Shuaiba Transition Zone Reservoirs: From Modeling Studies To Field Development Challenges

Abstract

Abstract Production data from several Shuaiba fields in the Sultanate of Oman demonstrate the oil recovery from capillary transition zones. This is underpinned by extensive detailed experimental and theoretical modeling studies describing favored oil mobility and residual oil saturation in transition zones in the Shuaiba formation reservoir. Nonetheless the economical development of capillary transition zone reservoirs, especially those with thin oil columns, remains an ongoing challenge and necessitates addressing uncertainties related to characterizing the lateral variations in initial oil saturation distribution, water flooding displacement, sweep efficiencies and accurate prediction of oil production. This paper discusses good practices for ongoing developments of transition zone fields and predicting their performance. The key enablers are:properly characterized initial saturation height functions for long horizontal well planning,implementation of consistent set of hysteresis saturation functions in the reservoir simulation models andgeosteering tools for accurate well placement and spacing. Examples of predicted well performances for a field compared to the actual production data will be shown. The results highlight the generic trends in transition zone oil production and the recommended procedures for static and dynamic modeling during field development planning. Introduction The reservoir interval from the oil/water contact (OWC) to a level at which water saturation is irreducible is referred to as the capillary transition zone where both the oil and water phases are mobile (Masalmeh et al., 2007). Thin reservoir units with low permeability formations are often totally located in the transition zone. The transition zone thickness for a reservoir of a given geological structure is mainly controlled by the low permeability rock of prevailing high entry capillary pressure, relatively wide pore throat distribution and small density difference between oil and brine at reservoir conditions. Thicker capillary transition zones compared to the reservoir unit thickness result in maximum water saturation at any given point in the reservoir that is ten's of saturation units more than the irreducible water saturation. Reservoirs that are characterized by thin, low-permeability, heterogeneous, transition zone, unfavorable aquifer support and high oil viscosity pose significant challenges for economical development. Nonetheless, large area and structurally relatively flat reservoirs may contain hydrocarbon volumes large enough to justify pursuing the field development despite some or all of the above mentioned challenges. The Shuaiba formation contains a significant portion of Oman's hydrocarbon volumes, with a significant part of these volumes located in the oil/water capillary transition zone. The subject of this paper is to address the field development aspects of one field with these characteristics and its associated different levels of reservoir and operational complexities. The field's reservoir geology is matrix dominated and it has relatively low oil viscosities (4- 5 cP, 28 API). The permeability of the oil bearing sections is in the range of 1–10 mD, while the porosity is in the range of 20–40% (Warrlich et. al, 2009). Some of the reservoir sections have a top thin layer of favorable reservoir properties with permeabilities in excess of 10 mD due to diagenetic leaching.