Mature Carbonate Heavy Oil Field Exploitation Strategies: The Cretaceous Ku Field, Mexico

摘要

This paper aims to present the strategy for future exploitation of the Ku oil field, based on its historical behavior of pressure-injection-production profiles and the advance of gas-oil and oil-water contacts. The production performance of all wells in Ku oil field was analyzed in order to evaluate the gas and water breakthrough and production. A pressure maintenance program was started in 2009 by injecting nitrogen at a rate of 160 MMsfc/D, which reduced the pressure decline from 4.45 to 1.36 kg/cm~2 per year. At the end of 2011, the nitrogen injection rate was increased to 250 MMscf/D in Ku field in order to maintain reservoir pressure and minimize advance of the OWC (oil-water contact), which is estimated at 3,140 vertical meters subsea (vmss). On the other hand, the GOC (gas-oil contact) continues to advance at a rate of 25 m/year. To maintain production, a staggered workover schedule is utilized to open new producer intervals in every well where the gas cap approaches. Ku is one of the offshore oil fields located in the Ku-Maloob-Zaap Asset. It produces mainly from naturally fractured carbonates in the Upper Cretaceous (UC), Middle Cretaceous (MC) and Lower Cretaceous (LC) formations. In addition to high net pay thickness and structural relief, UC rocks have a very high permeability, which ranges between 4 and 9 Darcies, because of high fracture density. This condition gives the formation a thief zone characteristic during drilling, cementing and completion jobs. In fact, during cementing of the last casing pipe on many wells, most or all cement slurry volumes are lost to the formation, creating an unrestricted channel to flow and thus an increased tendency towards water coning. The remaining oil reserves volume in Ku oil field will be recovered from the oil column located between the contacts. Therefore, it is necessary to understand the production performance under these conditions in order to establish the optimal operative ranges under which wells will be producing. The water coning and fluids channeling effects depend upon the rock – fluids properties, well geometry and design, and, finally, the production conditions under which the wells will be operated.