The drilling of a sour, high-pressure fractured reservoir in the Tengiz field of Kazakhstan displayed complex and hazardous well control situations. While drilling the carbonate reservoir termed Unit 1, multiple fractures were encountered resulting in severe lost circulation followed by hydrocarbon influx into the well-bore.
Severe and uncontrollable lost circulation in the target reservoir prompted the use of special practices and equipment; thereby enhancing the safety and efficiency in the development drilling of the field.
A rotating control device (RCD) was employed to drill ahead blind (no fluid returns) by pumping a sacrificial fluid (water) down the drillpipe. Simultaneously, a pressurized mud-cap on the annulus with weighted oil based mud OBM controlled hydrocarbon influx. The overall process of pressurized mud-cap drilling will be described by the term Closed Hole Circulation Drilling (CHCD).
The successful application of CHCD has resulted in Tengiz well T-7252 being the first well to completely penetrate Unit 1 where lost circulation could not be controlled by conventional methods. After all Unit 1 fractures had been penetrated, a liner was installed to isolate the fractures using the same CHCD well control procedures employed while drilling the interval.
This technical paper will describe the process used to successfully plan, develop, and implement the CHCD technique in the Tengiz field.
The Tengiz field is located on the south side of the 500,000-km2 Caspian Basin on the northeastern edge of the Caspian Sea. It is one of several large carbonate formations found around the edge of the basin. The field is approximately 110 km2 at the top and 400 km2 at the base. Tengizchevroil (TCO) operates the field in a joint venture with the Republic of Kazakhstan.
Field delineation and development consisted of drilling new wells and working on existing temporarily suspended wells that range from 4,500 - 5,700 m. Severe lost circulation can be encountered while drilling the pay horizon at approximately 4,000 m in what is termed Unit 1 on the Rim and Flank areas of the field. Please see (Fig. 1: Location Map Showing T-7252) and (Fig. 2: Cross-section of Rim Region). Drilling fluid losses in excess of 800 bbl/hr have been encountered while drilling with as little as 0.3-ppg overbalance. Numerous attempts to control the drilling fluid losses with conventional means have been unsuccessful. This challenging environment had prevented TCO from penetrating the entire reservoir in extremely fractured areas of the Rim and Flank.
Drilling of 1 - 7 m vertical fractures or cavities has been observed with an associated reduction in annular fluid level resulting in well control events. Due to the sour/corrosive content of the reservoir, it was desirable that hydrocarbons from the reservoir not be allowed to reach the surface under any drilling operations.
Drilling ahead without returns has been attempted by TCO with OBM being pumped simultaneously down the drillpipe and annulus. This was obviously very uneconomical and could not be sustained for extended periods of time. Drilling without returns-using water as the drilling fluid and holding a pressurized column of OBMin the annulus-had not been implemented in any of the Tengiz wells.
Historically, the challenges of drilling the objective oil reservoir have included severe lost circulation followed by sour gas kicks. This obviously presented a significant safety risk due to the high H2S content in the oil.