Technical and product enhancements in well cementing have allowed the introduction of non-foamed cement systems mixed at densities as low as 7.5 ppg that attain ultimate compressive strengths as high as 1,000 psi. These non-foamed ultra low-density slurries are simplifying blending and mixing operations. In California oilfields and oilfields throughout the world, drilling wells into weak and low-pressure formations, has always been an industry challenge. Successful cement placement in these types of well environments is also a challenge. Lost circulation and partial fluid returns is major culprit to poor cement bonding in addition to higher well costs. Operators in many California fields are forced to drill wells "blind", for example, drilling without mud returns. Drilling "blind" does not preclude the challenging need to properly cement these wells to obtain zonal isolation.
The process of foaming cement systems to reduce slurry densities has been applied successfully over the years in wells throughout the world. Though foamed-cementing has been successful, the design and execution add components of operational and technical challenges. These would include accurate metering of nitrogen, uncertainty of actual cement density within the wellbore, additional equipment and personnel requirements, and special drilling rig components. The need for simplicity has allowed the introduction of non-foamed ultra low-density cement systems. In California, successful cement jobs were pumped using non-foamed systems at densities as low as 10.0 ppg. In Colorado, 8.9 ppg slurry was successfully pumped that had ultimate compressive strength of over 1,400 psi. In both cases, these systems provided simpler and an economic alternative choice. These ultra low-density slurries can be designed to meet wide range well temperatures and depth applications. The paper will discuss these systems along with field applications and case histories from California and other parts of the world.
Introduction
Foamed cement has been successfully applied as a well cementing technique since the late 1970's. The solution of foaming cement slurry to reduce its density has solved well-cementing challenges throughout the world. The need for lower density slurry is necessitated by the need to cement across weak zones, low reservoir pressure formations, naturally fractured rocks, and highly permeable sands. The foam cementing process requires adding nitrogen (or air in some circumstances) to cement slurry to generate foam. Typically the cement slurry density is kept constant and pumped at a constant rate and nitrogen rates are increased during the job. Increasing the nitrogen rates during the job is done to compensate for the compression of the nitrogen bubble. Nitrogen gas is mixed with cement slurry at pre-designed ratios and the foam is stabilized by the addition of surfactants. Although adding nitrogen to reduce cement density has been successful well cementing process; it requires additional pumping equipment, an intricate slurry design and a complicated pumping schedule.
Another method of reduccing cement density is to incorporate low density materials, such as low-density spheres, into the system. Even though low-density spheres have been available to the industry for some time, their use has not been widely applied. Recent applications, and current understanding of the pozzolanic (also known as ceramic spheres) and the borosilicate type spheres (also known as glass spheres), makes it possible to reduce the density of cement slurry; maintain desired properties, yet simplify design and field execution. Applications of the low-density sphere cement slurries have also proven to be very competitive to foamed systems. Low-density, high compressive strength cement slurry produces high performance system, allowing for the design application of a one-slurry cement job instead of multiple systems.
