Application Of Foam Cements In Alberta
Authors: Olanson Michael T. 
Published: March 2010

The demand for light-weight cement in areas of low-strength formations has led to the increased use of foamed cement in PanCanadian's wells.

Foamed cementing involves mixing surfactant upstream of the cement pumper and injecting nitrogen into the pumper discharge line, with a foam generator installed downstream. The "constant foam density method" was used. It involves increasing uniformly the nitrogen/cement ratio, to establish a constant slurry density of the cement column. In most wells cemented to surface a nitrogen-free cap of 100 m to 200 m of lead cement was maintained above the foam, to prevent nitrogen from breaking out to surface.

Although exhibiting relatively high strengths and reasonably low permeabilities, the foamed cement was used only as a filler cement. All the zones of interest were covered by a pad of neat tail-in cement.

Recent field tests by PanCanadian confirmed that foamed cementing is only slightly more expensive than conventional cement jobs but less costly and involving less mechanical risk than multi-stage jobs. At the same time, it can be considerably less expensive than glass or ceramic microspheres cementing.

Introduction

During the last year, PanCanadian Petroleum used foamed cement slurries in primary cementing on a total of 11 wells (Fig. 1) located in areas where traditionally two-stage jobs were used: southeast Alberta, Grande Prairie and Lindbergh.

Conventional filler cements which meet compressive strength regulations, as pozzolan-cement, gel-cement and pozzolan-cement- gel, are often too dense for the weaker formations, resulting in lost circulation while cementing. The use of lightweight fillers (perlite, gilsonite) or of water extending additives which allow increased water ratios (bentonite, diatomaceous earth, silicate extenders, etc.) give little strength satisfaction at slurry densities below 1 400 kg/m). In the province of Alberta, ERCB Regulations for cement fillers require a compressive strength of at least 3 500 kPa after curing for 48 hours at the temperature of the uppermost potential hydrocarbon-bearing zone. Other effective methods of overcoming cement problems in wells having low fracture gradient formations are multi-stage cementing, or the addition of high-strength microspheres (HSMS). The first method involves some mechanical risks related to the actuation of the stage tool. The use of HSMS is simple and secure, but considerably more expensive.

PanCanadian's foam cementing field tests included a wide depth range (from 615 m KB in Lindbergh to 2 238 m KB in Wembley) as well as individualized slurry design and operational procedures.

Operationally, it has been found that foam cementing is relatively straightforward and can be done with standard equipment and no special personnel. When properly designed and conducted, if the planned foamed slurry downhole densities are not lower than 1 000 kg/m3, a foamed cement operation can be as effective and successful as any other conventional cementing job.

The foam quality and stability are very sensitive to even some small changes (in the field) of slurry parameters and/or in operational procedures. During the cement operation neat (unfoamed) cement slurry density being warranted, the proper and continuous monitoring of cement slurry pump rate, nitrogen delivery rate, and surfactant/foam stabilizer injection rate, is crucial for a successful job.