(A) the choice of cementing cement
The selection of cementing cement should be based on drilling depth, cementing ring strength and process method. Petroleum cementing cement is mainly classified according to the applicable well depth and temperature, and ordinary cement is classified according to the age strength of cement stone.
The American API standard divides oil well cement into nine grades: A, B, C, D, E, F, G, H and J. Among them, A, B and C are matrix cement, D, E and F are allowed to add regulators during firing, G and H are allowed to add gypsum, and J must meet specific standards.
Grade a: applicable depth 0 ~ 1828.8m, temperature 76.7℃. The common type is only 1, and there is no special performance requirement.
Grade b: the applicable depth is 0 ~ 1828.8 m, which belongs to medium-heat cement with a temperature of 76.7℃. There are two kinds of medium sulfur resistance (sulfate resistance) and high sulfur resistance.
Grade c: applicable depth 0 ~ 1828.8 m, temperature 76.7℃. It belongs to high early strength cement and is divided into three types: ordinary, medium and high sulfur resistance.
Grade d: applicable depth 1828.8 ~ 3050 m, temperature 76 ~ 127℃. Adding retarder into matrix cement can be used at medium temperature and medium pressure, which can be divided into two types: moderate sulfur resistance and high sulfur resistance.
Grade e: the applicable depth is 3050 ~ 4270 m, the temperature is 76 ~ 143℃, and the retarder is added to the matrix cement, which can be used under the conditions of high temperature and high pressure, and can be divided into two types: medium sulfur resistance and high sulfur resistance.
Class F: The applicable depth is 3050 ~ 4880 m, and the temperature is10 ~160℃. Retarders are added to the matrix cement for ultra-high temperature and ultra-high pressure conditions, which can be divided into two types: medium sulfur resistance and high sulfur resistance.
G and H grades: applicable depth 0 ~ 2440 m, temperature 0 ~ 93℃. They are two kinds of matrix cement, which can be widely used after adding regulators, and can be divided into two types: moderate sulfur resistance and high sulfur resistance.
Grade J: Applicable depth is 3660 ~ 4880 m, temperature is 49 ~ 160℃, only ordinary type, used under ultra-high temperature conditions.
China classified oil well cement into four categories, namely, 45℃, 75℃, 95℃ and 120℃. The quality standards are shown in Table 3-9. Ordinary cement is divided into six categories, namely 42.5, 42.5R, 52.5, 52.5R, 62.5R and 62.5r. See Table 3- 10 for the classified strength index of ordinary cement.
Table 3-9 China Temperature Series Cement Quality Standard
Table 3- 10 Graded Strength Index of Ordinary Cement in China
For geological core drilling, ordinary cement with grade of 52.5 ~ 62.5 r can be used for cementing generally without special requirements, and oil well cement with corresponding well depth should be used for cementing under ultra-deep conditions.
(II) Determination of cementing slurry performance
The performance of cement slurry that affects cementing operation mainly includes density, water cement ratio, thickening time, setting time (initial setting and final setting time), cement slurry strength and cement slurry corrosion resistance.
1. Cement slurry density
The density of cement dry ash is generally 3.05 ~ 3.20 g/cm3. The water required for complete hydration of cement is about 20% of the cement quality, but the cement slurry can hardly flow at this time. In order to make the cement slurry flow, the amount of water added should reach 45% ~ 50% of the cement quality, and the density of the prepared cement slurry is 1.80 ~ 1.90 g/cm3, which is generally the selected density during cementing.
2. Water cement ratio of cement slurry
Water cement ratio refers to the ratio of water to dry cement in cement slurry. The greater the ratio, the smaller the cement slurry density (the thinner the cement slurry), and the cement slurry density can be adjusted by changing the water cement ratio. When fixing holes, the water-cement ratio is generally controlled between 0.45 and 0.5.
3. Thickening time of cement slurry
After the cement slurry is prepared, with the hydration reaction, the cement slurry gradually thickens and the fluidity becomes worse. In the process of injecting cement slurry by pump and replacing it, it may become more and more difficult for cement slurry to flow or even be pumped in. At this time, although the cement has not solidified, it can no longer be pumped for replacement. Therefore, the whole cementing process must be completed before the cement slurry thickens, and the thickening time determines the cementing construction time. For deep hole cementing with long construction period, a long thickening time of cement slurry should be ensured.
