Integration of geological, PVT and SCAL data to reduce reservoir uncertainty of an unconventional field in Kuwait

Copyright 2014, International Petroleum Technology Conference The study involves a complex sandstone reservoir characterized by relatively thin stratified viscous oil-bearing net pays separated by localized shales and baffles in between. Some of the reservoir intricacies include mappable gas cap intervals overlying net pays at places, water-bearing intervals on top of oil, long transitional zones, and lateral as well as vertical variation in oil viscosity and API. Based on the stratigraphy and geological understanding derived from log interpretation of some initially drilled appraisal wells, the reservoir was divided into four oil-bearing layers: Upper-A, Upper-B, Lower-A, and Lower-B. Upper Sands are separated by Upper Shale Baffle and Lower Sands are similarly separated by Lower Shale Baffle; and Middle Shale acts as a regional barrier between Upper and Lower sand units. Initial reservoir description postulated that all the four oil-bearing layers are separate uncormected units, with no vertical communication. Several hundred wells have been drilled as of now; in many wells, it was found that Upper Shale is discontinuous, with Upper-A and Upper-B sands merging into a single net pay layer. The present study attempts to analyze and integrate various reservoir parameters to understand the realistic and credible "shaliness" of the Upper and Lower Shales. Analysis includes PVT and SCAL data from over 100 wells including viscosity, API gravity, compositional data, and volatiles. Steamflood experiments were conducted on plugs from Upper and Lower Shales. Many plugs were found to have appreciable permeability and porosity with limited oil saturation. All these data suggest that Upper and Lower Shales do not seem to be effective shale barriers and vertical fluid migration can occur. During cyclic steam stimulation in one of the pilot wells, steam was injected in Upper-B layer. Subsequent temperature survey suggests that steam has passed through Upper Shale and migrated into Upper-A Sand. This further corroborates that Upper shale is not acting as an effective barrier. It is thus concluded that to understand geological heterogeneities and to reduce reservoir uncertainty, integration of PVT, SCAL, and other reservoir information along with geology is required for optimum development of an unconventional reservoir.