Methane Pressure-Cycling Process for Thin Heavy-Oil Reservoirs

Abstract

This article, written by Assistant Technology Editor Karen Bybee, contains highlights of paper SPE 88500, "Methane Pressure-Cycling Process With Horizontal Wells for Thin Heavy-Oil Reservoirs," by Mingzhe Dong, SPE, PTRC, and Sam Huang, SPE, and Keith Hutchence, SRC, prepared for the 2004 SPE Asia Pacific Oil and Gas Conference and Exhibition, Perth, Australia, 18-20 October. The methane pressure-cycling (MPC) process is an enhanced-oil-recovery (EOR) method used in some heavy-oil reservoirs after termination of primary or waterflood production. The object of the process is restoration of the solution-gas-drive mechanism. This restoration is accomplished by reinjecting an amount of solution gas, mainly methane, and then repressuring the gas into solution by injecting water until the original reservoir pressure is reached. This recreates the primary-production conditions. This recovery technique targets the large portion of heavy oil in thin reservoirs. Introduction Heavy oil in reservoirs thicker than 10 m commonly is produced by use of thermal recovery methods. These methods generally are not suitable for thin reservoirs because of heat losses to the overburden, underburden, and bottomwater zones. Saskatchewan accounts for 62% of Canada’s total heavy-oil resource including 1.7×109 m3 of proven and 3.7×109 m3 of probable reserves. Of the province’s proven initial heavy oil in place, 97% is contained in reservoirs with pay zones less than 10 m thick and 55% is in reservoirs with pay zones less than 5 m thick. Primary and secondary methods recover only approximately 7% of the proven initial oil in place (IOIP). The incentive is strong for the development of EOR techniques that will maximize the recovery of these thin heavy-oil reservoirs. The MPC process is an EOR method intended for application after termination of primary or waterflood production in some thin reservoirs. The objective of the pressure-cycling process is restoration of the solution-gas-drive mechanism that provided primary production. Restoration is accomplished by reinjecting an amount of solution gas and then repressuring the gas back into solution by injecting water until original reservoir pressure is reached. This recreates the primary-production conditions. The process uses infill horizontal production wells located between the existing vertical wells that are used for methane and water injection to pressure up the depleted reservoir. This pressure cycling is repeated until additional economic recovery is no longer feasible. The Saskatchewan Research Council/Petroleum-Technology Research Center has been investigating the pressure-cycling process for the past few years. Experimental Pressure/Volume/Temperature (PVT) Studies. PVT studies were conducted for oil samples collected from four Saskatchewan heavy-oil reservoirs (Senlac, Golden Lake, Plover Lake, and Cactus Lake North) in contact with methane. The purpose of these studies was to investigate the PVT properties of heavy-oil/methane mixtures and to provide input data on the PVT behavior of live oils for numerical simulations. Heavy-Oil Coreflood Studies. The heavy-oil coreflood apparatus consists of a 30.5-cm triaxial-stress core holder, a Ruska dual-cylinder positive-displacement pump, an injection system, a production system, and support units. A backpressure regulator installed at the production end maintained the operating pressure. Oil production was determined on a mass basis.