The thickening time of cement slurry refers to the time required for cement slurry to reach the specified value from the beginning of preparation. For example, according to API standard, the time from the beginning of mixing to the consistency of cement slurry reaching 100BC (cement consistency unit) is the thickening time of cement slurry. API standard stipulates that the initial time 15 ~ 30min thickening value should be less than 30BC. The good thickening condition is that the consistency of cement slurry is within 50BC within the total cementing construction time.
4. Setting time of cement slurry
Hydration of cement slurry begins when mixing, and the time from liquid to solid is the setting time of cement slurry. This time is different from the thickening time. Generally speaking, the setting time of cement slurry is longer than the thickening time. The setting time of cement slurry has a great influence on the construction, that is, the time from cementing to bearing a certain load after casing sealing determines the time from cementing completion to the next process, and it is usually expected to wait for solidification for about 8 hours after cementing.
5. Strength and corrosion resistance of cement stone
The strength and corrosion resistance of cement paste are mainly controlled by choosing the type of cement, that is, the chemical composition of cement.
In order to meet the performance requirements of drilling and cementing, field sampling test and performance test of cement slurry formula should be carried out before cementing, and additives can also be added to adjust the performance of cement slurry. Through the field performance test data to guide the hole cementing operation.
(3) Calculation of parameters related to water injection mud
In order to ensure the cementing quality and return the cement slurry to the predetermined height, the length of the cement plug in the casing meets the design requirements. Before cementing, the dosage of cement slurry, dry cement, displacement slurry and clean water, and the corresponding maximum pump pressure and cementing time must be calculated according to the pore size, casing diameter, running depth and cement return height.
1. Cement slurry dosage
The dosage of cement slurry is calculated according to Formula (3-5):
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Where: V is the cement slurry quantity, m3; D 1 is the average diameter of the borehole, m; D is the outer diameter of casing, m; D is the inner diameter of casing, m; H is the return height of cement slurry, m; H is the height of cement plug, m; K 1 is the additional coefficient of cement in open hole section. According to the actual situation of each region, K1=1.05 ~1.10 is obtained by experience.
The actual aperture is irregular. When calculating, the average pore diameter must be obtained by subsection according to the electric logging diameter, and the annular volume must be calculated by subsection, and then the annular volume of the whole borehole can be obtained by accumulation. The smaller the line segment, the smaller the error.
2. Dry cement dosage
The dosage of dry cement is calculated according to formula (3-6):
G=K2Vq (3-6)
Where: g is dry cement content, bag; V is the cement slurry quantity, m3; Q is the number of dry cement bags required for preparing 1m3 cement slurry; K2 is the ground loss coefficient, generally 1.03 ~ 1.05.
3. Water consumption
The consumption of clean water is calculated according to Formula (3-7):
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Where: Vs is the consumption of clean water, m3; V 1 is the water required for each bag of cement, L/ bag; G is dry cement content, and the unit is bag.
4. Replacement amount of cement slurry
When the rubber plug is pushed to the floating hoop position, the slurry exchange is completed. Therefore, the required amount of mud replacement is the sum of the internal volumes of casings with different wall thicknesses above the float collar:
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Where: Vnj is the displacement of cement slurry, m3; D 1, d2, …, dn is the inner diameter of the casing with different wall thicknesses above the cement plug, m; L 1, L2, …, Ln are the lengths of casings with different wall thicknesses above the cement plug, and m and K3 are mud compressibility factors, with values of 1.03 ~ 1.05.
In order to simplify the calculation, formula (3-9) is commonly used on site:
vnj = K3(q 1l 1+Q2 L2+…+qnLn)(3-9)
Where: q 1, q2, …, qn is the internal volume of casings with different wall thicknesses above the cement plug, L/m.
5. Maximum pump pressure
In the process of cementing, because the density of cement slurry is generally higher than that of mud, with the injection of mud, under normal circumstances, the pump pressure is from large to small (almost zero), and then from small to large. At the end of mud replacement, the pump pressure reaches the highest. The maximum pump pressure consists of the pressure difference between the liquid columns inside and outside the pipe and the resistance of cement slurry and mud flowing inside and outside the pipe, namely:
p=p 1+p2 (3- 10)
Where: p is the highest pump pressure, MPaP 1 is the pump pressure needed to overcome the flow resistance, MPaP2 is the pump pressure needed to overcome the pressure difference between the liquid column inside and outside the pipe, MPa.
