with 1% standard saline, and the experimental results are shown in Table 2. As can be seen from Table 2, when the velocity of standard brine in coal samples increases, the permeability not only does not decrease, but increases, indicating that there is no velocity sensitivity. When the flow velocity is large, the outflow of fine coal is observed in the experiment. Because the particles are too small to block the seepage channel, the permeability of coal and rock is increased.

Table 2 Experimental results of speed sensitivity

1.3 water lock is damaged.

The micropores in the coal seam can be regarded as countless tortuous capillaries, while the coal seam is generally weakly hydrophilic, and the foreign liquid will produce strong water absorption when it contacts the coal seam. The invasion of liquid is very harmful to reservoir permeability. Experiments show that when the liquid saturation reaches 10%, the gas permeability is damaged by 50%, while when the liquid saturation reaches 30%, the gas permeability is almost zero.

1.4 solid intrusion

There are micro-cracks in coal and rock, and solid and liquid phases are easy to invade during operation. If effective measures are not taken, solid phase and liquid phase will invade the reservoir in large quantities, and with the subsequent operation, the amount and depth of invasion will continue to increase, which will greatly reduce the permeability of the reservoir and seriously pollute the reservoir.

Because of the low pressure of coal reservoir, developed fractures and bedding, the invasion of drilling fluid into the reservoir is the main damage mechanism, so low-density drilling fluid system should be used as much as possible to prevent drilling fluid from invading the reservoir in large quantities.

Study on low density drilling fluid for protecting coal reservoir

2. 1 Selection of density reducer

From the cause and mechanism analysis of coal reservoir damage, it can be seen that pressure difference is an important factor affecting coal reservoir damage, and the greater the pressure difference, the more serious the coal reservoir damage.

Hollow glass bead is a kind of single cell alkali calcium borosilicate material, which is white powder in appearance, chemically inert and resistant to high temperature and high pressure. The real density of drilling fluid is low, which can be reduced to 0.6~ 1.0g/cm3. This technology is simple, low risk and good reservoir protection effect, which can completely meet the drilling and completion construction of low-pressure coalbed methane wells and some underbalanced wells. The research and application of this technology will enrich the types of drilling fluids in low-pressure coal reservoirs and change the present situation of serious damage to coal reservoirs.

2.2 Performance evaluation of hollow glass microspheres

(1) density of hollow glass microspheres

After many tests in the laboratory, the true density of hollow glass beads is 0.37 ~ 0.45 g/cm3.

(2) Particle size and distribution range of hollow glass microspheres

The particle size of hollow glass microspheres was analyzed by laser particle size analyzer, and it was found that the particle size of 90% hollow glass microspheres was less than123 μ m.

(3) Mechanical fracture strength and compressive strength of hollow glass microspheres.

Mechanical rupture strength refers to the highest pressure at which hollow glass microspheres per unit volume can be directly compressed under mechanical pressure device, while compressive strength refers to the highest pressure at which hollow glass microspheres with a certain concentration will not rupture and precipitate under external pressure in water with different constant temperatures. For drilling fluid, the performance of the latter reflects the stability of the material and is more important. The experimental results of the strength of hollow glass microspheres are shown in Table 3.

Table 3 Strength of Hollow Glass Microspheres

As can be seen from Table 3, hollow glass microspheres have good compressibility and will not break under the pressure of 30MPa.

(4) The relationship between the content and density of hollow glass beads.

Different amounts of hollow glass microspheres were added to tap water, and the density of the added liquid was determined. With the increase of the content of hollow glass beads, the liquid density decreases, and when the content is 40%, the density can be reduced to 0.75g/cm3.

2.3 Evaluation of the Influence of Hollow Glass Beads on Drilling Fluid Performance

Preparation of (1) bentonite slurry

400ml water+12g bentonite +0.06g soda ash, stirred for 20min and aged for 24h.

(2) Influence of hollow glass beads on drilling fluid performance.

Figure 1 shows that the filtration rate of drilling fluid decreases after adding hollow glass beads. Before the content of 10%, the filtration rate decreased the fastest, and when the content of 10%~30%, the decline rate slowed down.

Figure 1 Relationship between water loss of drilling fluid API and hollow glass bead content

It can be seen from Figure 2 that the plastic viscosity of drilling fluid increases with the increase of the content of hollow glass beads, but the plastic viscosity does not increase much when the content is less than 30%, and it increases obviously when it is greater than 30%.

