How Much Oil You Can Get From CHOPS
Authors: Han G. , Bruno M. , Dusseault M.B. 
Published: March 2010

Cold Heavy Oil Production with Sand (CHOPS) has been applied with very good success to enhance heavy oil production in Canada, China, Venezuela and Kazakhstan. Based on existing laboratory and field information and case histories, the most important physical processes enhancing cold production have been reviewed, summarized and quantified.

Several sanding models are developed, including a new porosity cap model for failure propagation, as well as a semi-analytical elastoplastic stress model coupled with an unsteady pressure model for foamy oils. The mechanisms for oil rate enhancement by CHOPS, such as porosity and permeability enhancement that arise from sand removal, propagation of the elastoplastic (remolded) zone, increase of oil velocity relative to the matrix and the effects of foamy oil behaviour, are quantitatively described and compared.

The proposed model can be applied to predict how much additional oil one might expect for a given amount of produced sand. It might also serve as a tool for optimizing cold heavy oil production while nevertheless keeping the sand flux at a low level, which could reduce operating expenses such as limiting sand disposal costs.

Introduction

There may be more than 0.95 × 1012 m3 of heavy oil in the World(1), compared to 0.28 – 0.37 × 1012 m3 of conventional oil, of which over 40% has already been produced(2). Because of high viscosity, primary recovery factors for heavy oils are generally low; if the viscosity is higher than 10,000 – 20,000 cP in situ and the permeability is less than 5 Darcy, it appears that commercial recovery using any conventional non-thermal method is not possible. With careful design and implementation, various thermal recovery schemes can be effective, but high operational costs restrict their applicability.

Though it has been long recognized that "...the maximum recovery of oil from an unconsolidated sand is directly dependent upon the maximum recovery of the sand itself...(3), " CHOPS was not widely implemented with commercial success until advanced pumping systems (such as the progressive cavity pumps) were perfected in the late 1980's for slurries containing sand.

Since then, because of reasonable recovery factors (~15 – 20%), production rates (3.2 – 47.7 m3/day), effective sand handling and disposal and no heat costs, CHOPS has grown to provide more than 20% of Canada's oil. In 2002, Canada's oil production from all sources was approximately 460,000 m3/d, of which more than 95,400 m3/d was CHOPS production. Heavy oil reservoirs suitable for CHOPS are located in unconsolidated or weakly consolidated sands where sand mobilization can be easily triggered and sand influx sustained for the productive life of the well.

Because of several unique characteristics of unconsolidated heavy oil reservoirs, well productivity may be 10 – 20 times higher in CHOPS wells than predicted by conventional Darcy's law flow equations(4). The mechanisms responsible for the enhanced production rate in CHOPS are(5):Porosity and permeability are enhanced as sand is removed from the formation, along with any mechanical skin that may have developed;The oil flow velocity relative to fixed coordinates is increased if the matrix is partially mobilized. Therefore, production rate increases, as predicted from Darcy's law;