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Test and Research on Settlement Stability of Oil Well Cement Slurry under Simulated Well Conditions

2022-06-07

Abstract oil

 

In cementing operations, the settlement stability of oil well cement slurry is one of the key factors that determine the quality of cementing, especially with the development of horizontal wells in recent years.

 

It is becoming more and more important due to the wide application of the well technology. This paper deals with the experimental research on the stability guillotine test of oil well cement slurry under simulated downhole high temperature, high pressure and inclination conditions.

 

The results of this study describe the structure and characteristics of this new type of device successfully developed by the author, and propose a complete set of evaluation criteria for the stability of cement slurry under simulated well conditions.

 

experimental procedure. The effects of temperature, pressure and inclination angle on the free water and stability of cement slurry were compared. The" test device and method

 

It can replace the commonly used API oil well cement slurry free water test specification, which is used to guide the cement slurry design during the field cementing operation in the oil field.

 

Practical value and application prospect.

 

1 Introduction

 

In the design of cementing formula, the settlement of oil well cement slurry caused by the instability of actual downhole conditions often has adverse effects on cementing quality. Especially in directional wells and horizontal wells, due to the influence of high temperature, high pressure and high dip angle, the oil well cement slurry forms more "free water" during curing and curing, and accumulates at the top of the cement column enough to form a channel in the annular space. lead to blow-by. At the same time, due to the uneven density distribution caused by the phenomenon of gravity settlement, in addition to the existence of free water, it is often manifested that the cementation strength between the cement stone and the formation and casing decreases due to the decrease in the density of the upper cement slurry. This will inevitably have a negative impact on the effective sealing of the annulus by cement in the cementing well; the potential problems mentioned above often occur at the upper interface between the annulus and the formation in inclined wells and horizontal wells. In severe cases, it will lead to the failure of the seal, making the cementing failure and the lack of success. The cementing industry has paid great attention to this phenomenon in recent years. J. Therefore, the effective control of the "free water" caused by the settlement phenomenon and the uneven density caused by the gravity settlement has been recognized as one of the key factors for the success of the horizontal well cementing slurry formulation design. Usually, the cement slurry prepared by stirring according to the API specification is placed in a 250mL vertical measuring cylinder with a scale. Stability is good or bad”-4j. Obviously, this method does not take into account the influence of temperature, pressure and dip angle, and its test conditions are far from the actual downhole conditions, and it often appears that the “free water” is very small at normal temperature and pressure. The cement slurry system produced a large amount of free water under high temperature and high pressure conditions], which has a serious adverse effect on the cementing quality. In view of this, research and development can truly reflect the stability of cement slurry under high temperature, high pressure and inclined conditions. Practical devices and methods become imperative.

 

Drilling Fluids Testing

 

2 Cement slurry settlement stability test device

 

In order to meet the urgent need to test the stability of cement slurry in oil well cement formulation design in oilfield cementing laboratory and related scientific research units, the author developed a new type of multi-cylinder combined cement slurry stability testing device in 1991. And has been approved as a Chinese patent [5]. The device can be used together with the general cement high temperature and high pressure curing kettle, and can test the stability of the cement slurry under the actual high temperature and high pressure conditions in the well.

 

The cement slurry of multiple samples of the formula is measured for the free water height after curing, so as to eliminate the limitation and chance of single-cylinder test. The stability performance comparison under the same experimental conditions (temperature, pressure) makes the test results more reliable. The device maximizes the use of the kettle body space of the curing kettle, adopts a reliable sealing method, is easy to operate and clean, and has a compact and practical structure. It also increases the use function of the curing kettle and improves the utilization rate of equipment. A two-cylinder or four-cylinder combined structure is used as required, as shown in Figure 1.

 

The test tube is designed as two half-pipes buckled together and clamped with a throat hoop to form a complete cylinder. This structure is easy to demould, and it is convenient to take out the cement specimen after curing and solidification, so as to ensure the integrity of the test block and the accuracy of the test. A gasket is added between the test cylinder and the base to prevent the cement slurry from leaking along the thread and causing adhesion at the thread. The slurry cap and the test tube are in sliding fit, which not only has good sealing performance, but also facilitates the removal of the slurry cap. The slurry cylinder cap transmits the pressure in the curing kettle to the cement slurry in the cylinder through the communication groove, without causing the phenomenon of mixing between the curing liquid and the cement slurry in the cylinder. The material of the pulp cylinder is copper, which is easy to transfer heat and ensures the accuracy of the simulated temperature. The thermocouple probe space is reserved in the center of the device, and the whole device is easy to operate, assemble and disassemble, and easy to clean.

 

In recent years, with the deepening of the understanding of the phenomenon that oil well cement slurries produce more free water in inclined conditions than in vertical conditions, it is necessary to design a cement slurry that can test and simulate downhole temperature, pressure and inclination angle conditions. Stability device. In 1992, BJ company developed a set of such device [o], the fly in the ointment is that the device can only be used with a specially designed curing container, and cannot be cured with a conventional curing kettle. Because of its high cost, inconvenient to manufacture and use, it is difficult to popularize.

