Abstract

Obagi is a mature onshore oil field in the Niger Delta discovered in 1964 which comprises of mainly oil-bearing reservoirs with some reservoirs having large gas caps. Over the years, significant oil production has taken place in the oil rims of the reservoirs with large gas caps. Time-lapse fluid saturation evaluation using openhole well logs and cased-hole saturation logs (CHSL) established flushed oil zones and the existence of significant remaining gas columns in two gas cap reservoirs.

Integrated reservoir studies identified watered-out wells that traverse these gas caps and are well placed to produce the gas zone with good clearance from the current water contact. In order to increase gas production potential from the Obagi Field at very low cost, a strategy of rigless intervention using coiled tubing for cement isolation and E-Line re-perforation to convert the watered-out oil wells to gas producers was adopted. Candidate wells for intervention were selected based on their location within the reservoirs, considering their proximity to the existing producers as well as the ease of connection to existing surface facilities.

An intervention was carried out on OB-P1 by isolating the existing perforations via coiled tubing by setting a plug in the tubing above the watered-out zone, performing a tubing punch and circulating cement in the annulus. An E-Line re-perforation was then performed shallower in the R1 reservoir gas cap with a ceramic screen installed for sand control.

A post intervention production test shows that OB-P1 has a gas production potential of > 250 kSm3/day (8.8 MMSCFD). This rate is constrained by damage (well still cleaning up) as well as mechanical skin due the completion type (ceramic sand screen) and the small effective wellbore diameter. Successful execution of three other planned conversion wells will replace, at least, one of the infill gas wells in the planned drilling sequence. This will result in a cost saving of about 65% of a new well.

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