What is oil well cementing technology?

In order to strengthen the wall of the well, to ensure the continuation of safe drilling, to seal off the oil, gas and water layers, to ensure the stratification of the oil test during the exploration period as well as the reasonable oil and gas production during the whole extraction process, the process of lowering the high-strength steel casing into the drilled well and filling the annular space of the wellbore and casing with cement is known as cementing operation.

Consolidation is a key link in the process of building oil and gas wells. Compared with other drilling processes, cementing has obvious special characteristics. It is a one-time operation, if the quality is not good, it is generally difficult to remedy; it is a hidden operation, the main process in the well, construction can not be directly observed. The quality is often determined by the accuracy of the design and good or bad preparation work, and is affected by a variety of factors, but also affects the subsequent works; it is a costly operation, casing costs account for about 25% of the total cost of drilling; it is a highly technical operation with short construction time, many work processes and large workload. Obviously, the quality of cementing is an important indicator of the quality of a well.

The main content of cementing technology includes casing column design and cementing process. This section will be introduced around them.

I. Casing column design Casing column design is based on the working conditions of the casing column in the well, selecting different steel grades or different wall thicknesses of casing to form the casing column, so that the external force and strength to meet a certain relationship, in order to ensure that the casing column safety and economy. Casing column design is one of the basic skills that drilling engineers must master.

1. Casing column structure casing column is assembled by casing with some special parts to ensure the smooth running of cementing operation.

1)The casing casing is a seamless steel pipe rolled from high-grade alloy steel. In order to meet the requirements of different well structure and working conditions on the strength of the casing, the casing has a variety of diameters, wall thicknesses, steel grades and different connection methods.

The diameter of casing is generally 114.3～508mm, and there are also larger sizes,*** there are more than ten kinds. Commonly used casing wall thickness of 8 ~ 13mm, thin-walled casing wall thickness of only 5.21mm, thick-walled casing wall thickness of 16.1mm. obviously, for a certain casing diameter, the greater the wall thickness of the casing the higher strength.

Casing made of different steel, strength is not the same. In order to facilitate the distinction, with the steel grade named. Domestic casing D40, D55, D75 three kinds of steel grade, the subscript number for the minimum yield limit of steel, the unit is kg/mm2. API standard casing steel grade code for N-80, C-95, etc., the number in the code multiplied by 1000 is the minimum yield strength of the casing value, the unit is lb/in2.

Most of the casing for the external thread at one end, and at the other end for the internal thread (plus connector). Internal threads (plus hoop), mostly round threads, 6 to 8 buckles per inch. In order to strengthen the connection strength, some high-strength casing using trapezoidal buckle, 5 buckles per inch.

For corrosive fluids such as sulfate formations, should be used with sulfur-resistant characteristics of the casing. Sulfur-resistant casing chemical composition control is strict, and after special heat treatment. At present, our country mostly use the Japanese anti-sulfur casing, its anti-sulfur performance is better.

2) lead shoe lead shoe is made of pig iron or aluminum conical short section, covered with small holes in the whirlpool. Its function is: (1) to guide the casing into the well and prevent the casing from inserting into the rock layer of the well wall; (2) to constitute a circulating channel for cement slurry when injecting cement, and the cement slurry will flow out and return upward through the rotating holes to improve the efficiency of top replacement. When re-drilling after cementing, the pilot shoe can be easily drilled off.

3)Casing shoeThe casing shoe is a special short section attached to the leading shoe, generally made of casing clamp. The lower end is processed into a 45 ° internal bevel angle, the role is to prevent the bottom of the casing from damage when drilling down to prevent drilling tool joints, drill bits touching the casing. At the same time, it becomes the measurement mark of casing depth when logging.

4)Casing return valveThe main functions of the casing return valve are: (1) to prevent the high-density cement slurry from flowing back into the casing after cementing; (2) to increase the buoyancy of the casing column during the process of casing lowering to reduce the load on the big hook of the drilling rig; and (3) because the ball seat baffle plate of the return valve is the support ring of the cementing plug, its position determines the length of the cementing plug and the height of the slurry in the annulus.

5)The casing retainerThe casing retainer is a steel elastic device mounted on the casing body. Its function is to ensure that the casing column is centered in the borehole and to improve the quality of cement bonding with the well wall. In wells with large changes in inclination and orientation, it is especially important to pay attention to the installation of the correcting device, and through complex calculations, to reasonably select the number of installations and the installation position.

