This paper deals with the use of Side Wall Cores as a tool to monitor In-Situ Combustion (ISC) process performance in Balol field of ONGC where ISC process is being applied on commercial scale since 1997.
Balol field is about 13 km in length from south to north & 1 km wide. It is an unconsolidated sandstone reservoir having pay thickness is in the range of 6 to 20 m and oil viscosity in the range of 150 to 1000 cP at reservoir conditions and increasing from south to north. The process is successful in enhancing the oil recovery from envisaged 13 % of OIIP to 19 % of OIIP. However, the performance of the process was different in different sector depending on the pay thickness & the oil viscosities. The oil recovery is 54 % of OIIP in southern part of the field where thickness and oil viscosity are moderate as compared to the oil recovery of 5 % of OIIP in northern part where thickness and viscosity are high. Various attempts viz. reduction in spacing between injectors and producers through infill drilling and drilling of horizontal wells were made to improve the ISC performance in this sector but there was not much success.
To understand the reasons for poor process performance in northern part of the field conventional coring along with Side Wall Core (SWC) were carried out in a replacement air injector which was just 13 m away from the old air injector. The results of conventional core and side wall cores were correlated. As the conventional coring is cost intensive and involves complication, only SWC was carried out in another replacement air injector falling in southern part of the field which is 80 m away from the old air injector.
The conventional core & SWC of the well from northern part indicated that in thick pays the burning is taking place only in top few meters of the pay and major part of the pay is unaffected by the process. The SWC analysis in the southern part well having moderate pay thickness indicated that the entire pay participated in the burning. This indicated very good vertical sweep and oil displacement.
Air injection design is based upon the oil characteristics, fuel deposition, pay thickness, porosity etc. The core analysis indicated that in thin reservoirs vertical sweep efficiency is very high as compared to thick reservoirs. In thick reservoirs burning occurs in top few meters and any increase in air injection rate result into the early flue gas breakthrough and subsequent loss in oil production. The core data & the process performance of the various sectors of Balol field confirms that ISC process is more effective in terms of volumetric sweep and oil recovery in moderately thin reservoirs as compared to thick reservoirs and application of the process in thick reservoirs need relook while designing of the process.