Abstract
Abstract
In-situ combustion is an effective thermal recovery process. A part of the oil is burned in-situ, heat is generated, oil viscosity decreases, and larger oil recovery results. Thus, a key mechanism is effective oxidation of a small fraction of the oil in place to generate heat and pressure. Common, water-soluble metallic salts are known to play an important role as a catalyst for some combinations of crude oil/brine/solid matrix. Metallic additives enhance oxidation and cracking of hydrocarbons and thereby affect the nature and the amount of fuel formed. The mechanism of the catalytic effect is, as of yet, unknown.
This paper describes an experimental study combining tube runs that gauge combustion performance and ramped temperature oxidation tests that measure the kinetics of combustion. We propose cation exchange of metallic salts with clay as a mechanism to create activated sites that enhance combustion reactions between oil and oxygen. Sand and clay surfaces are examined with scanning electron microscopy for evidence of cation exchange and alteration of surface properties by metallic salts. The oils studied are heavy and light oil from Cymric (Kern, Co., CA). Effluent gas analysis is conducted and temperature profiles are measured. Additive improved performance in all cases including lower activation energy, greater oxygen consumption, lower temperature threshold, and more complete oxidation. In tube runs, Cymric light oil (34 °API) did not exhibit sustainable combustion, but gave sustained combustion with the addition of iron nitrate. Thus, metallic additives have potential to expand the range of candidate reservoirs for in-situ combustion.
Introduction
Thermal recovery is the principal method to recover viscous, dense crude oil because crude oil viscosity declines substantially as temperature increases. Thermal techniques include steam injection[1], in-situ combustion[2,3], and electrical heating[4]. All methods have restrictions. Steam stimulation may not be suitable for deep reservoirs because heat losses from uninsulated boreholes to the formation are very high. In some cases, up to half of the steam enthalpy is lost before it reaches the reservoir. Furthermore, for steam injection it is necessary for the steam swept region to remain at steam temperature so that the injected steam does not condense before reaching the steam-oil front. Heat losses from the steam swept region continue even after the front has passed. Also because of heat losses to the neighboring layers, steam is not applicable for thin reservoirs. Similar to steam injection, insitu combustion stimulates oil production by lowering oil viscosity and increasing its mobility.
In contrast, in situ combustion makes use of air to oxidize a fraction of oil within the porous medium and liberate heat. Heat remaining behind the front preheats the air, but the process does not rely upon air to carry heat from the injector to the displacement front. Wet combustion also takes advantage of remaining heat by injecting water along with air. Water is heated within the swept reservoir behind the front, vaporizes to steam, flows to the combustion front, and then heats the reservoir downstream of the front.
Combustion is usually initiated by an ignition device that raises the temperature of the zone surrounding the borehole. Once ignited, combustion is generally self-propagating given that sufficient air is injected. The production mechanisms of in-situ combustion include a series of complicated chemical reactions and displacement processes: oxidation and cracking of hydrocarbons, front displacement, gravity drainage, miscible flood, distillation, steam and hot water flooding[3].