Quality requirements for cement injection

1. Regulations on cement return height, cement plug length and artificial bottom hole

For oil and gas reservoir cementing, the designed cement return height shall be 150 meters higher than the top boundary of oil and gas reservoir, and the actual sealing height shall not be less than 50 meters above the top of oil and gas reservoir. Among them, for the qualified cement ring section, for the wells with a depth of not less than 2000 meters and not less than 10 meters, the wells with a depth of not less than 2000 meters shall not be less than 20 meters.

2. In order to ensure the sealing quality of casing shoe, when double plug cementing is adopted for oil layer casing, the distance between choke ring and casing shoe shall not be less than 10 meters, and that of technical casing (or completed well in advance), generally 20 meters, and the position of casing shoe shall be close to the bottom of well as much as possible.

3. The distance between the bottom boundary of oil and gas reservoir and the bottom of artificial well (the position of cement surface and float collar in the pipe) is not less than 15m, which is mainly to meet the demand of oil production.

4. The second one is to prevent the liquid membrane slurry on the inner surface of the casing from polluting the cement slurry when the upper rubber plug goes down, which will affect the cement sealing quality near the casing shoe.

Quality appraisal of cement sheath

At present, the quality appraisal of cement sheath in China is generally based on the acoustic amplitude logging (CBL). If the relative value of acoustic amplitude is less than 15% (10% is also used in some oil fields), 30% is qualified; if the relative value of acoustic amplitude is more than 30%, it is regarded as unqualified cement sealing. Acoustic amplitude logging is generally carried out within 24 hours to 48 hours after cement injection, but the time of acoustic amplitude logging for special wells (such as liner cementing, using retarding cement cementing, etc.) can be determined according to the specific situation, and now variable density logging (VDL) is also gradually popularized.

1. Well temperature logging

The hydration reaction of cement is exothermic reaction. The heat released in the setting process is transmitted to the liquid in the casing through the casing, which can make the temperature in the well rise a certain value, such as 6℃ ~ 20℃. The temperature in the well section without cement in the annulus is normal. By using this property, the return height of cement slurry in the annulus can be measured, that is, the top depth of cement.

2. Acoustic amplitude logging (cement bond logging)

Acoustic amplitude logging is based on acoustic principle. In the well, the acoustic wave is emitted from the logging tool. The acoustic wave radiates around the well with a nearly spherical wave front, and then propagates to the receiver through different media and routes. The first to reach the receiver is the refraction wave generated by the sliding wave (called casing wave) propagating along the casing, and the second is the formation wave propagating back after it is transmitted to the formation. Although the propagation distance of the sound wave in the drilling fluid is the shortest, the drilling fluid wave (mud wave) arrives at the latest due to the relatively low sound velocity in the drilling fluid. Acoustic amplitude logging records the first wave amplitude of the first arriving casing wave.

In the process of casing wave propagation, the energy should be dissipated to the casing and the annular space, that is, the sound amplitude should be attenuated (decreased). The attenuation degree of casing wave is related to the nature and period distribution of medium inside and outside casing. Because the distribution and properties of drilling fluid in the pipe are constant, the energy lost to the pipe is constant. On this basis, the attenuation degree of casing wave depends on the cementation of cement and casing outside the pipe. The experimental results show that there is a linear relationship between the logarithm of the amplitude of the first wave of casing and the percentage of the uncemented part of the cement around the casing to the circumference of the casing, that is to say, the more cement cemented with the casing, the higher the received acoustic amplitude, while when the outside of the casing is all drilling fluid, the largest received acoustic amplitude.

The received sound amplitude is transformed into a voltage proportional to it by the measuring circuit and recorded. By testing from bottom to top along the depth of the well, we can get a acoustic amplitude logging curve that reflects the cementation of cement and casing along the depth of the well. The application of acoustic amplitude logging curve to the detection of cement sheath quality is based on the relative amplitude (based on the acoustic amplitude of the free casing section in which all the drilling fluid is in the annulus).

Relative amplitude = amplitude of amplitude curve of target section / amplitude of amplitude curve of free casing section * 100%.

Acoustic variable density logging

Acoustic amplitude logging records the amplitude of casing head wave. Acoustic variable density logging (VDL) is a kind of logging method which uses receiver to measure and record the amplitude of acoustic waves such as casing wave and formation wave according to the time of arrival, and then displays them by a certain method to evaluate the quality of cement sheath. When variable density logging is carried out, acoustic amplitude logging is carried out at the same time. Because variable density logging can measure and record formation waves, it can reflect the cementation between cement and formation (commonly known as the second interface). The quality of cement sheath can be evaluated comprehensively by comparing the results of variable density logging with that of acoustic amplitude logging.