Field Implementation of Water Shutoff Treatments in Fractured Carbonate Wells by Microgel and Gel Injections

Abstract

Abstract With the increasing numbers of mature fields, the problem of water control becomes a major issue for the oilfield operators. A new technology based on microgel and gel injection has been successfully applied in producing wells from fractured carbonate reservoirs with water encroachment from active aquifer. This technology has been implemented firstly in a horizontal well four years ago, and, since then, several treatment campaigns occurred leading today to 21 water shutoff treatments. This paper summarizes the main results with a track record of several years. It includes progresses made on candidate selection, treatment design and main trends over a long post-treatment period of time (including sustainability). Most wells have open hole completion and no indication about production profile along the lateral length (no PLT available). The challenge was to place, by bullheading, a gel in the fracture with minimal leak-off in the matrix. Moreover, the treatment should have a minimal impact on the Productivity Index of the well. Alternating microgel and gel injection was shown to achieve all these conditions, the microgel playing the role of protective and placement fluid, while the gel creates a blocking zone in the fracture. The microgel is a pre-gelled species having a size of around 2μm. The gel combines acrylamide copolymer with an organic crosslinker. Final gel has a consistency of strong elastic gel. Gel time was adjusted to initiate the gelation after 1-2 days in order to ensure easy pumping and avoid premature gelation in the borehole. Nineteen wells have been submitted to such chemical water shutoff treatment, with three of them twice. The wells have been classified in three categories. Category 1 (4 wells) did not show any significant change (below 5000bbl of extra oil production). Category 2 (7 wells) showed higher additional oil production between 6 to 12 months. Category 3 (8 wells) showed additional oil production beyond 12 months and sometimes for several years. Chan's WOR plots helped to identify the type of water production mechanism. The wells showing strong channeling responded the best, while the wells showing more matricial production exhibited contrasting results. No well was damaged by the treatment and, in most cases, the gross production rate could be maintained as before the treatment. The sustainability of the treatment was more related to reservoir issues than to chemical issues, as shown by the remarkable longevity of some treatments. The additional oil production is estimated as 300,000bbl after 3 years and keeps increasing with time. The new microgel-gel WSO technology can be now considered as proven in fractured carbonate reservoirs under aquifer support. It can be deployed in the bullhead mode in horizontal wells, with almost no risk of damage. Next step could be applications in waterflooded wells.