Abstract: Identifying Candidates for Horizontal Drilling Through Understanding Reservoir Heterogeneity

Ross A. Clark

Horizontal drilling has proven to be a successful technology for exploiting unrecovered mobile oil in existing fields. Planning a successful horizontal drilling program requires a detailed understanding of reservoir heterogeneity while remembering that a horizontal well is basically a controlled, directional, completion technique. Also, in any reservoir being considered for horizontal drilling, heterogeneity has prevented the efficient and economic development of the pool by vertical wells.

Reservoir heterogeneity can be of two basic types: operational and depositional. Operational heterogeneities are inherent in pool-producing practices such as coning or operator bias. Depositional heterogeneities compartmentalize tile reservoir through some form of permeability anisotropy such as fractures or mud drapes. All reservoir complexities can be classified into either of these two major types or a combination of them. These two groups of complexities can be subdivided into 20 categories based on depositional architecture, hydrocarbon type, and reservoir drive mechanism. This system of classification simplifies the identification process.

Other criteria of significant importance were compiled from an analysis of several hundred horizontal wells from many different applications. The most important of these criteria are that heterogeneity creates the opportunity, permeability makes it work, and storage makes the opportunity commercially viable. Heterogeneous high-permeability reservoirs, with good vertical permeability, make excellent horizontal drilling candidates. An example would be high-permeability fractures (heterogeneity) that drain a reservoir volume with good storage. In general, low-permeability reservoirs do not make good candidates.

Lithology also plays an important role in screening potential reservoir candidates for redevelopment using horizontal drilling. Top preference should be given to carbonate reservoirs because they often exhibit highly variable porosity and permeability within laterally continuous beds. Also, well-bore damage in carbonate pools can be more easily stimulated than in sandstones, and they can often be completed open hole. Sandstones are a lower priority than carbonates because they typically are less competent, are more prone to damage, and are difficult to stimulate. Finally, shales should be given the lowest priority because of difficult borehole stability problems and abundant fines production.

The use of this classification scheme as illustrated from numerous examples of successful and nonsuccessful horizontal drilling projects can significantly improve overall performance and recovery efficiencies in most reservoirs.

AAPG Search and Discovery Article #90985©1994-1995 AAPG Distinguished Lecture