The Geothermal Industry's largest remedial budgetary cost involves the removal of scale from its existing completions. From it's inception forty years ago, the geothermal industry has tried many different systems and applications for the removal of very hard scale from their injection and production wells. Today the most widely used and accepted method for hard scale removal has been the use of a workover rig utilizing a bit and scraper. This method, although partially effective, does not fully address the impediment of scale in the well conduit - a bit and scraper simply cannot remove what it cannot reach. In addition, with the advent of high alloy materials to combat the corrosion effects of these scales, along with pressure depleted formations, this method is proving to be inadequate as it can cause severe damage to the expensive wellbore tubulars. This paper reviews and discusses several case histories of a newly developed technique for removing these very hard geothermal scales.
Geothermal energy generation involves the production of steam or superheated water from geologically heated reservoirs. Wells producing steam are used to drive turbines that power electricity generators. Wells producing hot water are used to heat low flash point fluids that in turn drive turbines and generators. All of the fluids produced from geothermal reservoirs are re-injected. The closed loop nature of this system results in the water becoming saturated with minerals such as silica, which are dissolved from the formation. Table 1 provides a breakdown of a typical scale sample as determined by X-ray diffraction. Superheated water flashing to steam and the cooling of hot water can cause metallic and mineral scales such as calcium carbonate to be deposited in producing wellbores. Cooling of injection water when pumped down hole can cause silica scale deposition in injection wells. The caliper survey in Figure 1 illustrates the localized nature of the scale deposition. Scale buildup in either production wells or injection wells can significantly limit the amount of electricity that can be generated from a given field. The length of time that a well is shut in for workover can also be very costly during periods of peak electrical demand.
Until recently, workovers have been limited to the standard rig jointed pipe, bit and scraper approach1. With this technique, the well must be killed. This cools down production wells making them more difficult to bring back on production after the cleanout. The low pressure of many geothermal reservoirs makes it necessary to nitrify the cleanout fluid system but this is a difficult process for jointed pipe workovers. The bit and scraper is a mechanical attack on both the scale deposit and the completion itself. In a significant number of cases the integrity of the casing or liner is compromised after five or six workovers. This can result in expensive casing repair programs as often as once every three to four years. This is particularly costly for operators using titanium casing strings to combat corrosive water. The fixed diameter bit is unable to clean out to both the casing and liner ID's in a single trip. Figure 2 illustrates the varying ID's which must be cleaned in a geothermal completion. In addition, a bit is not able to clean deposits in the slots of the liner no matter how it is configured. This new technique utilizing coiled tubing solves many of the problems associated with the rig technique.2,3