Abstract

This paper describes the enhanced oil recovery (EOR) studies conducted for the low permeability Kharaib B carbonate reservoir of the Al Shaheen field, located offshore Qatar. The reservoir is characterised by large lateral variation in oil properties (Lindeloff et al., 2008), lateral uniformity in geological and petrophysical properties over large distances, a relatively thin oil column in a transition zone, and the presence of local gas caps. The Kharaib B reservoir is currently undergoing water-flooding, which is reaching maturity in parts of the field.

The EOR screening process will be described together with the performed experimental programs and the results of the calculations performed to reduce the uncertainty in assessment of the efficiency of various recovery processes. Four EOR processes were evaluated in the screening study: gas injection (hydrocarbon gas and CO2), alkaline-surfactant-polymer flooding, microbial EOR, and air injection.

In the process of qualifying the EOR processes, a 6-month gas injection trial was performed to determine whether the degree of reservoir heterogeneity was as predicted. The results from the gas injection trial showed no premature breakthrough of gas. Furthermore, the modelling studies and the enhanced oil recovery potential for the different studied processes are presented with results showing that the microbial EOR process is currently not considered an applicable EOR process, whereas the remaining EOR processes could be beneficial from a subsurface point of view with gas injection showing greatest potential. Evaluations of the feasibility of implementing each of the EOR processes in the offshore environment are discussed.

Introduction

The Al Shaheen field is one of the giant oil fields of the Middle East situated in Block 5, offshore Qatar (Figure 1). Maersk Oil has been developing the field with Qatar Petroleum since 1992 with start-up of production in 1994 and implementation of water injection in 1996 (Thomasen et al., 2005). The Kharaib B reservoir is one of the five producing reservoirs in the Al Shaheen field and it has been producing since the production start-up in 1994. Water flooding is mature in parts of the fields where injection water breakthrough has been observed in producing wells. The development philosophy of the Kharaib B reservoir centres on long horizontal wells, the older wells placed in radial well patterns and the newer wells in parallel line drive patterns of alternating water injectors and oil producers. Well placement is designed to avoid fractured regions within graben zones of the reservoir. The successful placement of the wells outside fractured zones is confirmed by the good water flood performance of the injector-producer pairs with high oil recovery and only few short circuits of injection water into producing wells.

The Kharaib B reservoir is characterised by a thin oil column (up to 80 ft), low permeability in the range 1–10 md and lateral continuous layering extending more than 25 km across the field. The geological and petrophysical characteristics of the reservoir correlate well within the field with the carbonate rock in the best producing parts of the formation being oil-wet to mixed-wet whereas the deeper layers and reservoir flanks are mixed-wet to water-wet.

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