Research and practice of shale gas oil-based mud cementing slurry technology for horizontal wells

Hongsheng Company has been studying the cementing slurry and prefixed fluid technology for shale gas oil-based mud horizontal wells since the beginning of 2013. On the basis of a large amount of preparatory work such as theoretical analysis, technical demonstration and full experiments, Hongsheng Company formally carried out on-site application in the second half of 2013. Four wells were cemented in that year, with satisfactory quality results. The cementing quality rate in the horizontal section was 100, and the cementing quality of two wells was excellent. In 2014, 23 oil-based mud horizontal wells were cementing, and the excellent rate of cementing quality in the horizontal section reached 100%. The two-year practice has proved that the cementing quality of oil-based mud horizontal wells constructed with cement slurry and preflushes provided by Hongsheng Company is relatively stable, and the cementing technology of oil-based mud horizontal wells of Hongsheng Company tends to be mature, which has also been recognized by the owner and the cementing company that provides us with service opportunities.

In the past two years, Hongsheng Company has gained very useful knowledge and experience in the slurry technology practice of shale gas oil-based mud horizontal well cementing.

Understanding of Key Technical Points in 1. Oil-based Mud Horizontal Well Cementing Technology

Compared with ordinary horizontal well cementing, oil-based mud horizontal well cementing is carried out under oil-based mud medium. In addition, others are the same as horizontal well cementing. In fact, we realize that there are four key points and key points of oil-based mud horizontal well cementing technology: first, good borehole conditions, no rock cuttings bed, requiring careful drilling and well flushing, and the standard for clean borehole flushing is that the screen margin on the vibrating screen should not exceed one washbasin per hour; Second, the casing should be high in medium, especially for horizontal casing, centralizer should be added to each casing, it can all be used with rigid or rigid elastic cross. Elastic or rigid centralizer should be added to every two casings in inclined well section and elastic centralizer should be added to every three casings in straight well section. The third is to adopt high-efficiency flushing fluid technology to completely remove mud cake on casing and well wall to provide good conditions for ensuring interface cementation quality. The fourth is to adhere to large displacement and high pump pressure construction, in most wells, the flushing fluid has already entered the annulus when the water injection mud is injected. Both the water injection mud and the replacement slurry require large displacement, so as to have good displacement effect and excellent flushing effect.

Among these four technical points, the first two are also the technical points of horizontal well cementing, and the latter two are especially for oil-based mud horizontal wells. To ensure the oil-based mud horizontal well cementing quality of these four indispensable, otherwise there will be no oil-based mud horizontal well cementing quality to speak.

In order to realize the implementation of the above four key technologies, only the efforts in cementing are far from enough, and there must be understanding and full support in drilling.

Flushing characteristics of 2. oil-based mud

Because the current shale gas horizontal well depth is not too deep, the vertical depth is generally about 3000m, which belongs to the medium temperature well section, and the cement slurry technology under the medium temperature condition is not difficult, so the cement slurry for oil-based mud horizontal well cementing is not a technical problem. The difficulty is scientific and reasonable flushing fluid technology.

The function of flushing fluid is to remove the oil-based mud cake on the casing wall and the well wall. On this basis, the spacer fluid further cleans the residual oil film on the casing wall and the well wall and makes the interface change from oil wetting to water wetting to realize wetting reversal, which provides good conditions for the cement slurry behind it to bond with the interface. In this way, the cementing quality of the one-two interface is guaranteed.

Experiments and practice make us realize that the flushing difficulty of oil-based mud is related to three factors. First, the flushing difficulty is different for different types of oil-based mud. The flushing difficulty of diesel-based oil-based mud is more difficult, and the flushing of asphalt-based oil-based mud is easier. This is because diesel is more active and permeable than asphalt, and diesel is more difficult to be washed away than asphalt, diesel has stronger adhesion than asphalt, and the mud cake formed is tougher and more stubborn, and is not easily washed away. Second, the service time of oil-based mud is different, the difficulty of flushing is different, the oil-based mud used for a long time downhole uses more agents because of the maintenance of performance, which makes the mud composition more complex and thus more difficult to flush, and the oil-based mud used for a short time is easier to flush. Third, the density of oil-based mud is different, the difficulty of flushing is different. Oil-based mud with low density is easy to flush, while oil-based mud with high density is not easy to flush. This is because greater vitality and flushing energy are needed to wash out the weighting materials in the mud cake.

3. flushing fluid

1. Main components:

Organic solvents, surface active materials, basic materials, anti-expansion materials, penetrating materials, stabilizers, etc.

2. Main functions:

Dispelling oil, destroying the false mud cake structure and taking away part of the false mud cake.