P 1 can be calculated by the following empirical formula:
When the hole depth is less than 1000m, P1= 0.001L+0.8 MPa;
When the hole depth is greater than1000m, p1= 0.001l+1.6mpa. ..
Where: l is the depth of the hole in meters.
P2 can be calculated by the formula (3- 1 1):
p2 = 9.8(H-H)(ρc-ρb)× 10-6(3- 1 1)
Where: h is the height of cement column outside the pipe, m; H is the length of cement plug, m; ρc is the density of cement slurry, kg/m3; ρb is the density of displacement mud, kg/m3.
6. Cementing time
The construction time of cement injection includes the time for the cement truck to distribute all the cement slurry, replace the cement slurry and pour the gate, open the stop pin and push the rubber plug. In order to ensure the smooth construction, the total time of cement injection must be less than or equal to 75% of the initial setting time of cement slurry.
T=T 1+T2+T3+T4 (3- 12)
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Where: t is the total time for cementing construction, minT 1 is the time required for cementing, minT2 is the mud displacement time, minT3 is the time for opening and closing the gate, opening the stopper and pushing the rubber plug, generally1~ 3min; T4 is the contact pressure time, ming is the total amount of cement injected, bag; M is the amount of cement injected into the bicycle every minute, bag/minute; N is the number of cement injection vehicles; Q 1 is the displacement of mud drainage pump, m3/min；; V is the displacement of mud, m3.
(4) hole fixing operation
1. Cement cementing equipment
Cement cementing grouting equipment should be selected according to the cementing depth and quality requirements. Oil drilling has special cementing equipment. In geological drilling, 1000m shallow borehole is generally grouted by mud pump, and 1000m deep borehole is also grouted by special cementing equipment. Cementing equipment mainly includes: cement truck, cement mixer, cement distributor, cement head, rubber plug, cement ash tank and so on.
(1) cement truck
Cement truck is a special vehicle specially used for cement injection and other extrusion work, equipped with cement (piston) pump, plunger pump, calibrated water tank, pipeline, mixer, auxiliary power equipment and gearbox and other special equipment for cement injection.
(2) Cement mixer
Cement mixer is a device that directly mixes water and dry cement into cement slurry. When injecting cement, the plunger pump sucks the water in the water tank of the cement truck into the pump body through the suction pipe, pressurizes it through the plunger, discharges it through the drainage pipe, flows under the mixer, ejects it from the nozzle at a high speed, and rapidly injects it into the water outlet pipe, thus forming a vacuum at the bottom of the mixer, thus sucking out the dry cement powder that enters the mixer from the lower ash tank, mixing it with water to form uniform cement slurry, and flowing into the cement slurry mixing tank through the outlet pipe, as shown in Figure 3-27. Then, the cement pump injects the cement slurry in the mixing pool into the hole. The new cement truck is equipped with a mixer and a mixing tank.
Figure 3-27 Flowchart of Grouting and Slurry Preparation
1-ash discharge tank; 2- cement slurry; 3- cement mixer; 4— Cement slurry mixing tank; 5- Cemented pipeline
(3) Cement head
The cement joint is connected to the top joint of the casing string and communicates with the surface pipeline and cementing equipment (Figure 3-28). Both sides are welded with short joints, equipped with high-pressure gates, and respectively connected with cementing pipelines by movable joints. At the same time, stop pins are installed on the other two sides to hold the rubber plug.
Figure 3-28 Schematic Diagram of Cement Head Structure
1- union; 2— Cement head body; 3- stop pin; 4- rubber plug; 5— Cement head cover; 6- Lifting rings
(4) Rubber plug
Rubber plug plays the role of isolation, scraping and buffering in cementing operation, as shown in Figure 3-29.
Figure 3-29 Schematic Diagram of Typical Rubber Plug Structure
(a) Solid rubber stopper; (b) hollow rubber plug; (c) Self-locking rubber plug
1 core; 2- rubber plate; 3-O type sealing ring; 4- circlip
2. Pre-grouting fluid
Pre-fluid is a kind of special fluid injected into the hole before mud is injected. Its function is to separate the cement slurry from the mud, play the role of isolation, buffering and cleaning, and improve the cementing quality.
(1) cleaning solution
The cleaning fluid is used to dilute and disperse the mud, prevent the mud from gelling and flocculation, effectively clean the hole wall and casing wall, and clean the remaining mud and mud cake; It plays a buffering role between cement slurry and mud in the hole, which is beneficial to improve cementing quality. The cleaning solution should have a low density close to water, and the density is about 1.03g/cm3. It has very low plastic viscosity, good flow performance, low shear rate and low flow resistance, and can realize turbulent characteristics at low speed, and its critical turbulent velocity is 0.3 ~ 0.5 m/s; Good compatibility with cement slurry and mud.
