New Ultralightweight-Cementing Technology Combines Benefits of Current Leading Methods

Abstract

This article, written by Assistant Technology Editor Karen Bybee, contains highlights of paper SPE 92970, "New Ultra-Lightweight-Cementing Technology Proven With Case Studies, Combines Benefits of Current Leading Methodologies," by D. Kulakofsky, SPE, O. Araujo, SPE, and F. Self, SPE, Halliburton; A. Mora, Petroleos Mexicanos; and P. Hendriks and P. de Mestre, Petroleum Development Oman, prepared for the 2005 SPE 14th Middle East Oil & Gas Show and Conference, Bahrain, 12–15 March. Historically, the industry has either foamed base cement with nitrogen or added microspheres to the slurry to create ultralight cement slurries. While both of these technologies are considered premium solutions and have been used successfully for a number of years, they each have distinct advantages and limitations when compared to one another. While each is ideal for certain situations, neither one can be used to cover the ever-increasing spectrum of challenges. Lightweight Cement Slurries There are three ways to make lightweight cement, by adding water or microspheres, or by creating a foam. The first method, addition of extra water, while the most common method for creating lightweight cements, cannot be used to create an ultralightweight cement. If water alone is used to reduce the density, the amount of water required to create an ultralight cement will dilute the cement so much that the cement will not solidify in a short period of time. This makes addition of microspheres or foaming the only two lightweight-cementing methods capable of generating a useful oilfield ultralight-weight cement. By use of microspheres, cement-slurry densities have been reduced to 9 and 10 lbm/gal. Case histories are available in which microsphere-containing slurries with densities as low as 7.5 and 8.5 lbm/gal have been pumped. As the slurry density decreases and micro-sphere concentration increases, cost increases. Foamed cements will set up at any density. Because density reduction is accomplished with a gas that costs less than cement, cement cost decreases with decreasing density. Logistics To be mixed and pumped effectively, lightweight cement slurries require a dedicated engineering effort. When using dry-blended cement (with lightweight solids), strict quality-control procedures must be followed to ensure that the blend is stable and meets programmed requirements. The lightweight components can separate from the cement powder, and often it is not convenient to reblend. When the blend is added to the mix water, if the design is flawed, the lighter components may rise to the surface and it will be difficult to trap them within the slurry matrix. Likewise, when lightweight components are added to the mix water, they also may separate immediately and the slurry blend is not re-established easily. Advantages and Limitations As with all slurry formulations, sometimes there must be a compromise in final slurry properties to meet specific objectives. With lightweight slurries, often there is less cement in the mixture and there are higher water/cement ratios. Because of this, the set cement can have a lower compressive strength. A high water/cement ratio can lead to slurry instability and excess free water. High water/cement-ratio cement slurries typically take longer to develop strength and can be susceptible to gas/fluid channeling. Permeabilities of the resulting set-cement column typically are higher than those of conventional slurries.