Abstract

We have conducted an experimental scaled model study to investigate the feasibility of two methods aimed at improving oil production from mature heavy oil fields. The first method involved the use of propane as a steam additive (propane:steam mass ratio of 4:100 was used in the study). The second involved the use of a novel well system consisting of a vertical injector and a smart horizontal producer. To minimize steam over-ride, the smart horizontal producer was divided into three sections, the section furthest from the injector being opened only after steam had broken through in the first two sections. The 16 in. x 16 in. x 5.6 in. physical model was scaled to present conditions in the San Ardo field in California and crude oil from the same field was used for the tests. The sides of the model consisted of 0.75 inch thick Teflon sheets to minimize heat loss, while the top and bottom consisted of 0.125 inch aluminum sheets to maximize heat conduction to the over-and under-burden. The physical model was placed in a pressure jacket containing nitrogen that provided an overburden differential pressure of about 20–40 psi during the runs. Superheated steam at 190-210°C was injected at 48 cm3/min (cold water equivalent) while maintaining the flowing pressures in the production wells at 50 psig. Liquid samples from each producer in the model were collected and treated to break emulsion and analyzed to determine water and oil volumes.

Main results of the study may be summarized as follows. First, for the vertical well system, addition of propane accelerated oil production by 53% and increased ultimate oil recovery by 7% of original oil-in-place (OOIP) when compared to pure steam injection. Second, use of the smart horizontal well system increased ultimate oil recovery to 49% OOIP compared to that obtained with the vertical well system (42% OOIP).

Introduction

As the ever-growing world energy demand strains the supply of conventional oil and gas, the role of unconventional resources becomes increasingly more prominent. Heavy oil, extra-heavy oil and bitumen constitute a large portion of the so-called unconventional resources, which include tight gas, coalbed methane and gas hydrates. Moreover, about 70% of the world's total oil resources is made up of heavy oil and bitumen.1 Most of the world's heavy and extra-heavy oil reserves are located in Canada and Venezuela, which some estimates point to a combined total of about 3 to 4.5 trillion barrels original oil-in-place.1 In the United States, heavy oil reserves are estimated to be around 200 million barrels, mostly located in California and Alaska. Steam injection remains the most widely used recovery process for heavy and extra-heavy oils.

The efficiency of steam injection is typically reduced by gravitational forces. Given the large density difference between steam and oil, steam tends to rise to the top of the reservoir causing a series of drawbacks in the steamflooding process. First, the steam creates a path of preferential flow at the top of the reservoir which in turn accelerates steam breakthrough in the production well. This establishes a preferential path for the steam to be recirculated from the injector to the producer. This situation reduces the amount of contacted oil in the reservoir, only heating its upper portion and the oil directly below it. Lastly, due to the accumulation of steam in the top of the reservoir, heat losses to the overburden by conduction are exacerbated.

To mitigate the problem of steam override and early steam breakthrough, numerous alternatives have been proposed, most prominently, the use of foams.2–4 An alternative method was proposed by Mamora and Sandoval5–6 in which the producing vertical wells in a nine-spot pattern are replaced by a smart horizontal well. They conducted a series of numerical simulation studies and showed that increased recovery was obtained with the use of the vertical injector-smart horizontal well system compared to the conventional vertical well system.6

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