Abstract
It is well recognized that casing cementing is the most important operation for long life geothermal production wells. However, it has been a very difficult operation to get the cement slurry to return to the surface, especially when formations have many lost circulation zones. Various methods have been developed to fill cement slurry from bottom to top of the casing strings. But still no guarantee of success can be planned prior to the operation. What often happens is the formation fractures and cement slurry is lost into open thief zones during cementing operations. Finally the top of the cement is found a few hundred meters from the surface within the annulus between the surface and inter mediate casing strings. It is not only very difficult to maintain but also dangerous to produce such a well. But if cement slurry is poured into the annulus from the surface, water will be trapped between two cement plugs and when steam production starts, water in the annulus heats up and finally collapses the casing. So some conventional top casing cementing methods have been devised. However, these methods have major disadvantages. The basic concept of this newly developed method is to remove the mud from the annulus, so that the annulus can be filled with cement without leaving water pockets. Then there will be no collapse of the casing when steam production starts. Therefore this new concept has been applied for one production well in which the primary cementing failed. The cement top depth was about 300m in the 18 5/8 in. and 13 3/8 in. casing annulus. To remove the annulus fluids, steam was circulated in the well to heat and dry the annulus. After the annulus was dried, water was circulated in the well to cool the annulus. Then cement slurry, with the specific gravity of 1.90, was pumped through the small diameter pipes and placed the annulus to the surface. This top-job operation was successfully conducted and no casing expansion nor casing collapse has occurred during extended well production. This new procedure has the following advantages compared to the conventional ones: (1). no possibility of water pockets, therefore no collapse casing, (2). even if the cement top is 300m or deeper this method is applicable, and (3). it is possible to fill the annulus with high density cement slurry which strengthens both yield strength of the cement sheath and bonding strength between the cement sheath and casing. A patent has been applied for relating to this method.
