Depletion gas drive, gravity drainage, aquifer inflow, and compaction are identified as naturallyoccurring production mechanisms in heavy-oil production. These are supplemented by potential gradients developed during flooding operations. Flooding processes usually are thermal in character: hot-water, steam, and in-situ combustion.

Production mechanisms during thermal recovery processes include: displacement; thermal expansion of fluid and minerals; distillation, transport, and condensation of light components: incrementai gas drive with increasing temperature: decomposition, fluid-fluid, and fluid-solid reactions: and changes in fluid-fluid and fluid-solid interfacial forces. Each of these alters the local potential field, and thus are mechanisms affecting flow. The mechanisms resulting in ‘foamy oil’ behavior (low produced gas/oil ratios, and apparently high oil mobilities and production rates sustained for prolonged periods of time) remain unexplained. The use of horizontal and conventional wells in different arrangements does not create new production mechanisms.

Steam is the most common injectant, usually started as cyclic for early income generation and switched to flooding to optimize gains. Primary production using horizontal wells (or high productivity index completions) may be commercial in areas of high oil mobility.


The term ‘heavy oil’, or viscous crude oil, does not have a universally-accepted definition. Briggs et al.' suggested that heavy oils be defined as those having no less than 100 cp (100mPa. s) at ambient reservoir conditions. That is the working definition used in this article.

All reservoir mechanisms present in light oil reservoirs are common to reservoirs containing heavy oils. Since most reservoirs (including tar sands deposits) have a maximum permeability of about 10000 md (10000 pm2), an approximate upper bound to the mobility of heavy oils is 100 md/cp (100 pm2/mPa s). This combination of high permeability and low viscosity is not common. A more representative upper bound would be about 10 md/ cp (10 pm2/mPa s), which is near the lower bound of economic attractiveness even for light crudes.

Additionally, heavy oil deposits are usually shallow (are at relatively low pressure). As a result, essentially all heavy-oil production is based on an improved recovery process.

The improved recovery processes based on injectants, the flooding processes, enhance the natural driving forces in the reservoirs. Other processes use wells with high productivity indices (e.g., horizontal wells, downhole heating, hydraulic fracturing) and make use of natural forces existing in the reservoir to not supplement the forces in the reservoir in significant ways, are stimulation-type applications. Here we take an extended view that the use of horizontal instead of conventi

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