Gas flow following cementing has hindered the completion of wells for a number of years. Research of this problem has indicated many solutions which have been either partially successful or unsuccessful in controlling gas flow following actual cementing operations.
A theory describing the cause of this problem and a novel solution to the problem were presented in SPE 8257. This paper summarized the results obtained during a fifteen-month field application of the control method. Results of 250 cement jobs performed world-wide are summarized and specific performed world-wide are summarized and specific examples are discussed in detail.
The problem of gas flow following cementing has plagued all operators who drill through high-pressure plagued all operators who drill through high-pressure gas zones. This problem has been the subject of a great deal of research, and many theories concerning the cause of annular gas flow and methods to combat the problem have been developed. These annular gas control methods have included application of back pressure to the annulus, control of fluid loss from the cement slurry, and increasing the weight of the cement slurry mixing water. None of these previously developed control methods have been totally successful in a wide range of applications.
A unique theory describing the cause of annular gas flow was developed and presented in SPE 8257. Basically, this theory states that a cement slurry passes through a transition state of finite length passes through a transition state of finite length Can estimated thirty minutes to four hours) between the fluid state and the solid state. While in this state of physical transition the cement, neither a fluid nor a solid, will not transmit hydrostatic pressure. Thus, cement hydrostatic pressure pressure. Thus, cement hydrostatic pressure controlling a formation gas is trapped across the formation. Volume decreases occurring during this transition state due to chemical reaction and fluid loss cause rapid decreases in the trapped pressure. If the pressure in the cement column falls below the pore pressure in the cement column falls below the pore pressure of the gas zone while the cement is still pressure of the gas zone while the cement is still in transition, the gas can percolate through the still unset cement.
A method for preventing annular gas flow based on the above theory was also presented in SPE 8257. This method controls gas flow through the unset cement by increasing the compressibility of the cement slurry. Increasing the cement compressibility allows cement in the transition phase to undergo the volume decreases without suffering the disasterous trapped pressure decreases.
The cement compressibility is increased by introducing a gaseous phase into a conventional cement slurry In the form of small, finely dispersed bubbles. The introduction of gas into the cement is accomplished by addition of a gas-generating material which generates the gas insitu.
Field application of the method outlined above has indicated that the method is successful in controlling gas flow through unset cement. Field results over a fifteen-month period show 85.2% success ratio for 250 applications.
Field application of the new compressible cement system requires a great deal more engineering design than conventional cement. The amount of gas required to increase the cement compressibility sufficiently to prevent gas flow is calculated based on well conditions and slurry parameters. The cement compressibility increase necessary to prevent gas flow is determined from the difference in pressure between the initial hydrostatic pressure and the pore pressure of the formation gas. pore pressure of the formation gas.