3. Displacement mechanism:

Organic solvents have a strong ability to dissolve fats and oils. Surface active materials can reduce the surface tension of the oil-water interface, weaken the adhesion of grease on the casing and the surface of the well wall, and the grease enters the flushing fluid to be emulsified, dispersed and suspended in it to be taken away. Under certain temperature conditions, the alkaline material reacts with the saponifiable oil to form a water-soluble fatty acid salt.

4. Features:

① Safety

Ground safety: mild performance, corrosive and irritating, organic solvent volatility is small, high ignition point.

Downhole safety: it has the function of preventing well wall from swelling and collapse, and has good compatibility with drilling fluid, spacer fluid and cement slurry.

② Environmental protection

Organic solvents are environmentally friendly, non-toxic, low volatility, no pollution to the environment and so on.

③ Efficient

The oil displacement mechanism of alkaline oil displacement, surfactant oil displacement and organic solvent oil displacement is integrated, which can flush the oil film on the well wall and casing wall in a short time, and make the well wall and casing wall change from "oil wet" to "water wet. At the same time, it also has good destructive ability to fake mud cake.

5. Evaluation of flushing capacity:

While understanding the flushing law of oil-based mud, many experiments have been carried out on the flushing effect of the flushing fluid. The flushing force of the flushing fluid is provided by the kinetic energy of the flowing flushing fluid. The kinetic energy (1/2)mv2 of the flushing fluid is proportional to the material density of the flushing fluid and the square of the flow rate. If the flow rate is doubled, the kinetic energy of the flushing fluid will be increased by 4 times, the scouring force can be increased by 4 times. The scouring force of 600 revolutions (the up-return speed converted into the annulus of the 215.9 borehole and the 139.7 casing is 1.26 m/s, and the displacement is 42.4L/s) is 4 times of the scouring force of 300 revolutions (the up-return speed is 0.63 m/s, and the displacement is 21.2L/s), and the scouring force of 200 revolutions (the up-return speed is 0.42 m/s, the equivalent displacement is 14.1L/s) is only 44% of 300 revolutions. The greater the scouring force, the better the scouring effect.

According to the specific conditions of several shale gas horizontal wells, the calculated annular air return rate is close to the surface linear velocity of φ 300 sleeve of 6-speed rotary viscometer, 0.625 m/s, and the contact time between the fluid of one side of the annular air and a certain point in the annular air is about 1min, so φ 300 rpm × 1min is used to evaluate the flushing capacity.

Simulate the mud cake flushing test on the borehole wall (use a water loss instrument to lose water to form a filter cake), as shown in Figure 1.

A lot of exploration and experimentation has made us realize that:

From the flushing effect, it is less difficult to flush the filter cake formed by the sleeve in the oil-based mud (simulating the mud cake on the casing), and it can be washed clean in three minutes. It is more difficult to flush the filter cake formed by oil-based mud losing water on the filter paper (simulating the mud cake on the borehole wall), and part of the surface mud cake is washed out in three minutes.

The mud cake on the well wall is very stubborn due to the special conditions of its formation. To wash off the mud cake on the well wall, the amount of flushing fluid should be appropriately increased to achieve good results.

It is not the best way to solve the problem of flushing mud cake on the wall by simply increasing the amount of flushing fluid, but also to use flushing agent to increase the flushing effect.

The scouring agent is preferably a material of high density, because the scouring force of the scouring liquid is provided by the kinetic energy of the flowing scouring agent, and the kinetic energy (1/2)mv2 of the scouring agent is proportional to the density of the substance. The scouring agent with high density provides large scouring force and good scouring effect.

Generally, the effective time of flushing and flushing is kept at 20-25 minutes, so that the cementing quality can be guaranteed.

The continuous improvement of the 4. site pre-liquid has achieved results

For more than a year, the different structures and quantities of pre-cementing fluids for oil-based mud horizontal wells have been explored and experimented, and the result is that the cementing quality of the horizontal section is of high quality. See the following table for the specific usage statistics of precementing fluid for oil-based mud horizontal wells that have been constructed:

