Type I fractured reservoirs are those in which the matrix is impermeable and fractures provide essentially all of the storage capacity and the fluid-flow pathways in the reservoir. This paper summarizes an analog reservoir study that compared the performance of dozens of such reservoirs to understand their behavior when strong water drives are present and particularly when oil viscosities are moderately high. Most such reservoirs are basement or volcanic rocks; however, numerous analogous clastic and carbonate reservoirs have also been identified.
Recoveries in "Type I" reservoirs access a larger fraction of the oil-in-place in the fractures and very little matrix oil. Fracture recoveries can be expected to be very high (60 to 90% of the swept fracture volume) with extremely low recoveries in the matrix. Fracture porosities vary widely in fractured reservoirs and analogs may be poor sources of this information. Values of fracture porosity in excess of 1.0% are uncommon; however, some fields may have values up to about 3% or higher for certain fractured cherts.
Strong water drives can lead to high initial flow rates and relatively early water breakthrough; true Type I reservoirs may have very poor recoveries as a result. Horizontal wells often accelerate total recovery and may in fact lead to substantial incremental recovery. Heavier oils exacerbate early water breakthrough. Few reservoirs with significant fracturing were consistently fractured areally and with depth. Heterogeneity in fracturing is the rule and has few exceptions. In some many cases identifying the driving cause of spatial heterogeneity in fracture occurrence is the key to developing the field properly.
Water shutoff efforts around the world in highly fractured reservoirs are generally failures. Treatments that successfully shut off water often greatly reduce oil production. This is particularly true of horizontal wells and most true of uncemented liners and open holes. Only in cased and cemented vertical or horizontal wells is there a routine chance of shutting off completion intervals. In many such cases, profile control improvements are temporary as the unwanted fluid bypasses the near-wellbore shutoff area. Gas influx is generally more severe than water influx.
Horizontal well placement is critical in developing Type I reservoirs and is a function of reservoir management and fracture characterization. Multiple laterals offer attractive operational and cost alternatives. Overdrilling similar reservoirs appears to have occurred in quite a few fields. Example fields from around the world are compared using a variety of approaches including some unique comparisons of temperature logs and pressure transient analysis tests.