A Critical Evaluation of a Steamflood Pilot in a Deep Heavy Carbonate Reservoir in Ikiztepe Field, Turkey

Abstract

Abstract This paper presents an evaluation of a 4,430-ft deep steamflood pilot being carried out jointly by the Japan National Oil Corporation (JNOC) and Turkish Petroleum Corporation (TPAO) in collaboration with the Japan EOR Research Association (JEORA) in the Sinan heavy oil reservoir of the Ikiztepe field, Turkey. The paper discusses pilot operations and performance highlighting the problems and the corresponding remedial measures/solutions adopted. Also presented is a discussion of results of an integrated simulation history match study. The pilot is in its final phase, and has so far yielded oil-steam ratios (OSR) of 3.42 and 0.32 during the initial cyclic steam stimulation phase and the steamflood respectively. Introduction Among thermal EOR processes, the steamflooding has been the most prolific both in terms of the oil production and field applications (Oil & Gas J., Sept. 26, 1994). Chu has summarized 28 major steamfloods which are being carried out or completed — mostly in sandstone formations. Only a few of them yielded an OSR >0.2. Thirteen projects have been in the field-scale, and data suggest that, for a steamflood to be successful, the reservoir should be shallower (<4000 ft) with an oil transmissibility (koh/ o) and content (So) greater than 5 md ft/cp and 0.08, respectively. With the exception of Shell's Brea steamflood no steamflood has been reported at a depth over 5000 ft. For carbonate reservoirs, the deepest steamflood reported is the Lacq Superieur flood, Algeria at a depth of 2,300 ft. There are many heavy oil reservoirs in the south-eastern Turkey, with an oil-in-place estimated at 2.5 billion STB. Therefore, primary objective of this project was to evaluate the potential of the thermal methods in enhancing oil recovery from such reservoirs. Location and Geology Since 1987, the JNOC and TPAO have been involved in the steamflood pilot project in the Ikiztepe field, Turkey in collaboration with JEORA. The field is located in south-eastern Turkey on the southern flank of the Taraus belt (see Figure 1). The 4430-ft deep target formation is the Sinan limestone of tertiary-cretaceous age, and is occasionally dolomitic and irregularly vuggy and fissured. The total Ikiztepe field production had been dismally low at 88 MSTB of an OOIP of 127 MMSTB due to the low oil mobility (high o), rapid decline rate and a high water-cut. There exists a thin gas zone (10 ft) but it is not strong enough to resist the rapid decline rate. The estimated GOC, WOC and other relevant reservoir data are provided in Table 1. The oil is heavy (10–12 API), and before the steamflood, only lour wells were on production for about 4–5 years with an average oil rate of 10–20 STB/D. The initial solution GOR was estimated to be 95 SCF/STB and the measured oil viscosity. under the reservoir conditions of 1841 psi and 120 F was 936 cp. The oil viscosity sharply increases as the solution gas is released, and exceeds 2000 cp at the stock tank conditions. Some core-derived porosity data are available from the top three of eight zones and they match reasonably well with log-derived porosities. The average porosity within the zone of interest is 15–23%. However, it was difficult to estimate the permeability as the slow separation of the solution gas resulted in a 2–5 fold increase in the oil viscosity. Nevertheless, based on the -k relationship, the average permeability was estimated to be within 50–400 md. Pilot Operations Feasibility Study. The feasibility study was based on available geological data, results of a simulation study with the primary production history, and some follow-up laboratory studies to characterize the rock-fluid properties. Three processes: gas injection, high pressure CO2 flood and steamflood were considered and sensitivity analyses were carried out for each process. The gas injection process was ruled out due to the adverse mobility ratio which would have resulted in a lower sweep efficiency. P. 267