Hour	mud density	Flushing fluid	Oil-based flushing fluid			barrier fluid
			Density	Quantity	base liquid
Coke Page 12-4HF	1.58	Diesel 3 square	1.65	28	clear water oil base	4 cubic meters 1.60 oil base liquid 5 cubic meters
Coke Page 12-1HF	1.60	Diesel 3 square	1.65	28	clear water oil base	11 square 1.70
Coke Page 1-4HF	1.58	Diesel 3 square	1.35	8	oil-based	30 square 1.71
YS108H1-01	2.15	--	1.90	10	oil-based	8.3 Party 2.2
			2.18	18	water-based
D-Page 1HF	1.62	Diesel 2.1	1.62	25	clear water oil base	8 square 1.70
Focal Page 16-1HF	1.54	Diesel 3.9	1.50	20	clear water oil base	8 square 1.80
Focal Page 42-2HF	1.45	Diesel 4 cubic meters	1.45	20	clear water oil base	6.4 Party 1.70
Focal Page 42-1HF	1.51	Diesel 2 square	1.50	20	clear water oil base	8 square 1.60
Focal Page 20-2HF	1.52	Diesel 1 square	1.40	20	clear water oil base	8 square 1.70
Coke Page 23-3HF	1.45	Diesel 0.8	1.40	20	clear water oil base	2 square 1.70)
Coke Page 23-1HF	1.65	-	1.52	24	clear water oil base	8 square 1.75
New 505H	2.08	-	2.09	25	clear water oil base	15 square 2.21
Focal Page 42-3HF	1.44	Pure oil base 4 square	1.35	20	clear water oil base	10 square 1.40
Focal Page 42-4HF	1.51	Pure oil base 4 square	1.40	20	clear water oil base	10 square 1.45
Coke Page 40-2HF	1.63	Pure oil base 4 square	1.45	20	clear water oil base	10 square 1.70
Coke Page 23-2HF	1.65	Pure oil base 4 square	1.60	20	clear water oil base	10 square 1.70
Focal Page 20-1HF	1.52	Pure oil base 4 square	1.50	20	clear water oil base	10 square 1.70
Focal Page 27-3HF	1.53	Pure oil base 4 square	1.50	20	clear water oil base	10 square 1.70
Gaosha 307-1HF	2.12	--	2.10	28	clear water oil base	15 square 2.10
Gold Page 1HF	1.5	(Qingshui oil base) 5 cubic meters	1.6	30	clear water oil base	10 square 1.65
YS108H3-2	2.2	(Qingshui oil base) 5 cubic meters	2.2	30	clear water oil base	12 square 2.25
Coke Page 26-3HF	1.52	Pure oil base 4 square	1.5	22	clear water oil base	10 square 1.70
YS108H11-2	1.96	--	2.00	30	clear water oil base	10 square 2.05
Focal Page 21-1HF	1.52	Pure oil base 4 square	1.5	25	clear water oil base	12 square 1.55
Focal Page 54-6HF		(Qingshui oil base) 5 cubic meters	1.35	20	clear water oil base	10 square 1.4
Coke Page 60-3HF	1.49	(Qingshui oil base) 5 cubic meters	1.56	20	clear water oil base	10 square 1.55
Focal Page 47-2HF	1.5	(Qingshui oil base) 5 cubic meters	1.4	20	clear water oil base	8 square 1.46

From the above pre-liquid usage can be seen:

All used oil-based scouring fluid, the quantity varies, but the majority of 20 cubic meters;

All the water-based isolation fluids are used, and the quantity varies, but most of them are 10 cubic meters;

There are 22 wells using flushing fluid, of which 9 wells use diesel oil as flushing fluid, 11 wells use oil-based flushing fluid, and 2 wells use water-based flushing fluid;

In the initial 9 wells, diesel oil was used as flushing fluid, and the number of wells used was gradually reduced until it was canceled.

There are 5 wells without flushing fluid, of which 4 wells directly use oil-based flushing fluid by self-dispensing and self-injection.

The single well dosage of oil-based flushing agent is about 8-10 tons.

Oil-based flushing agent is used much more (10T) in well Ding Page 1 and well Jiao Page 16-1, which are almost of high quality.

The above different structures of the pre-liquid use effect: the horizontal section of the cementing quality is very good.

In addition, among the 27 oil-based mud horizontal wells that have been constructed, there are 5 wells with the density of well slurry above 1.90g/cm3 and 22 wells with the density of well slurry below 1.65g/cm3. The density of most wells is below 1.65g/cm3.

Several high-density oil-based mud horizontal wells are:

Hour	mud density	Flushing fluid	flushing liquid			barrier fluid	Preparation method of flushing liquid
			Density	Quantity	base liquid
YS108H1-01	2.15	--	1.90	10	oil-based	8.3 Party 2.2	Tank preparation
			2.18	18	water-based
New 505H	2.08	--	2.09	25	clear water oil base	15 square 2.21	Self-matching and self-note
Gaosha 307-1HF	2.12	--	2.10	28	clear water oil base	15 square 2.10	Self-matching and self-note
YS108H3-2	2.2	3 cubic meters (clear water oil base)	2.2	30	clear water oil base	12 square 2.25	Self-matching and self-note
YS108H11-2	1.96	--	2.00	30	clear water oil base	10 square 2.05	Self-matching and self-note

It can be seen from the above table that the amount of flushing agent used in several horizontal wells with high density oil-based mud is similar to that used in horizontal wells with slurry below 1.65g/cm3, the effect is the same, and the cementing quality in horizontal section is excellent. It shows that the amount of flushing fluid used in high-density oil-based mud horizontal wells is appropriate, and the amount of flushing fluid used in low-density oil-based mud horizontal wells is slightly rich.

