Producing wells in Middle East are faced with downhole corrosion problems as they mature. External and internal corrosion of production casing and tubing can lead to underground blowout as it was the case in well 159. Well 159 was drilled in 1959 in Bahrain and was completed within the shallow sandstone reservoir. Thirty-six years after the well was completed, a routine annulus survey was conducted and indicated tubing leak in 5” production casing. Several attempts were made to work the well over to get near the producing perforations after pulling part of the original completion. These conventional methods failed and attempt to kill and cement off flow using a snubbing unit failed as well.
In the interval within the shallow aquifer from 300′ to 700′, casing had shown signs of severe corrosion. The casing was parted and shifted below 681′ and the wellbore was no longer accessible from the surface below this depth. Blowout flow rate was estimated at 8 MMscfpd of gas. This caused significant safety problems (Flak L.H. et al, 1995): 1) sour gas venting out of the charged aquifer in offset well cellars; 2) subsurface charging and associated shallow gas hazard. Relief well 515 was drilled as a solution to kill underground blowout in well 159, plug and abandon well 159 and then serve as a replacement well. Few challenges have to be considered in pre-planning process (Flak L.H. et al, 1995):
1) Surface separation of blowout and relief wells limited by shallow reservoir depth to 85′;
2) No borehole surveys of any kind available on the blowout;
3) Safely drilling and casing of the shallow actively charged aquifer;
4) Maintain well control and borehole stability of the relief wellbore;
5) Near well intercept (<3') to assure blowout kill and well 159 abandonment; 6) Using relief well to replace blowout after the kill. Figure 4 Assumed downhole status (Flak L.H. et al, 1995) Figure 4 shows planned relief well and critical points and depths in order to intersect well 159. The 85' well separation was principally based on the requirement that the shallow aquifer is cased off prior to well kick off, directional drilling and open-hole ranging. Total loss returns and dry drilling are very often encountered while drilling charged shallow aquifer. It was likely that when returns are lost gas would start blowing at the surface as the hole is drilled. Because of that, the system was designed to divert the gas while drilling using a rotating head. Gas would be diverted out and burned. The reservoir was only 2038' vertical depth. The surface casing had to be set at 1000' to cover shallow aquifer and massive loss zone prior to directional drilling and open-hole ranging. Another problem was that personnel had to face with total lack of wellbore surveys. Bottom hole uncertainty required effective magnetic ranging and relatively close initial well proximity. Several conclusions can be made after successfully drilled the relief well and killing the Well 159 (Flak L.H. et al, 1995). · The plan for handling the shallow gas hazard using a rotating head and multi-bowl wellhead was very effective. · Sour gas was safely diverted and flared while drilling and casing of the charged zone. Directional drilling with tools with site adjustable bent housing motors and MWD systems proved to be very effective. · Good borehole stability was achieved with the use of asphalt-treated lime lignosulfonate mud. No stuck pipe was experienced during the entire operation. · The initial intercept and first well kill were not avoidable given the requirement for close proximity between the two wellbores at the top of the blowout reservoir. Plans must be made to handle this likelihood as were made here. It is unfortunate that communication could not be re-established even after milling a slot in the Well 159 casing. Having a back-up plan to go for the blowout reservoir under 5'' casing became very critical for the success of this project. · Several other wells in Bahrain had the similar problems as Well 159 due to mature fields and external casing corrosion in uncemented areas of the production casing. A regular well monitoring is required. When relief well is used as the control method in underground blowouts, the key is gaining direct communication. Once it is gained, well killing is generally not very difficult.