A New Methodology for Modelling of Sand Wormholes in Field Scale Thermal Simulation
Authors: Tan T. , Slevinsky R. , Jonasson H. 
Published: March 2010

Sand wormholes created by the concurrent production of sand and heavy oil are instrumental in improving oil rates and ultimate recovery. The wormholes form high permeability conduits for flow of oil and more sand. The wormholes may extend several hundred metres away from each well and connect with other wells' wormholes. Efforts have been made to predict the wormhole network based on growth of each wormhole's tip.

In this paper, we use a fractal geostatistical method to pregenerate the wormhole network. This wormhole network is then incorporated into a full field thermal simulation model. A dual porosity-dual permeability grid is used, one for the reservoir pay and one for the wormhole network. The wormhole grid cells have the same pore volume as the wormholes and a sparse network structure.

With this wormhole representation in the simulation model, we were able to match thermal responses of cold production wells from a steam injection pilot due to the wormholes conducting hot fluid from the steam injectors.

As a result, this modelling concept for wormholes may be used to investigate post cold production thermal recovery methods as well as for thinner reservoirs not suitable for Steam Assisted Gravity Drainage (SAGD) where the wormholes may act as natural horizontal wells.

Introduction

Sand production occurs with heavy oil production in unconsolidated sand formations. If sand production is stopped with screens or filters, this often results in near total loss of production from the well. With the use of progressive cavity pumps, sand production can be encouraged, resulting in sand cuts that can be as high as 30 – 40% initially before dropping to about 5%. The production of sand results in open holes, whimsically called wormholes, that stretch far into the formation away from the well. The productivity of the well rises from the average 4 to 5 m3/d to as high as 15 to 20 m3/d as the wormholes form high permeability conduits for flow of oil and more sand. This production process is called Cold Heavy Oil Production with Sand or CHOPS.

Many papers have been published on the mechanics of sand wormhole development. Recently, Y. Wang and C. Chen(1) describe a reservoir model coupled with a detailed wormhole propagation model. The model also incorporates slurry transport with three phase fluid flow. This appears to be a single well model. Research work at the Alberta Research Council on modelling cold production was published by Sawatzky et al.(2). They provide a review of cold production technology as well as their modelling efforts in combining a comprehensive sand production with a fluid flow simulator. It would appear that this too was a single well model predicting the development of wormholes from a well using criterion for sand failure at the tip of a wormhole. These two papers also list extensive bibliographies which will not be repeated here.

We are, however, more concerned with the effect of a developed network of wormholes on recovery from a reservoir than with trying to predict the growth of each wormhole based on stresses at its tip.