In addition to the above basic components of the casing column, there are many other special components installed to meet different requirements, and will not be discussed in detail.

2. Casing column design method is that the casing column, which is lowered into the well, is subjected to various external loads in the process of cementing and subsequent production, the main loads of which are axial pulling force and external squeezing force. Mechanical analysis results show that: axial tension in the wellhead at the maximum, the bottom of the well at zero, along the depth of the well gradually decreases and is a straight line distribution; external extrusion pressure is the smallest in the wellhead, the bottom of the well is the largest, along the depth of the well gradually increases and is also a straight line distribution, as shown in Figure 5-9.

Figure 5-9 Casing column structure and force diagram

The casing itself has a certain ability to resist external loads, called strength. Under the action of axial tension, the casing has tensile strength; under the action of external extrusion pressure, the casing has extrusion strength. As long as the casing column on any cross-section of the casing strength is greater than the external load, the casing column is safe.

Because the casing column force is two big (the maximum axial tension at the wellhead, the bottom of the maximum extrusion force) in the middle of the small, so you can choose a different strength of the casing, constitute the strength of the two big in the middle of the casing column, in order to ensure the safety of the casing column at the same time to reduce the cost of the casing column. According to this principle, the design of the casing column is called composite casing column.

Two, cement injection process in order to fix the casing, seal the oil, gas and water layers, after the casing must be injected between the casing and the well wall. The method of injecting cement into oil and gas wells is different from that of surface construction, in which the cement slurry has to be pumped to the bottom of the well hundreds or even thousands of meters below the ground, and then returned to the annulus outside the tubing and topped off with the drilling fluids therein. The cement has to fill the entire annular space, and after solidifying and hardening, it forms a hard and dense cement ring. These characteristics determine the performance difference between oil well cement and ordinary construction cement. Oil well cement paste is required to have good fluidity and proper setting time, and high early strength. The cement ring outside the casing will be in contact with groundwater for a long time and should have good impermeability and resistance to groundwater corrosion.

1.oil well cement 1) oil well cement classification at present commonly used oil well cement is mainly high grade silicate cement. Limestone or calcareous tuff, clay or shale and a small amount of iron ore, etc., according to a certain proportion of raw materials, calcined at a high temperature of 1450 ℃ to become a silicate as the main ingredient of clinker, and then add the appropriate amount of gypsum, together with a certain fineness of the powder will be made into this oil well cement. Due to the temperature and pressure in the well with the increase in well depth and rise, must produce a variety of oil well cement, in order to adapt to the requirements of different well depth conditions.

2)Cement paste performance adjustment of a large number of cementing practice has proved: the existing variety of oil well cement is still difficult to fully meet the requirements of various downhole conditions, which must be added to certain additives to further regulate the performance of the cement paste.

(1) density adjustment. Commonly used aggravators are barite powder, ilmenite powder and galena powder; commonly used reducing agents are clay, diatomaceous earth, fly ash, hard asphalt, expanded perlite and so on. High-density cement paste can also be obtained by using the method of low water-cement ratio plus reducing agent.

(2) Adjustment of thickening time. In order to meet the requirements of the construction time, it is sometimes necessary to use retarder to extend the thickening time of cement paste. Commonly used retarding agents are sodium tannate, dragon gum powder, iron chromium salt, sulfonated tannin, lignosulfonate, tartaric acid, carboxymethyl hydroxyethyl cellulose and some complexes. You can also use quick-setting agent to shorten the solidification time. Commonly used quick-setting agents are calcium chloride, sodium silicate, formamide, potassium chloride and gypsum hemihydrate.

(3) Reduce water loss agent. Excessive water loss will cause: ① early thickening of cement paste, fluidity and even block the ring air, resulting in holding the pump; ② accelerate the reduction of the pressure of the cement slurry column; ③ mud shale water absorption and expansion, spalling, affecting oil and gas production; ④ a large number of free water up and back, affecting the sealing of the cement ring. Therefore, a suitable water loss reduction agent should be added into the cement slurry. Commonly used water loss reduction agents are carboxyethyl cellulose, carboxymethyl cellulose, carboxyethyl synthetic dragon glue powder and S-24 composite agent.

(4) Resistance-reducing agent. The drag reducing agent, also known as dispersant, turbulence agent, belongs to the cationic surface-active substances. It only plays the role of surface physicochemical, not hydration reaction with cement. The drag reducing agent has wetting and lubricating effect, which can accelerate the hydration of cement; it can prevent the cement slurry from dehydrating in the first stage and improve the early strength after cementing, and it can make the cement cake thin and impermeable, which is helpful for the protection of oil and gas layer.

