Integration of Geological, PVT and SCAL Data to Reduce Reservoir Uncertainty of an Unconventional Field in Kuwait

Abstract

Abstract 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 unconnected 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. Introduction Proper understanding of a reservoir is very important for the success of an EOR process. It becomes crucial when a thermal process such as steam injection is used as an EOR process because cost per barrel by thermal methods is much higher than for other EOR processes. If a reservoir is complex then it is a must prior to application of any thermal method. The reservoir under consideration is a complex one. Its thickness is relatively less with baffles and shales in between. At places it is having gas cap which is mappable, some water is also found on top of oil. Laterally as well as vertically there is an appreciable variation in oil viscosity and API. After considering all the thermal EOR processes, steam injection has been finalised as the optimum method for the development strategy. Considering the cost of steam, it is always desirable that it should be applied thoughtfully in view of reservoir complexity. Therefore, proper understanding of the reservoir with an integrated approach is critical prior to steam injection in the field for commercial development strategy. Sometimes, only log response of some of the wells may not be enough to understand the reservoir. An integrated approach is needed to understand it completely. In the present work, an integrated approach was used to understand the exact nature of various shales and cemented rocks and interconnection between layers by considering all inputs such as geological, PVT, SCAL and log correlations. This would help in developing the reservoir in an efficient and disciplined manner.