Cement pulsation is a novel technology for enhancing zonal isolation by applying low frequency, hydraulic, pressure pulses to the wellhead immediately after cementing. The treatment maintains the slurry in a liquid state, which transmits hydrostatic pressure downhole, and keeps the well overbalanced thus preventing early gas flow after cementing.
The paper summarizes several stages in the development of cement pulsation technology including comparison to other methods, physical principles, process analysis, mathematical modeling, computer-aided design, laboratory testing, and field performance.
The paper supports published information on cement pulsation with data from research and field studies that was instrumental in developing the technology. Emphasis has been given to the analysis of the pulsation process, description of design model and software, and an updated account of field applications.
Described is the MS Windows software for pulsation design. Two examples demonstrate the computer-aided design. The examples show that the software could be used to find the pulse size and treatment duration for a constant-pressure treatment. Alternatively, a variable-pressure treatment with controlled treatment depth could be designed.
Data is presented from pulsation of over 80 wells in drilling areas notorious for early gas migration after cementing. Field applications of the technology in 80 wells provided significant evidence of the success of cement pulsation in preventing early gas leakeage in cemented wells.
Introduction - Top Cement Pulsation
In 1982, a landmark field experiment performed by Exxon revealed hydrostatic pressure loss in the annuli after primary cementing in wells1. Since then, hydrostatic pressure loss after cement placement has been considered a primary reason for gas migration outside wells. As the annular cement - still in liquid state - loses hydrostatic pressure, the well becomes under-balanced and formation gas invades the slurry and finds its way upwards resulting in the loss of well's integrity.
Cement slurry vibration using a low-frequency cyclic pulsation is used by the construction industry for improving quality of cement in terms of better compaction, compressive strength, and fill-up. (Cement gelation or transmission of hydrostatic pressure is not a concern in these applications.)
In the oil industry, the idea of keeping cement slurry in motion after placement has been postulated a promising method for prolonging slurry fluidity in order to sustain hydrostatic pressure and prevent entry of gas into the well's annulus. The idea was based upon experimental observations that cement slurries in continuous motion remained liquidous for a prolonged period of time2,3.
Manipulating the casing string would move the cement slurry. Thus, early concepts considered keeping cement slurry in motion through casing rotation or reciprocation4,5,6. The motion should improve displacement of drilling mud and placement of cement slurry in the annulus.
The use of forced casing vibrations for gas flow control has become subject of several inventions in the 80's and 90's7,8,9,10,11,12. For example, "enhanced filling of annulus with cement slurry without rotating or reciprocating the casing" was considered the main advantage of the first casing vibration method with mechanical vibrator placed at the bottom of the casing string7. All these methods have been already experimentally studied and patented. However, none of them have been used commercially because of difficulty involved in manipulating the entire casing string. Apparently, heavy equipment and installatioin needed to vibrate a long and heavy string of casing makes these methods not feasible, even onshore.
