History Matching Thermally Assisted Gas/Oil Gravity Drainage In Fractured Reservoirs
Authors: Denney Dennis 
Published: February 2015

This article, written by Technology Editor Dennis Denney, contains highlights of paper IPTC 11411, "Key Learnings From History Matching a Thermally Assisted Gas/Oil Gravity-Drainage Pilot in Fractured Reservoirs," by A. Ikwumonu, K. Rawnsley, M. Habsi, and R. Penney, PDO, prepared for the 2007 International Petroleum Technology Conference, Dubai, 4–6 December. The paper has not been peer reviewed.
Globally, large volumes of heavy oil exist in shallow low-permeability reservoirs. If the reservoirs contain sufficient natural fractures, thermally assisted gas/oil gravity drainage (GOGD) could be a viable recovery process. In this process, steam is injected into the fracture system, heating oil in the matrix, reducing the oil viscosity, and accelerating gravity drainage. The economic viability of this process is determined mainly by fracture spacing. Large blocks take longer to heat than small blocks, and the correct description and thermal simulation across the reservoir are critical to decision making.

Introduction
Developing the appropriate reservoir description of the fractures for full-field-simulation modeling requires input of the geological-matrix and fracture-model scenarios, a spacing-averaging method for each gridblock, and the geometric-shape-factor term for each gridblock. Each item contains its own uncertainties. The full-length paper details a study to assess each parameter and characterize its effect on recovery.
For a given heterogeneous geological description, determining the appropriate average spacing for each simulation gridblock is a separate operation. Several techniques have been assessed including arithmetic and area-weighted averages. In addition, several thermal shape factors used in dual-permeability/-porosity simulators were tested against single-porosity results. The effect of the parameters on recovery and produced-fluid temperatures was investigated through multiple reservoir-simulation models.
A history-matching approach was used in matching a pilot steam-injection scheme. This study included matching the measured temperatures, oil-rim position, and production data gathered during a pilot.

Basis
In naturally fractured carbonate reservoirs, the matrix contains most of the oil and is surrounded by a system of fractures that have very little volume but have permeabilities several orders of magnitude higher than that of the matrix. In such a reservoir, it is difficult to apply pressure differential in the matrix to cause the oil to flow out by a conventional displacement process between injectors and producers. The injected fluid tends to flow through the fracture system bypassing oil in the matrix. If, however, gas is introduced into the fracture system such that the gas/oil contact (GOC) in the fracture system is deeper than the GOC in the matrix, then a hydrostatic imbalance is created. The oil in the matrix above the fracture GOC is surrounded by gas and will drain downward because of its higher density, ultimately into the fracture oil rim. As the oil drains from the matrix, it is replaced by gas, and then the oil collected in the fracture system can be produced. This process is called GOGD.