Concepts And Opportunities In Using Horizontal Wells In High Quality Reservoirs
Authors: Mcintyre F. 
Published: March 2010

A considerable fraction of Alberta's, and the world's, oil has been produced from high quality reservoirs dominated by gravity drainage. However, almost all of these reservoirs in North America have been completely flooded with gas, solvent or water, and are nearing economic limits with vertical wells. Often reservoir simulations show little economic incentive to drill horizontal wells for incremental recovery of sandwich or attic oil, because very high permeability minimizes cone heights. Nevertheless, many of Husky's horizontal wells, which have been produced from these types of reservoirs, have been extremely successful(1).

Heterogeneity

The most obvious reason for an unexpected success in high quality reservoirs is the effect of higher than anticipated heterogeneity on oil drainage. Even though average permeability at a well may be in the darcy range, the consequence of low permeability layers on the cone shape C3n be extreme (Figure 1). The existence and effect of these low permeability sheets is often a surprise, because log, core and production data alone do not provide one with the full story of heterogeneity and fluid flow in the reservoir. Estimating the lateral extent of tight zones shown on logs is difficult when interwell spacing is less than the extent of these tight zones. Also, many thin low permeability layers are not clearly identified by most logging tools. Stylolites, and other very thin impermeable zones, are often avoided in selection of core for analysis, but can extend far enough laterally to seriously restrict vertical fluid flow(2). A simulation history match of production data can be accomplished using a high residual oil saturation, instead of a representation of bypassed oil zones caused by reservoir heterogeneity. Therefore, a very careful analysis of log, core and production data is required to identify heterogeneities.

Other methods which can help one determine the location and extent of permeability "barriers" are pressure interference testing, interwell tracing, gathering fluid type and saturation data from infill wells and testing intervals of existing wells.

Lateral heterogeneity can also play a major role in oil drainage. For example, porosity stringers at the margin of a reservoir are often poorly drained (Figure 1). The location of these stringers can sometimes be determined with more advanced seismic techniques.

Mechanics of Interval Isolation

Another reason for horizontal well success is simply the challenge of producing from a thin oil interval with a vertical well. Formation damage, poor cement, and equipment malfunctions all contribute to hinder vertical well production (Figure 2). Recent technological advances have reduced the severity of these problems in some newly drilled and completed wells, however, many high quality reservoirs are being produced through older wells.

Low Drawdown, Wellbore Crossflow and Fluid Separation

Horizontal, and in some cases vertical, wells can produce at economic rates from very high penneability zones at almost no drawdown. Producing 100 m3/d of light oil from a 1,000 m long horizontal well in a massive reservoir with 2 darcies permeability will result in a drawdown of only 3 kPa3.