Abstract
Abstract
Drilling activities in diverse areas can pose significant challenges for conventional cement systems and circulation methods. To overcome these challenges, a unique cementing system and circulation method has been tested in the field with great success. This paper details how these technologies and techniques were successfully used in the cementation process of one casing string in a relatively shallow geothermal well in central California and a relatively deep string in central Wyoming.
Globally, a great many wells experience lost-circulation problems that require the reduction of circulation pressures to allow proper placement of cement to provide annular coverage. To help minimize these circulation pressures, cement was pumped into the annulus directly, significantly reducing placement pressures and allowing placement of the cement with no apparent losses - a process referred to as the reverse circulation placement technique (RCPT). In each of the deep and shallow applications discussed, the placement technique and the individual slurry designs address temperature issues.
Introduction
Drilling and completion of geothermal and deep, hot wells pose a number of challenges, including very high temperatures at great depths and at relatively shallow depths. The high temperatures pose challenges while the well is being drilled and completed, and after construction has been completed. In a geothermal well, CO2 may be encountered in some of the formations, which can be a serious corrosive threat to both the cement and the casing in the well.
The most common problem faced when drilling wells in geothermal fields is lost circulation. Because of the typically fractured nature of formations in a geothermal field, lost circulation can cause serious problems during drilling and while casing is cemented across lost-circulation zones. Lost circulation that occurs during a cement job can lead to incomplete sealing of the annulus, which may require remedial cementing in an attempt to fill the annulus.
Traditionally, when lost circulation is a problem, operators rely on conventional lost-circulation treatments before cementing. Additionally, various lightweight cement systems are employed to help minimize the pressure while cement is circulated into an annulus, which can help minimize the potential for lost circulation and associated problems.
In central Wyoming, a 15,932-ft string of 10 3/4-in. pipe was successfully cemented using the RCPT. The positive results have led to additional applications of the technology in other parts of the U.S.A.
A central California geothermal well was cemented using the technology as well. The reverse circulation technique was employed to cement a 13 3/8-in. casing string at a total depth of 3,720 ft.
Reverse Circulation Placement Technique
In the cementing technique known as RCPT, spacers and cement are pumped directly down the annulus. Using RCPT has a number of potential advantages, along with some challenges.
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