Numerical Simulation of Tertiary CO2 Injection at Ivanic Oil Field, Croatia

Abstract

Abstract Tertiary CO2 injection is planned at Ivanic oil field, Croatia, in order to recover some of more than 12×106 Sm3 (75 MMSTB) of remaining oil in place. The field was discovered and put to production in 1963, secondary waterflooding was started in 1972, and today the field is facing a considerable decline in oil production and an increasing water cut. A revised geological model of Ivanic field, based on 3D seismic data acquired in 1998, was used as a ground for building a full field numerical simulation model. 40 years of production history were matched sucessfully by using black oil formulation. Compositional simulation was applied for 18 tertiary injection scenarios, which investigated impact of number and position of injection wells, number of CO2 slugs, and prior repressuring of the reservoir on the predicted production profile and final tertiary oil recovery. All tertiary injection cases were compared to the base case of continued waterflooding. Simulation results indicate possibility of recovering more than 2.5×106 Sm3 (15.7 MMSTB) of incremental oil in 20 years of future production. One of the possible sources of CO2 is the gas treatment plant in Molve, Croatia, which daily releases into atmosphere large quantities of carbon dioxide extracted from natural gas produced in northern Croatian gas fields. Sequestering a considerable amount of this CO2 in Ivanic oil field is a welcome additional benefit of the planned tertiary injection project. Introduction Ivanic oil field is situated in the north-western part of the Sava depression, about 35 km to the east of the Croatian capital of Zagreb. It was discovered in 1963 by drilling the exploratory well Iva-4, which confirmed the existence of commercial quantity of 33.4°API oil in Miocene sandstones inside an assymetrical brachianticline, at the absolute (subsea level) depth of approximately 1600 m. Porosity of the reservoir rock is in the range 21.5–23.6%, permeability is in the range 14.6–79.6 mD, initial reservoir pressure was 183 bar, and reservoir temperature is 97.7°C. Original oil in place was estimated at 21.62×106 Sm3. Total of 88 wells were drilled during the field history. Currently there are 43 oil producers (all equipped with sucker rod pumps), 11 observation wells and 14 water injectors; 20 wells were abandoned. 7 oil producing sandstone intervals, named Gamma, were identified during the field development phase. Starting from the topmost, they are Gamma 5, 4, 3, 2/1, 2/2, 2/3, and Gamma 2/4. A presence of initial gas cap was confirmed in Gamma 5, while PVT analyses of a number of downhole and surface fluid samples taken from Gamma 4 through 2/4 indicate that oil is undersaturated, with saturation pressure of 137.2 bar common in those intervals. Initial PVT properties of reservoir oil are shown in Table 1. Production started in late 1963, and peaked in 1966 with slightly more than 1200 Sm3 of oil per day. Soon after that, lack of aquifer support caused a substantial decrease of reservoir pressure and, consequently, oil production decline typical for inefficient solution gas drive. It was therefore decided to support the reservoir energy by water injection. Waterflooding was started in 1972, and oil production response was noted early in 1973, together with water production increase. Secondary production peak was reached in 1977 with oil production reaching almost 1200 Sm3/day, which was followed by a more or less regular trend of oil production decline and water cut increase. Cumulative oil production until end of year 2003 is 8.68×106 Sm3. Oil and water production history is shown in Figure 1. Ivanic as an EOR Candidate During the late 1970s and beginning of 1980s, majority of Croatian oil fields were comprehensively evaluated in terms of applicability of various EOR technologies. The basic screening criteria applied were taken from the available technical references.1,2 During this screening process, Ivanic oil field was identified as a good candidate for application of CO2 injection. This lead to an extensive laboratory research program aimed at quantifying the thermodynamical interaction between CO2 and Ivanic reservoir oil and the ability of CO2 to mobilise the capillary trapped oil from the pore space of actual reservoir rock samples.3 Several sets of slim-tube tests performed by injecting CO2 into sandpacks saturated by recombined Ivanic oil according to Yellig and Metcalfe laboratory procedure4 showed that minimum miscibility pressure (MMP) is in the range of 190–200 bar, close to the initial reservoir pressure.