Much work has already been undertaken by various operators throughout the industry to explore frontier areas and drill into ever-deeper geological horizons. High pressures and temperatures in the range of 15,000 psi and 450-F need to be handled to safely drill and produce these targeted reservoirs.
This paper illustrates the experience of an E&P operator in the exploration process of a deep Ultra HPHT field offshore east India.
The consortium led by Gujarat State Petroleum Corporation Limited was awarded the production sharing contract (PSC) for the KG-OSN-2001/3 Offshore Block in the state of Andhra Pradesh, by the Government of India on 4th of February 2003 (Figure 1). The consortium committed to drill a number of wells within the first 2.5 years of the PSC.
At the initial stages of the project evaluation in the Krishna Godhavri block, the acquired seismic indicated highest reservoir potential into or below the early Cretaceous formations. The project geared up to drill wells with maximum drilling depth of 4200 m TVD SS. The Saipem PN3 JU rig was mobilized to the field on July of 2004. Two offset wells were drilled into the block in late 80's. This constituted the only data available for the initial basis of design of the KG wells.
After the first two less successful wells drilled in this block the project team had to re-evaluate the exploration strategy and understanding of reservoir potential and trapping mechanisms.
The KG-08 well was the first deep well planned on the structure, planned to drill basement at 4850 m SS. Interpretation of pore pressure from seismic data indicted maximum formation pressures equivalent to 11.5-ppg, and BHST of 366-F. Figure 2 illustrates the well design of the KG-08 well.
Consistent with the assumption of pore pressure, the casing design was selected to allow conventional DST with kill weight packer fluid. The wellhead equipment purchased at the beginning of the drilling campaign was rated for 10M. The drilling rig was contracted with a 10M BOP and choke manifold. A 7″ tieback string option was made available to drill ahead higher pressures (contingency string), with a 15M spool and 15M BOP mobilized prior to well spud.
At deep burial depths, seismic accuracy in mapping structures and faults becomes less, and thus pore pressure prediction becomes less accurate too. The problem could be exacerbated by presence of tectonic stresses. Substantial amount of time was dedicated into reading and deciphering the offset data available. From the reports made available there was an indication of possible presence of overpressures. The only offset well drilled to around 4000 m TVD used up to 15-ppg-mud weight. However from reading into the daily operations or the drill stem test data for the said well, it was not clear whether the mud weight was increased as a matter of fact or if this was done in response to genuine indicators of the presence of overpressure.
As stated earlier the project was not geared to drill deep HTHP wells, and with the current steel industry status non-standard size or weight tubular delivery schedules are more than 12 months. Conventional and standard casing sizes had the advantage of using readily available bits, stabilizers, downhole completion tools etc. Therefore with available casing in stock the well construction team made the conscious decision of going ahead with drilling the well to its geological targets, with a "regret clause" in place that the well may not be tested in case bottom hole pressure exceeded a particular value. The casing design was based on predicted pore pressure profile, while a contingency plan was in place in case of drilling through high pressure. For pressures above 11.5-ppg but below the critical value testing was to proceed using an underbalanced packer fluid, basically treated seawater in this case. This technique proved to be best option when it comes to HPHT testing, which present the obvious advantages of testing with solids free clear fluids, thus pressure transmissibility to down hole test tools is and guaranteed, the casing is subjected to much lesser burst loads when tubing leak load cases are being considered.