There are also some commonly used cement slurry additives that can improve the various properties of cement slurry. Such as defoamer can reduce interfacial tension, inhibit the generation of gas bubbles; quartz powder is commonly used in high temperature well cementing, can improve the corrosion resistance of cement stone; salts can improve the cementing strength of rock salt layer cementing cement, etc..

2.Cementing 1)Cementing ground equipmentCementing ground equipment includes cement storage tanks, cement trucks, cement mixing funnels, pressurized air units, and water-mud pools. According to the requirements of the cement injection process, the cement slurry pipeline, water pipeline, gas pipeline will be connected to each other, constitute the cement injection construction equipment. The cement truck is the most critical among the ground equipment. The number of cement trucks depends on the number of ash storage tanks, which is determined by the amount of cement injection. Every additional set of cement trucks should increase the corresponding other equipment and pipelines. Generally every two or three ash storage tanks with a pressurized air unit.

When injecting cement, the pressurized air blower blows the cement in the ash tank into the mixing funnel through the air pipeline, and at the same time, the water is also pumped into the funnel to mix with the cement to form cement slurry, which enters into the slurry pool. The cement truck then sucks in the cement slurry and pumps it through the pipeline to the cement slurry sink. The cement slurry pumped by all the cement trucks is pooled and delivered to the cement head at the wellhead for cement injection circulation.

2)Cementing processAfter the casing is lowered to the predetermined well depth according to the design, the cement head is mounted and the drilling fluid is circulated to prepare for cementing. Figure 5-10 shows a typical construction procedure for cementing with double rubber plugs. The uppermost device of the casing column is the cement head, which contains the upper and lower rubber plugs. The lower rubber plug, together with the isolation fluid, separates the cement slurry from the drilling fluid and prevents the drilling fluid from affecting its performance after contacting the cement slurry. The lowermost end of the casing column is equipped with a pilot shoe to facilitate the lowering of the casing. The floating hoop is actually a one-way valve that prevents the cement slurry in the annulus from flowing backward into the pipe and seats the rubber plug. When the ground equipment is ready, cementing can begin.

Figure 5-10 Diagram of cementing process

1-Pressure gauge; 2-Upper plug; 3-Lower plug; 4-Drilling fluid; 5-Floating hoop 6 -Pilot shoe; 7-Cement slurry; 8-Isolation fluid; 9-Drilling fluid (1) Hit the isolation fluid. Before injecting water slurry into a certain amount of special liquid (usually CMC solution), the drilling fluid in the well and the injected cement slurry separation, and play a role in flushing the wall of the well and casing wall, as shown in Figure 5-10 (b).

(2) Open the lower rubber plug stopper and press the rubber plug. The lower rubber stopper is hollow, and the rubber membrane on its top can be ruptured under pressure, as shown in Figure 5-10(b).

(3) Use the cement truck to inject the mixed cement slurry into the casing through the cement slurry pipe sink and the cement head, as shown in Figure 5-10(b).

(4) After cement slurry injection, close the water injection mud access, open the upper rubber plug stop pin and top access, and use drilling fluid to push the upper rubber plug into the casing, as shown in Figure 5-10(c).

(5) Pump the drilling fluid into the casing. At this time, the drilling fluid pushes the upper rubber plug, the upper rubber plug pushes the cement slurry, the cement slurry pushes the lower rubber plug and the isolation fluid, and the isolation fluid pushes the drilling fluid. After the cement slurry reaches the bottom of the well, it flows out of the rotary hole on the casing column's lead shoe and returns up the annulus, as shown in Figure 5-10(d).

(6) Bumping. Because the upper rubber plug is solid, when it goes down to the floating hoop or back pressure valve seat, blocked can not continue to go down. At this time, the pump pressure surge, so that you can judge the cement slurry has topped the predetermined position, immediately stop the pump, as shown in Figure 5-10 (e).

After the cement injection process is completed, the well needs to be shut down to wait for condensation. Generally, the well should be shut down for 48h, and after the cement slurry condenses in the annulus, the cement head is removed, and sonic logging is carried out to check the quality of cementing. If the quality is qualified, then install the casing head and wellhead equipment, and drill down to remove the cement plug below the floating hoop, after which the next drilling can be carried out.