Light-Oil Air-Injection Performance: Sensitivity to Critical Parameters

Abstract

Abstract The objective of this study is to determine parameters that have major impact on the recovery from air injection into low permeable fractured chalk reservoirs that produce light oils, such as the Ekofisk field in the Norwegian North Sea. Earlier works on air injection into light oil reservoirs mainly focused on mathematical modeling of air injection and also on laboratory studies. In this paper, we present results from numerical simulation study. Results show that when recoveries from low permeability and high permeability reservoirs are compared, recovery is accelerated in the former. In addition, for maximum recovery, air should be injected into all layers containing hydrocarbons. The ultimate recovery from air injection was observed to be insensitive to the amount of water injected during the secondary recovery phase. Also, for optimum recovery, there exists a minimum temperature at which air should be injected, this temperature being a function of the injection rate. Lastly, we observed that residual oil saturation to gas does not have any significant effect on the recovery factor from air injection. These results are only valid for single porosity systems and may not all apply to dual porosity systems. Also, no economic analysis was undertaken in this study. Introduction In recent years, High Pressure Air Injection (HPAI) has proven to be a valuable EOR process, especially in deep, high pressure, low permeability fields, where other recovery processes are uneconomic. As stated by Moore et al [1], HPAI is loosely defined as an EOR process in which compressed air is injected into a high gravity, high pressure oil reservoir, with the expectation that the oxygen will react with a fraction of the reservoir oil at an elevated temperature to produce flue gas. The produced flue gas usually comprises 10 to 14% CO2, with the rest being N2. In its simplest implementation, the process is initiated simply by injecting air, which will spontaneously ignite the in-place oil due to the high temperature and pressure conditions in the reservoir. The desire to obtain optimum recovery from an air injection project requires the engineer to employ competent field development practices. This necessitates the knowledge of parameters that have significant effect on the performance of a reservoir. The possible application of air injection to light-oil reservoirs in the North Sea has been the subject of research in recent years. The giant structures of North Sea oil fields and the large amount of residual oil after secondary recovery make the consideration of air-injection as an enhanced oil recovery (EOR) mechanism worthwhile. Currently, air-injection is being considered as a possible EOR mechanism in the Ekofisk field, Norway. We present result of the first part of a series of studies focusing on the possible application of air injection in the Ekofisk field. While further studies would be conducted using dual porosity systems, this initial study utilizes single porosity reservoir models. The Ekofisk Field The Ekofisk field, discovered in 1969, is located in the southern part of the Norwegian sector of the North Sea. The field consisting of two producing reservoirs, one each in the upper Ekofisk formation and the lower Tor formation, is a naturally fractured chalk field with low matrix permeability and high porosity. In between the two producing reservoirs is a layer commonly referred to as the tight zone, which acts as an impermeable barrier throughout most of the field. Original oil in place was estimated to be 6.4 billion barrels of oil. The original fluid was an undersaturated, moderately volatile crude oil with a 36º API gravity and a 1,530 scf/stb solution GOR.Reservoir porosity ranges between 25% and 40%. Matrix horizontal permeability varies between 0.1 and 5md depending on porosity. Fractures enhance overall permeability and yield effective permeability that is an order of magnitude higher. Vertical effective permeability is between 0.001 and 0.1 times that of horizontal effective permeability.The field is currently being waterflooded as a means to improve oil recovery.