Subsurface Uncertainty Management and Development Optimization in the Harweel Cluster, South Oman

Abstract

Summary In the past 5 years, there has been a string of exploration successes in discovering some 350 million m3 (2 billion bbl) of oil in the Harweel Cluster in south Oman. This oil is deep, under high pressure, and comes with a high gas/oil ratio (GOR), and the fluids contain 15% CO2 and 5% H2S. Of the nine Precambrian carbonate reservoirs, six are lithostatically pressured, and three are hydrostatic. The Harweel Cluster is more than 70 km from existing infrastructure. If gas can be reinjected, miscible displacement is expected in many of the reservoirs. The Harweel Cluster is one of Oman's major opportunities for "green field" oil development. There is a large degree of subsurface uncertainty in gross rock volume and oil/water contact (OWC), reservoir architecture, faults and fractures and reservoir compartmentalization, degree of dolomitization, permeability level and heterogeneity, fluid properties, and so on. However, little is known about the scale and relative importance of each of these uncertainties. Some uncertainties, such as compositional grading, might turn out to be more important than anticipated when evaluating development options because of non-linearities and dependencies. Analyzing the effects of all uncertainty combinations in all reservoirs is not a realistic option. Therefore, a structured scenario-based approach has been developed to analyze and assess all potential factors influencing recovery schemes, reserves, and productivity. After evaluating the range of possible values and calculating the impact of each uncertainty, realizations were created to represent the range of possible values in each new reservoir. Field-scale equation-of-state models were used to simulate both primary depletion and gas reinjection. Development scenarios were optimized for each discrete realization. The resulting reservoir models serve as a basis for the development-concept selection and the subsequent field-development plans.