Simulated Production Behaviour of Heavy Oil Pools With Gas Caps
Authors: Baker R.O. , Bialowas S.A. 
Published: March 2010

A number of thin heavy oil pools in Western Canada are being produced in which gas coning is unavoidable, even with long horizontal wells. The petroleum literature tends to have an abundance of cases where there is an associated gas cap over a thick, low viscosity oil leg that can theoretically be produced without appreciable gas coning. Gas/water coning evaluation is often restricted to the analytical calculation of breakthrough times and critical coning rates. Thus, work in the literature has limited applicability in thin oil columns of heavy oil pools. This paper reviews the unique production characteristics of those pools where gas coning is typical and discusses the underlying reservoir mechanisms associated with that production signature. In the case of a thin heavy oil pool, there may exist four distinct periods during a well's life. The first period is pre-breakthrough, the next is post-breakthrough where there is a rising gas-oil ratio, then a period identified by a decreasing gas-oil ratio and, finally, a period of relatively flat gas-oil ratio. Wellbore dynamics, standoff distance, gas cap thickness and areal extent, oil zone thickness and drainage area, vertical and horizontal permeability, oil viscosity, solution-gas oil ratio, and relative permeability all combine to control the production signature.

Introduction

Historically much of the literature(1-11) dealing with coning has focused on critical coning rates and breakthrough time, but very little discussion has been devoted to post-cone breakthrough response. There are many situations where production rates must exceed critical coning rates to be economic. This is particularly true in reservoirs with thin oil columns. Although horizontal wells may increase oil recovery and decrease GOR production signatures in thin oil columns, they generally cannot totally alleviate gas/water coning.

TABLE 1: Summary of layer properties in reservoir model. Available in Full Paper.

This paper investigates the behaviour of heavy oil pools in which small gas caps exist. One very unusual characteristic of these pools and drive mechanism is that gas-oil ratio (GOR) may decline with time or pressure depletion. This is in contrast to solution gas drive systems or in large gas cap reservoirs where, for the large majority of project life, the GOR is increasing. This work uses simulation as a tool to understand oil production and GOR profiles.

Description of Simulation Model

Simulations were performed for a horizontal well on 400 m spacing. The objectives of the simulations were to predict GOR behaviour in horizontal wells and to examine the sensitivity of the GOR profile to: gas cap vs. no gas cap, gas cap size, oil viscosity, pay thickness, etc. A 40 ? 21 ? 6 grid was used, representing a 800 m ? 399 m ? 4 m volume of reservoir. A uniform area grid was used. All grid blocks had areal (xy) dimensions of 20 m ? 19 m. The thicknesses (z) of the grid blocks were 0.8 m for the top layer and 0.64 m for the remaining five layers. Tables 1 and 2, as well as Figures 1 and 2, show the reservoir and model parameters.