It can be seen from Figure 3 that the dynamic shear force of drilling fluid increases with the increase of the content of hollow glass beads. When the content is 40%, the dynamic shear force increases from 3Pa to nearly 5. 1Pa.

The final permeability recovery rate of cores contaminated by water-based drilling fluid containing hollow glass microspheres can reach 95%, while the final permeability recovery rate of cores contaminated by drilling fluid without hollow glass microspheres is less than 60%. Therefore, the hollow glass bead drilling fluid is beneficial to protect the reservoir and the mud cake formed is easy to remove.

2.4 Study on Hollow Glass Beads Low Density Drilling Fluid

(1) Single dose screening

A certain amount of tackifier was added to the base slurry, and its room temperature properties were determined after 20min minutes of high-speed stirring. Then it was aged at 120℃ and 150℃ 16h respectively, and its properties were measured after cooling to room temperature. Among all kinds of viscosifiers evaluated, DSP-2 has good temperature resistance, can improve viscosity and shear force, and also has good filtration loss reduction effect, so DSP-2 is selected as the viscosifier of drilling fluid system. LY- 1 has good filtrate loss reduction effect after aging at room temperature or high temperature, which shows that it has good temperature resistance and can be used as filtrate reducer for drilling fluid system. Amino polyol AP- 1, sodium silicate, potassium silicate and high concentration sodium formate all have good inhibitory effects, and the combination of amino polyol AP- 1 and some salts has good inhibitory effects; The plugging and anti-collapse agent FF-2 has a good plugging and anti-collapse effect. Several surfactants can reduce the interfacial tension, among which SP-80 has the best effect, and the surface tension of SP-80 has little change with temperature, which shows that it has good temperature resistance.

(2) Study on drilling fluid formula

(1) drilling fluid formulation and performance optimization

After determining the tackifier, filtrate reducer, inhibitor and surfactant, the dosage of each treatment agent is optimized by using its characteristics, so as to obtain a drilling fluid formula that not only meets the requirements of drilling engineering, but also helps to protect reservoirs. After a lot of experiments, the optimized drilling fluid formula and performance are shown in Table 4.

Fig. 2 Relationship between plastic viscosity of drilling fluid and content of hollow glass beads

Fig. 3 Relationship between dynamic shear force of drilling fluid and content of hollow glass beads

Table 4 Optimum drilling fluid formulation and performance

It can be seen from Table 4 that the preferred drilling fluid has good rheological property and filtration performance, and the mud cake is thin and compact, with API filtration less than 5ml and high temperature and high pressure filtration less than 15ml. After aging at 120℃ 16h, the drilling fluid is stable, indicating that it has good temperature resistance.

Adding different amounts of inferior soil powder into the optimized formula, the optimized drilling fluid has stable performance before and after pollution and good anti-pollution performance.

The recovery rate of the optimized formula is much higher than that of clean water, and the linear expansion is much smaller than that of clean water, which shows that the optimized formula can effectively inhibit the hydration expansion and dispersion of shale and has good anti-sloughing performance.

(2) evaluation of plugging performance

It can be seen from Table 5 that the optimized formula has good plugging effect on sand layers with different permeability.

Table 5 Experimental data of sand layer plugging

(3) Evaluation of settlement stability

The experimental results show that the optimized formula has good high-temperature settlement stability. After standing for 48 hours, the difference between the upper and lower densities of drilling fluid is only 0.02g/cm3.

(4) Performance evaluation of drilling fluid for reservoir protection.

As can be seen from Table 6, the permeability recovery rate of the core is high, indicating that the optimized drilling fluid has a good protective effect on the reservoir.

Table 6 Permeability recovery experiment

3 Field test study of drilling fluid

On the basis of indoor theoretical and experimental research, 1 well was tested in Qinnan demonstration area.

3. 1 Basic information of test well

The drilling depth of the exploratory well is 690.00 meters, the drilling horizon is Carboniferous Taiyuan Formation, and the target coal seam is 3# coal seam (639.00 ~ 645.00 meters) of Shanxi Formation of Lower Permian.

3.2 Field test

The drilling fluid prepared by field test has a density of 0.95g/cm3, a viscosity of 55Pa·s, and a pH of 8. Because the depth of the well is 590 meters, drilling fluid has been used until drilling, and the construction is smooth.

The field test results show that hollow glass beads can reduce the density of drilling fluid. The density of drilling fluid is 0.95g/cm3, and the water loss of drilling fluid is small. The glass beads with smaller particle size also have a good plugging effect, temporarily blocking the roaring passage of coal seam and forming a protective film, effectively preventing the liquid from polluting the coal seam.

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