A New Combustion Process Utilizing Horizontal Wells And Gravity Drainage (1994)

Abstract The Combustion Override Split-production Horizontal-well (COSH) process has been conceived to combine the high recovery potential of gravity drainage with the energy efficiency of combustion processes while minimizing problems associated with combustion operations. COSH should be less sensitive to oil composition and combustion kinetics and it has minimal water supply and disposal requirements. The mechanisms for the COSH process have been identified within the limitations of numerical simulation. A simulation study indicates that the COSH process has technical performance approaching that for the highly rated steam-assisted gravity drainage (SAGD) process, and has much lower energy costs. Introduction In-situ combustion has been studied and piloted for several decades because it has the potential of the highest energy efficiency and lowest production cost among thermal processes(1) Unfortunately, overall field experience has been disappointing because of numerous problems such as slow production response difficulty in sustaining combustion, early oxygen breakthrough, severe sanding, gas locking of down-hole pumps, corrosion, and scaling. The literature on combustion studies relates primarily to frontal displacement of the combustion zone. This involves linear or radial areal displacement from vertical injection wells to vertical production wells with the pay-zone visualized as being divided into successive zones including a combustion zone followed by a condensing zone and an oil bank which is displaced ahead of the combustion front. Many elegant and sophisticated studies have been conducted on the dynamics of the combustion process for linear displacement, particularly with combustion tubes(2). Unfortunately, it is very difficult to model in the laboratory the three-dimensional effects of gravity segregation of fluids and poor vertical and horizontal sweep efficiency which are at the heart of many of the problems in field combustion projects. It is also difficult to analyze the process mechanisms in combustion field projects which have been reported to increase oil production. In particular, for field projects it is difficult to determine the relative importance of thermal effects, gravity drainage (particularly in dipping reservoirs) and complex three-phase relative permeability effects caused by the large volumes of gas and water used. This paper will present a new approach to take advantage of the theoretical efficiency of combustion by combining the best features of successful steam pilots that the Alberta Oil Sand Technology and Research Authority (AOSTRA) has participated in while minimizing the negative features responsible for problems reported in earlier field combustion projects. COSH Process The main features in the Combustion Override Split-production Horizontal-well (COSH) process(3) are described as follows, with reference to Figures I and 2:Gas containing oxygen is injected via rows of vertical injection wells. The gas may be air, oxygen or recycled gas with oxygen added. The gas is generally injected in the upper part of the pay-zone. Maintaining continuous gas injection into the reservoir and cold water circulation within the wells will prevent combustion damage. The vertical wells can also serve to define the geology and locate the base of the pay-zone accurately prior to drilling the horizontal production well and they could be used to monitor temperature profiles in the lower part of the gas chamber with cemented-in thermocouples.