Numerical Modeling of Radio Frequency Heating Process for Enhanced Oil Production

Abstract

Abstract Radio frequency heating (RFH), a novel technique for enhancing oil production, is discussed as a solution to problems associated with steam assisted gravity drainage (SAGD) operations in reservoirs containing highly viscose conventional heavy oil. Numerical modeling using the finite-difference time domain (FDTD) technique, is used to present specific power absorption rate (SAR) pattern data, produced by RFH in a heavy oil reservoir with and without the presence of bottom water. The engineering and economics of a commercial RFH system are also discussed. Introduction Reducing the viscosity of conventional heavy oil in a reservoir with a focused pattern of RF energy may provide a means for stimulating greater oil production. Currently, horizontal wells and SAGD (Ref. 1) have proven useful for removing heavy oil from unconsolidated sands. The presence of bottom water, however, can lead to breakthrough, lowering productivity. Breakthrough occurs when the mobility of the bottom water is greater than that of the oil. Heating reduces the viscosity of heavy oil increasing it's mobility, thereby decreasing the dominance of the bottom water and preventing breakthrough. RFH offers advantages over steam for this application because it preferentially heats the heavy oil reservoir without wasting energy on heating the bottom water. The opposite is true for steam as it follows the path of least resistance and channels energy to the bottom water. RFH may also be useful for starting the flow of heavy oil prior to SAGD operations and for cleaning produced sand. FDTD electromagnetic numerical modeling techniques are employed to present the results of two simulations of RFH in an unconsolidated heavy oil reservoir, one with bottom water present and the other without. Both RFH simulations are conducted with the antenna tool deployed within a horizontal wellbore. A comparison between the power absorption patterns from each reveals that the proximity of bottom water concentrates RF energy closer to the wellbore, increasing the heating of heavy oil. A description of an system, including a calculation of the power and energy required to reduce the viscosity of a conventional heavy oil throughout a given volume, is discussed from both an engineering and economic point of view. The development of RFH and other thermal technologies for enhancing production of oil from heavy oil and tar sand deposits has been pursued for many years. RFH works by transferring energy at the molecular level to the formation; its use for EOR dates back to the early 1970's. Field testing by KAI Technologies, Inc. was performed in the summer of 1992 in a shallow diatomite reservoir located in the Texaco Denver Producing Division's Midway area, N. Midway Field, CA (Ref. 2). More recently, RFH technology has been applied to enhancing environmental remediation (ref 3.). Numerical Analysis Technique: FDTD Electromagnetic Analysis RF reservoir heating is a dynamic process; one that requires the in-depth understanding provided by numerical modeling to optimize the design of the RFH system including the antenna tool. A code operating in the time domain employing FDTD techniques is inherently practical for this purpose (Ref. 4). During RFH the physical, chemical, thermal and electromagnetic properties of the medium are in a state of constant flux. FDTD allows properties of the medium such as density, specific heat, and dielectric properties to change over time in response to temperature, water content, removal of oil, etc. FDTD codes can also be modified to simulate heat and mass transfer (e.g., the flow of vapor and liquids). The FDTD code used in this paper is capable of calculating the electric field and SAR distributions throughout the reservoir as well as the antenna tool's driving point impedance. Knowledge of the electric field and SAR provides insight into the way energy is distributed by RFH; P. 501^