Cementing of Fragile-Formation Wells With Foamed Cement Slurries
Authors: Harms W.M. , Febus J.S. 
Published: December 2007

Summary
Substantial regions of hydrocarbon production in California consist of formations that are very fragile, which imposes a density limit on the cementitious system used to cement the well pipe. Also, troublesome fallback problems have been experienced for years in these areas. problems have been experienced for years in these areas. Fallback can possibly reduce production by causing formation damage. Usually, the use of foamed cement offers a low-density cementitious material that develops adequate compressive strength while avoiding fallback problems that are caused by density. After hardening, foamed cement has reduced density, and it usually provides the advantages of temperature stability and heat insulation properties. In this paper, the properties of foam cements are discussed and paper, the properties of foam cements are discussed and more than 60 cementing jobs completed with foam cement are summarized.

Introduction
The recent availability of foam cement as an oilwell services product has offered a new procedure to circulate cement slurry past formations having exceptionally difficult lost circulation tendencies. Certain areas of California hydrocarbon production are overlain by several highly permeable, unconsolidated sand and gravel bed permeable, unconsolidated sand and gravel bed formations that have very low reservoir pressure and low fracturing gradients. Also prevalent are naturally fractured shaly sands and shale formations that consistently pose lost circulation problems during the drilling and cementing operations on wells located in Kern County, CA. If circulation is maintained to the surface during cementing and cement returns are achieved, fallback often occurs such that the top of set cement is found many feet (often hundreds) below ground level. Therefore these formations are incapable of supporting the hydrostatic load exerted by conventional-density cement slurries throughout the cement hardening period. In the past, numerous attempts have been made to cement these types of fragile formations. The use of ordinary lightweight cements extended with bentonite, fly ash silicates, perlite, diatomaceous earth oil emulsions, or gilsonite, as well as the use of thixotropic cement slurries, has been attempted, but when applied to exceptionally pressure-sensitive lost circulation zones these slurries have provided limited success. To a large degree, the conventional slurries have not been successful because their densities can be lowered only to approximately 11 to 12 Ibm/gal [4.9 to 5.4 kg/m 3] (1.32 to 1.44 specific gravity) before their useful strength and permeability properties are compromised. Multistage techniques that properties are compromised. Multistage techniques that use conventional lightweight cement slurries also have afforded limited success. In addition, the total cost of this type of approach can be prohibitive. Two types of cement slurries currently are available that can achieve densities much lower than 11 Ibm/gal [4.9 kg/m 31. The first type incorporates pressure-resistant hollow microspheres into cement slurries. High-strength microsphere slurries continue to be used successfully in oilwell cementing. The relatively high bridging ability of the hollow beads enhances their effectiveness in controlling lost circulation. The second available type of ultralow-density cement slurry results from the preparation of a stabilized, gas-containing foam cement slurry. preparation of a stabilized, gas-containing foam cement slurry. Foam cement is successful because of its ability to keep cement slurry density below the hydrostatic breakdown gradient of the sensitive formations. A general comparison of typical physical properties of conventional and foam cements is given in Table 1. It was reported earlier 12 that foam cement could be used to seal underground storage caverns, insulate wellbores, and perform remedial squeeze jobs. Subsequent reports substantiated the basic properties of foam cement. Relatively little information properties of foam cement. Relatively little information has been published that describes the use of foam cement in primary cementing applications. This paper will focus on the use of foam cement as a primary cement in severe lost circulation zones in California.
Properties of Foam Cement Properties of Foam Cement A true foam cement is created when a gas is chemically and physically stabilized as microscopic cells within an ordinary cement slurry. Two uncommon factors are needed to prepare stable foam cement. First, the cement slurry should contain a high calcium ion and high-ph-tolerant foaming surfactant and foam stabilizer. Second, the slurry should be conveyed through an effective mechanical foam-generating device that imparts sufficient energy and mixing action with pressurized gas to prepare uniform gas bubbles of the correct size. Foam cement so prepared is essentially stable, unlike nitrified cement or drilling fluid; the gas does not coalesce and separate from the slurry if the slurry remains under the pressure conditions for which it was designed. Nitrogen usually is the first choice of gas since current nitrogen servicing equipment is more than capable of providing sufficient quantities of gas at suitable rates for cementing purposes. Fig. 1 illustrates the quantity of nitrogen per barrel of cement slurry needed to prepare low-density foam cements.
JPT
P. 1049