 

In view of this, the author improved the device shown in Figure 1, and designed a device that not only maintains the characteristics of the original device that can simulate high temperature and high pressure, but also adds the ability to test from 0. Apparatus for the stability of cement slurry samples at different inclination angles between 900 and 900 (see Figure 2). Since the test cylinder is in a state of inclination, in order to avoid the leakage of cement slurry in the cylinder, and at the same time to facilitate the accurate transmission of the pressure of the external curing liquid into the test cylinder, the top of each pair of test molds composed of two half pipes is A floating plug with an O-shaped sealing ring is added, which is slidingly matched with the inner wall of the pulp cylinder, and can slide along the axis of the pulp cylinder until the upper and lower pressures of the floating plug are balanced. The adoption of this special structure ensures the authenticity of the simulated pressure conditions. In the center of the floating plug, there is a threaded overflow hole, so that when the floating plug is placed, the excess slurry and the air at the top overflow from it, cover the slurry cylinder cap, and screw on the overflow hole sealing screw. Put the slurry cylinder into the test stand with the spring locking mechanism, adjust it to the required test angle, then put the whole test device into the high temperature and high pressure curing kettle which is used routinely, and take it out after curing according to API oil well cement test specifications. mold, and test its free water and stability.

 

Drilling Fluids Testing

 

After the cement slurry to be tested is built in the device, it needs to be cured in a curing kettle that simulates the high temperature and high pressure conditions in the well. During the curing period, the settlement effect of different cement slurry systems is different. Therefore, the free water height generated at the top of the slurry cylinder and the density of the cement stone formed along the height direction are also different. The stability of the cement slurry is measured by measuring the free water liquid and density difference after curing. The specific test operation steps are as follows: ① Coat the inner surface of the test mold and the matching surface with sealing grease; ② Prepare the cement slurry according to the API specification; ③ Install the mold (each sample shall not be less than two); , heat up and pressurize according to the API test specification, and maintain for 24 hours; ⑤ The results of the test are detailed as follows.

 

3.1 Free water test

 

After curing, fill the test tube with clean water until the water surface is flush with the upper surface of the test die, and measure or weigh the volume and weight of the free water at the top of the cement column in the test tube, which is the amount of free water.

 

3.2 Density settlement test and calculation

 

Use a colored marker to number the cement columns and indicate the up and down orientation. Use a diamond saw to cut the cement column from top to bottom into 1 cm long columnar specimens and number them sequentially. Weigh and record the weight (W;) of each cement column, then clamp the specimen with tweezers and put it into the boiling wax solution and take it out quickly to coat the surface with a layer of wax film, then measure the specimen again The weight after waxing (IV7-u, c+W...), after that, use a specially designed device to weigh the weight (Ww) of the wax-coated test block in water, so as to calculate each test block according to the following formula The actual volume of:

 

 

 

Usually the density of the wax. , is 0.9. The density of the cement column is

 

 

In the formula: m is the weight of the cement test block} IV' is the weight of the cement test block after waxing; w. The weight in water of the cement block after waxing. Calculate every

After the density of a test block, the average density of the entire cement can be calculated as follows

 

After that, the degree of density settlement can be represented by the maximum density difference (4.x), the maximum relative error of density (&max) and the average relative error of density (6), etc., namely:

 

 

The above results can be used to guide the design of oil well cement slurry. By adjusting the cement admixture formula, the cement slurry system can achieve the minimum degree of free water and density settlement.

 

4 Experimental results and discussion

 

In this project, the oil well cement slurry system prepared with grade G oil well cement and tap water is used as the experimental research object. Through the comparative experiments under different conditions, the temperature, pressure, inclination angle, settlement tube diameter, water-cement ratio, The effect of mixing time and some typical cement admixtures on free water and density settlement. This paper only briefly discusses the effects of temperature, pressure and inclination angle.

 

4.1 Temperature

 

In order to verify the effect of temperature on the stability of cement slurry, the stability test device shown in Figure 1 was used for curing under a pressure of 14 MPa (2000 Psi) for 24 hours, and the free water content of APIG-grade cement slurry at different temperatures was measured respectively. to measure. The test results are shown in Figure 3. The stability of the cement slurry shows a trend of increasing and then decreasing with the increase of temperature. This shows that the amount of free water produced by the cement slurry increases with the increase of temperature, but when the temperature continues to increase, The thickening time of the cement slurry is bound to be shortened, which shortens the transition time for the generation of free water, resulting in a decrease in free water as the temperature continues to rise thereafter.

 

4.2 Pressure

 

In general, the effect of pressure on free water is not as significant as other factors such as temperature. It was long ignored by the consolidation community for a long time. Pressure was only taken into account later. Nonetheless, in order to approximate According to the real downhole conditions, we also carried out experimental research on the influence of pressure on stability to verify its influence on the stability of cement slurry. As shown in Table 1, pressure is also one of the factors affecting the free water of cement slurry and its stability. 1. The test condition is a vertical settlement test at normal temperature (37℃) to eliminate the influence of temperature and inclination angle on the stability test results.) All experiments use blank APIG cement with a water-cement ratio of 0.44.

 

 

At the same time, the influence of pressure on the density distribution was also tested and calculated, and the average relative density error (to) and the maximum relative

 

Density error (8..) to measure its stability, the results are shown in Table 1.

 

4.3 Inclination

 

Although the use of vertical subsidence experiments to simulate downhole temperature and pressure conditions is very helpful in guiding cement slurry design;