Reviving Oil Production in a Mature Offshore Field Through a Downhole Coiled Tubing Completion

Abstract

Production in offshore Mexico mature fields is mostly driven by gas injected from surface. With time, this injected gas flows directly through the natural fractures of the low-pressure carbonate reservoirs, leaving oil trapped in the low-permeability matrix. Over the past few years, the gas-oil contact (GOC) has rapidly moved across those fields—up to 6.5 ft per month in some wells—, making conventional gas shutoff techniques either unsuccessful or uneconomic. A new rigless intervention method using coiled tubing (CT) equipped with real-time telemetry has been used to revive oil production in gas-invaded wells. The objective was to achieve better results than the mixed success that had been obtained with gas shutoffs using chemicals and to reduce the cost and time typically involved with traditional workover operations. The approach consists of the modification of the downhole completion through CT. After perforating a new interval in the lower oil-bearing zone, a CT string is conveyed down the well and precisely hung by another CT at the end of the existing production tubing. Equipped with slotted bars, the string allows directly tapping into the newly opened zone while bypassing the former intervals that are mostly producing gas. From a CT standpoint, such interventions in an offshore environment present significant challenges, ranging from logistical to operational. During the first implementation of this new technique, the available section for anchoring the CT hanger was only 11.5 ft of 3 1/2-in. tubular inside a 5 1/2-in. completion. In addition, the projected tail of the CT hang-off was very close to the well total depth (only 10 ft from the bottom). The use of CT with real-time telemetry was thus critical to an accurate depth control that would allow not only determining the actual length of the CT string to be cut, but also for precisely hanging that string. In addition, the joint use of real-time telemetry and a downhole tension and compression module was key to ensuring proper actuation of the anchoring mechanism downhole while protecting the integrity of the new and former completion components. As a result of that intervention, the gas/oil ratio was reduced by 96%, while oil production increased more than twofold. In addition, the use of the CT measurement system and its flexibility saved a major workover intervention and the associated deferred production to the operator. This rigless methodology, leveraged by CT real-time telemetry, brings a new, viable, and economical alternative to gas-control treatments. Use of the technique can significantly extend the life of producers facing major gas-control issues in environments where the complexity of the reservoir and its dynamics tend to make shutoff treatments complicated and their outcome uncertain.