Cleaning solution is usually made by adding surfactant or diluted mud to fresh water. Commonly used cleaning solutions include CMC aqueous solution, surfactant aqueous solution and seawater.
(2) spacer
The function of spacer fluid is: it can effectively isolate mud and cement slurry in borehole; Can form a plane propulsion displacement effect; It can buffer the low-voltage leakage layer; It has high buoyancy and drag to enhance the displacement effect.
Spacer is usually a viscous liquid. Its viscosity is higher than that of cleaning solution, its density is slightly higher, and its static shear force is also slightly higher. After the cleaning liquid is injected, the slurry is injected after the spacer liquid is injected.
When drilling and cementing geological cores, clean water is usually used to replace the front fluid.
3. Drilling operation flow
Drilling cement cementing is a one-time construction technology, and strictly controlling the quality of each process is the key to ensure the success of cementing once. The main flow of cementing operation: installation, debugging and inspection of cementing equipment → adjustment of circulating hole mud → injecting spacer into the hole → injecting mud → pressing rubber plug → replacing mud (using mud first, then using clean water) → waiting for cement slurry to solidify → washing floating mud in the pipe with mud → waiting for cement stone → sweeping cement plug, float collar and guide shoe → moving to the next process.
4. Key points of cementing operation process
1) Before cementing and grouting, the construction personnel should be carefully organized, and the post division should be clear, so as to ensure the smooth cementing work.
2) Before grouting, be sure to check and debug the grouting equipment and connect the ground manifold of the grouting equipment; According to the design, prepare enough clean water, slurry change and isolation liquid for hole fixation.
3) Inject spacer fluid and mud. When pouring the gate, a certain amount of cleaning liquid and isolation liquid are injected by cement truck to separate the slurry from the cement slurry to prevent calcium from invading, and at the same time, the cleaning hole wall and the outer wall of the casing are washed to improve the bonding strength between the cement slurry and the outer wall of the casing and the hole wall. After injecting spacer, inject cement slurry.
4) Glue pressing plug. After grouting, open the stop pins on both sides of the cement head, and then press down the rubber plug with a certain amount of trailing liquid with the cement truck.
5) replace the slurry. After the rubber plug is pressed down, the mud pump is started to replace the mud, and the pump pressure gradually increases. When the rubber plug sits on the casing support ring, the pump pressure suddenly rises and the safety pin is cut off (commonly known as bump pressure). Before collision, the pump can also be stopped, the cement truck can be started, and the clean water can be replaced by a small displacement of about 1m3/h for collision.
6) Wait for solidification. In order to prevent the floating hoop from being damaged due to excessive pressure, the method of holding back pressure and waiting for solidification should be adopted for deep holes and bad holes closed with counter-pressure balls. In the process of holding back pressure and waiting for coagulation, the pressure in the hole increases due to the exothermic solidification of cement, so pay attention to the wellhead pressure. If the pressure is higher than the set value, part of the pressure can be released from the back pressure valve of the orifice plate device to ensure balance. Generally, the waiting time is about 24h, and the orifice pressure gauge can be reset to zero.
7) When the borehole is shallow, it is easier to use mud pump for grouting. After the amount of cement slurry is injected accurately, the displacement slurry and clean water are sent in, and the cement slurry returns to the orifice, so that a cement slurry column of about 10m is left above the float collar, and then the orifice valve can be closed to allow the cement slurry to solidify.
8) After the cement slurry is solidified, run the drill pipe, flush the excess laitance in the pipe with a large amount of cement slurry to 3 ~ 5m above the float collar, and wait for the cement stone. When flushing the floating slurry in the pipe, rotate the drilling tool as little as possible to reduce casing jumping.
9) After the cement stone is formed (generally it takes 5 ~ 6d to get from cement to stone), a roller bit or an all-round alloy bit can be used to penetrate the cement column, sweep the floating hoop and guide the shoes.
10) When sweeping the floating hoop and guide shoes, gently press and turn slowly. Before sweeping the floating hoop and guide shoes, drill, grind and punch holes with a drilling rig with larger outer diameter to remove the sediment at the bottom of the hole and ensure the pipe diameter, so as to prepare for the next construction.