Abstract

While discussing various Improved Oil Recovery (IOR) techniques, the word "heterogeneity" is open used in a negative context, the assumption being that performance (recovery/ rates) would necessarily suffer due to any departures from the "ideal" attributes (i.e. homogeneous/isotropic) of the pay zone.

To test this hypothesis, anticipated IOR performance in Different types of heterogeneities (homogeneous, numerous shale/ anhydride intervals numerous vertical fractures, fining upwards and fining downwards) was analyzed. The results provided insights into screening/ design of various IOR schemes in different geological settings.

It was seen that oil recovery definitely benefits from some heterogeneities. Consequently, one needs to make a distinction between heterogeneities potentially improving IOR performance, and those resulting in a poor response. It was also reinforced that the same heterogeneity could be a bad news in the context of one IOR technique, and good news for some others. Therefore, screening and design of IOR schemes (simulation, horizontal wells, coning mitigation, water flood and enhanced oil recovery) must take into account the prevailing site-specific heterogeneity/stratification.

The main contribution of this paper is an examination of interactions between different IOR mechanisms and variations within the pore matrix leading to an identification of good and bad heterogeneities in the context of specific exploitation modes.

Introduction

There is a growing realization that most pools deposited under various geological environments are usually more heterogeneous and complex than commonly perceived. Although it is known that one may have to characterize heterogeneities on micro, macro and meso scales, a quantitative characterization of heterogeneities is lacking as there are few parameters for describing them, other than the Dykstra- Parson's coefficient1, which is based on dissimilarity of permeability in different intervals of pay zone. Likewise there can be heterogeneity in porosity, an example being intercalation by shaly/ conglomerate/ chert beds. More importantly, due to different minerals, grain sizes, pore shapes and wet ability, they may have different flow properties. There can also be variations in saturation, dissolved gas content, oil viscosity and pressure at different horizontal or vertical locations within a pool. Trapped gas saturation can also vary regionally and with depth within a given pool2. Some of these may be beneficial in achieving economic injection/ production rates in specific cases.

Complexities and dissimilarities within a reservoir/ Site- specific nature of heterogeneities

Because of environment of deposition and digenetic changes, different parts of a sedimentary formation may have different levels of heterogeneity. Therefore, it is prudent to characterize variations at a specific well in the context of the overall variations within a pool. Consequently, some production problems may be more acute at some wells as compared to the others.

Potentially beneficial and adverse impacts of heterogeneities upon oil production and recovery

It is surmised that some heterogeneities definitely benefit oil recovery. For example, in very low permeability reservoirs, naturally occurring permeability heterogeneities (streaks with high permeability) or induced heterogeneities such as hydraulic fractures, can help achieve an acceptable productivity under primary production mode. Similar heterogeneities can also help achieve acceptable injectivities in an otherwise low injectivity situation.

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