Reliable Characterization and Modeling of the Capillary Transition Zone and Flow dynamics in the Oil Rim Reservoirs

Abstract

The oil rim reservoirs are referred to the reservoirs with limited oil column underlain with aquifer and overlain by a gas cap. In such reservoirs, the whole oil column can be in the capillary transition zone with a complex flow dynamics. In some cases, oil might have been displaced by imbibition process over geological time during the fluid migration history following the initial primary drainage. Understanding the capillary transition zone, displaced oil zone, saturation modeling, mobile oil distribution and flow dynamics can influence the resource estimation, field development planning and ultimate recovery. In this paper, we present an extensive review on the concept of the transition zone characterization and the current limitations and challenges in modeling static and dynamic properties in oil rim reservoirs. Various actual field data and examples are presented to better understand the flow characteristics in capillary transition zone. To perform the study, we utilize sector modeling from real oil rim fields with displaced oil zone, the actual drainage and imbibition SCAL measurements, and the historical field performance data. Modeling approaches based on sole drainage and sole imbibition for initialization, imbibition and hysteresis impact on the flow dynamics and residual oil saturation (Sor) dependent to the initial oil saturation (Soi) have been investigated. The study show that the sole drainage based approaches could underestimate the potential of the transition zone, which can make the project less commercially attractive. This method can lead to significant errors in residual oil saturation and inevitably unrealistic history match assumptions. The resulted model could be very mis-leading in the production forecasting. It is shown that proper description of representative relative permeability and capillary pressure data and residual/initial oil saturation dependency will lead to a more realistic history matched model. This could play a major role in the field performance predictions including the value of the tertiary oil recovery, in particular, for heterogeneous reservoirs.

Continuous Incremental Reserves in an Oil Rim Marginal Field Development through Integrated Technical Initiatives and Technology Advancements

“…Evidences from different fields presented in our previous work [1] showing the actual residual oil saturation could be far less than the reported Sor from the SCAL tests at the maximum oil saturation. Residual oil saturation significantly affects the mobile oil saturation and the amount of oil recovery.…”

“…The field background, capillary transition zone characterization and modeling concepts for the studied field were detailed in earlier publications [1,2]. Initial contractor for the field development concluded that the field was marginal due to low recovery factor (RF) based on the complexity of the flow dynamics, initial interpretation of appraisal wells and the technical limitations of the early 70s.…”

Challenges and Rewards in the Resource Evaluation and Flow Modeling of Transition Zone

“…Differentiation between capillary and apparent transition zones and reliable understanding of the volumetric assessment based on determination of their boundaries play a major role in project economy and the success of field development [1,2,3] . The failure of correct identification of CTZ leads to by-passing oil-rich footage of a producible zone or on the contrary, early breakthrough of water or miss-guided infill wells that could pave the way to economic failure of the project.…”

“…The capillary transition zone is controlled by the balance of the capillary and buoyancy forces during the primary drainage process of oil migrating into an initially water filled reservoir trap. The height of the transition zone and its saturation distribution are determined by the range and distribution of pore sizes within the rock, as well as the interfacial force and density differences between the two immiscible fluids [1,2] .…”

On the Concept and Challenges of Water Saturation Determination and Modeling in Carbonate Reservoirs

“…It should be mentioned that the challenges on the Sw characterization in capillary transition zone is elaborated in detail elsewhere [12]. The original Sw log tends to predict more gas saturation with very negligible amount of irreducible water saturation and hence believed to be too optimistic.…”