This paper describes the various steps involved to safely suspend a HPHT well and to repair a number of leaks in the production tubing. The tubing leaks were the result of corrosion and occurred over a two hundred meter interval.
Thorough analysis and associated difficulties to resolve this problem lead to the approach of cementing the tubing in place. Thus allowing the operator to test this gas well and increase the knowledge of the reservoir to develop this field. The process and solution will be described in great detail and will serve as the basis for similar problems in the future.
The exploration group of the operator uses coiled tubing as a means to abandon exploration wells, which have been tested. The deep gas wells have high bottom hole static temperature and pressures and significant H2S and CO2 properties. Any operation in these wells cannot be considered to be routine and need to be planned in great detail.
After going through a learning curve, the current technique of squeezing the perforations off with cement through coiled tubing is well established and has become a standard mode of operation. Being comfortable with the cementing practices, it was felt that cementing the tubing in place, followed with a cleanout for full bore access could safely repair the tubing.
This well was originally drilled end 1997 and tested on two formations (Buah and Amin) in 1998. The well was then suspended with the perforations cemented over the lower formation (Buah) and a deep-set plug in the top nipple at 4297.6 mahbdf. The Amin perforations were left open. The well bore was filled with CaCl2 kill brine, with a density of 13.5 kPa/m, the tubing retrievable sub-surface safety valve (TRSSSV) shut and a back pressure valve (BPV) installed in the well head.
In September 1999 the well was re-entered with the aim of acidizing and consequently re-testing the lower reservoir (Buah) for gas inflow and reservoir performance. The deep-set plug was found to have been leaking, which resulted in half the tubing to be filled with gas. Debris was found on top of the plug and several attempts were required to pull the plug. An injectivity test was performed followed by a cement squeeze on the upper reservoir (Amin) into the perforations to isolate that reservoir.
Milling with coiled tubing then started past the upper formation. After milling past the upper formation, a gas influx was observed into the tubing and a small pressure increase was observed on the annulus (~5500 kPa). A series of injectivity tests were performed with brine, which indicated significantly higher level of injectivity than expected (up to 200 l/min at a tubing head pressure of 7000 kPa) into the upper formation. During this period no change was observed in the annulus pressure. Although not optimal, the test programme did allow for a small influx from the Amin when re-testing the Buah so no further action was required at that moment in time.
After completing the milling operation to beyond the old perforations of the lower reservoir (Buah) the reservoir was re-perforated. Going through the final checks before producing the well, it was observed that the annular pressure increased to 18,000 kPa with gas coming to surface. The testing operations were suspended and the tubing and annulus were killed by lubricating 13.5 kPa brine in the annulus. With the well stable, a flow test indicated that the pump rate on the annulus of 260 liter per minute (l/min) with the annulus and tubing pressure at a constant 4000 kPa.
At this point the decision was taken to abort the test and suspend the well again. See appendix A for a well schematic.
Securing the Well.
A common practice for this operator is to suspend exploration wells that have been production tested, with a cement plug set with coiled tubing. Since the original plan was to test the lower reservoir and to shut off the upper reservoir a sand plug was placed across the lower reservoir and the upper reservoir perforations were re-squeezed with cement. This suspension allowed another possibility to isolate the Amin from the Buah in case of further attempts to access the reservoir later.