C. Foam Cement

Foam cement is ultra-low density cement, which is mixed with a certain proportion of bubbles in cement slurry to form foam cement slurry, so as to reduce the density of cement slurry.

There are two ways to prepare foam at present.

(1) mechanical method: injecting air or nitrogen and foam stabilizer into cement slurry, mixing gas in cement slurry, and using mechanical method to evenly disperse gas in cement slurry, forming stable foam cement slurry.

(2) chemical method: when foaming material and control agent are used in dry cement, when cement is mixed with water for a period of time, the substance react with some chemical components in cement to produce gas and form foam cement slurry.

The mechanical method can adjust the gas volume at any time according to the actual situation of cementing construction to meet the requirements of cement slurry density, but it needs special aeration equipment, and the construction is complex; The chemical method is simple, but it takes a certain time for the gas generating substances to react with some chemical components in the cement slurry to produce gas, and the gas generation is not easy to control. Therefore, mechanical method is the most used method in the world.

Research and use of foam cement is mainly to solve the problem of low pressure and easy leakage. When foam cement slurry is used to cementing, foam cement slurry is required to meet the needs of construction and production.

2. Graded cementing

Graded cementing is a cementing method that uses a graded cementing device to divide the cementing operation into two or three levels at one time in a well.

Staged cementing technology is used for cementing in long sealing and solid section, which can reduce cementing pressure, prevent low-pressure leakage and protect oil and gas reservoir. In the cementing of long sealing section in high pressure oil and gas reservoir, the water channeling of oil and gas reservoir caused by cement weight loss can be reduced.

A. Function of graded cementing

(1) Reduce the pressure of annulus liquid column to prevent leakage and damage to oil and gas reservoir.

(2) In high-pressure oil and gas wells, a variety of cement slurries with different thickening times can be injected by stages. After the completion of the first stage cement injection, open the circulating hole and circulate the drilling fluid to form the back pressure on the formation until the first stage cement is solidified, which can prevent the channeling of high-pressure oil, gas and water in the annulus.

(3) If the two oil and gas reservoirs are separated for a long time and the formation pressure is low, the use of two-stage cement injection process can not only reduce the construction pressure, but also save cement.

(4) Due to the shortening of cementing section, many measures conducive to improving cementing quality can be implemented. For example, a larger displacement can be used to replace the cement slurry to improve the displacement efficiency, so as to better ensure the cementing quality.

B. Structure and operation of graded cementing device

Throw the gravity opening plug into the opening seat of the grading hoop and form a sealed new artificial bottom hole. When the wellhead is crushed, the pin of the opening sleeve is cut off to force the opening sleeve down and expose the circulating hole. Circulate the drilling fluid and carry out the second stage cement injection. After the completion of mud injection, press in the shut-off plug and replace the drilling fluid until the shut-off plug sits on the shut-off seat and hits the pressure. Continue to press, cut off the pin of the closing sleeve, force the closing sleeve down and close the circulating hole.

C. Process requirements

The application of graded cementing technology requires not only the graded cementing device with good performance and reliable quality, but also the key points of the process.

(1) Grading hoop installation shall be determined according to the design cement return depth and oil distribution.

(2) In order to ensure that the circulation hole of the grading hoop can circulate normally after being opened, 1-2 casing centralizers must be placed up and down respectively.

(3) The cement slurry shall be injected at a depth of more than 100-200m from the first level of cement casing, and the cement slurry shall not be injected at a depth of more than 200m from the second level of cement casing, so as to prevent the cement slurry from returning to the first level of cement casing.

(4) The following factors shall be considered when designing the first slurry of the first stage cement injection: in case the first slurry returns above the grading hoop, ensure that the cement slurry can be recycled to the ground.

(5) When circulating after completing the first stage of cement injection and opening the annular hole, fully consider the downhole conditions and carefully select the start circulation time and circulation displacement. It is necessary to consider the possibility of the first grade cement slurry returning above the grading hoop and the bearing capacity of the formation to prevent well loss.

(6) Strictly implement the operating procedures for graded cement injection, and pay special attention to:

a. Before entering the well, carefully check whether the inner diameter of the opening seat and the closing seat and the size of the rubber plug are consistent.

b. The first level cement slurry shall be measured accurately so that the cement surface is below 50m below the grading hoop to prevent the cement slurry from returning above the grading hoop, polluting the drilling fluid and affecting the cementing quality above the grading hoop.

c. The thickening time of the first slurry of the first grade cement slurry shall be > the construction time, the gravity plug opening and falling time + the time required to circulate the drilling fluid above the grading hoop.

D. the closing pressure should be greater than the sum of the cement slurry column pressure and the shear pin pressure.

e. The floating hoop and shoe must be sealed reliably, otherwise it may lead to the failure of two-stage cement injection process.