5. successful attempt to improve the injection mode of oil-based scouring fluid

On May 12, 2014, the new 505H well constructed by Southwest Cementing Team 1 was first used to improve the injection method of oil-based flushing fluid, and the self-injection experiment was successfully carried out. On July 31, it was then successfully applied in Gaosha 307-1HF well constructed by Zhongyuan Cementing Company, and then the self-injection method of flushing fluid was successively applied in Jinye 1 well, YS108H3-2 well and YS109H11-2 well. The advantages of the injection method are: the dry mixing of the flushing liquid, the base liquid can be prepared in advance, the site through the cement truck to complete the injection, convenient operation is conducive to continuous construction, the flushing agent is effectively used without waste, does not produce waste, reduces sewage, avoids environmental pollution, reduces the labor intensity of on-site construction. In the future, when conditions permit, the cementing company shall be persuaded to use this method to inject flushing fluid.

6. continuous improvement of cement slurry formulation has achieved results

At the initial stage, it was found in the field construction that the cement slurry in horizontal wells with oil-based mud was mostly thick and the density could not be increased. The analysis showed that the reason for the high dosage of fluid loss reducer was that the dosage of dry mixing material BS100 was reduced from 2% to 0.5 in the cementing of the 23-3HF well in Jiaoye. After the optimization of the cement slurry formula, the field construction solved the problem of thick cement slurry, then in the coking page 20-2HF well cementing continue to dry mix material BS100 addition is also reduced to 0.5, the site construction effect is the same, to solve the slurry thick, density can not go up the problem. After that, the dry mixing materials of oil-based mud horizontal well cementing all controlled the dosage of BS100 at about 0.5, which not only facilitated the on-site slurry mixing, but also ensured the density of cement slurry, and met the intuitive requirements of on-site personnel for slurry.

Use of 7.-elastic ductile cement

After cementing of horizontal wells, large-scale fracturing construction will be carried out in the horizontal section. The cement ring in the fracturing section will rupture. If the cement ring is hard and brittle, the fracturing crack will spread far away, even causing the cement ring in the inclined section and the vertical section to rupture. Shale gas will go up to the ground through the cracks, resulting in pressure at the wellhead. If elastic ductile cement is used, the tiny ductile deformation of the cement ring can absorb the energy transmitted during fracturing, so that the fracturing crack will not spread very far, the cement ring of some inclined well sections and the whole straight well section can remain in good condition, and shale gas will not move up.

The so-called elastic toughness cement, that is, plasticizing and toughening micro-expansion anti-gas channeling cement slurry, is cement stone with high strength and certain toughness. In order to make the cement stone have elastic toughness, the main measure is to add plastic agent. We have added plastic agent in the oil-based mud horizontal well cementing of coke stone. The cement ring has elastic toughness, so that both high compressive strength (up to 30MPa) can withstand the extrusion of fracturing and certain toughness can recover the deformation generated after fracturing.

1. Main components:

Cement, water, plastic agent, expansion agent, fiber, multi-functional water loss agent, wide retarder, surfactant, etc.

2. Features:

① to meet the requirements of site construction, with high viscosity, high dynamic cutting, is conducive to improve the efficiency of displacement;

②Surfactant improves the interface affinity, which is beneficial to improve the quality of interface bonding;

③ The micro-expansion of cement stone volume and the stress of casing and borehole wall are beneficial to improve the quality of interface bonding;

The plastic agent and fiber fully increase the elasticity and toughness of the cement stone, which can reduce the damage degree of the cement ring by the operation such as the sweep plug;

⑤ BS200R wide retarder can make the cement slurry right angle thickening, through right angle thickening and lattice expansion to achieve better anti-gas channeling effect.

⑥ Fiber can greatly reduce the invasion of the leading slurry to the reservoir and effectively protect the reservoir.

⑦ BS200R type wide retarder, the sensitivity to temperature is small, can make the cement slurry in the thickening time to meet the design requirements of the top strength of the rapid development.

3. Quality effect:

Recently, from the relevant information, the use of Hongsheng company's cement slurry solid well fracturing late wellhead gas blow-by is less, these facts show that the use of elastic toughness cement is successful.