Reservoir connectivity, and its inverse, compartmentalization, is a critical area of petroleum industry research and business application. However, significant differences in how it is defined, measured, and modeled exist among companies. For some, connectivity is defined relative to an entity such as a well or set of perforations in a reservoir. Others prefer reservoir connectivity indexes, using a set of often subjectively defined criteria to gauge how problematic a field will be to develop or exploit.
We have developed a technology called "Reservoir Connectivity Analysis" (RCA) to investigate field compartments and associated connections (Vrolijk et al. 2005). A compartment is precisely defined as a trap which has no internal boundaries which would allow fluids to reach equilibrium at more than one elevation. Compartment boundaries include sealing faults, channel margins, shale-draped clinoforms, paleokarst fractures and other diagenetic boundaries. These can separate hydrocarbons and aquifers within a field or discovery. Connections between compartments include fault juxtaposition windows, erosional scours between channels, and capillary leakage. Compartment boundaries include spill and breakover points, defined on topseal and baseseal maps.
We also find that it is important to separately define and investigate "static" and "dynamic" connectivity. Static connectivity describes the native state of a field, prior to production start-up. Evaluation of static connectivity is the basis for proper assessment of original hydrocarbons in place and prediction of fluid contacts in unpenetrated compartments. Dynamic connectivity describes movement of fluids once production has begun. Initiation of production actually perturbs the original fluid distributions as pressure and saturation changes proceed in a non-systematic fashion across field compartments. Analysis of dynamic connectivity is essential to estimating ultimate recovery from a field.
An example of RCA application in the Gulf of Mexico is provided to illustrate how RCA can generate testable fluid connectivity scenarios and explain troubling production anomalies.
Reservoir connectivity and its inverse, reservoir compartmentalization, is a growing area of oil industry research and business application, as large offshore discoveries go through development and established producing fields progress through their maturity cycle (Smalley and Hale, 1996; Elshahawi et al. 2005). The size and scale of the associated monetary investments requires three key foundations be established:
strict definitions of connectivity;
explicit workflows for analyzing connectivity; and
suitable analogs to be used as guides for investigating new cases.
This paper provides each of these in order below.
Surveying the general terrain of reservoir connectivity reveals significant differences among companies in how it is defined, measured, modeled, and acted upon.
Schlumberger (2004) compared two unnamed North Sea fields (Fig. 1). Without specifying the connectivity measure, it was noted that the "highly connected" submarine fan reservoir and "poorly connected" deltaic reservoir fields differed substantially in terms of estimated oil in place, ultimate oil recovery (reserves), and cumulative oil production. Particularly striking is the drop in oil reserves (ultimate recovery) in the "poorly connected" deltaic reservoir field, just a short time after production start-up. By contrast, the highly connected submarine fan reservoir had showed a steady climb in oil reserves through time.