Abstract

The lateral reach of earlier symmetric fork-type dual-lateral wells were limited by torque and drag. Attic zones are also left between wells leading to a potential 7% reduction in ultimate recovery. After an in-house torque and drag modelling of several well profiles, the assymetric fork multilateral well profile was seen to be capable of eliminating the shortcomings.

Six of these assymmetric-fork multilateral wells have since been drilled: 2 of them to world record total horizontal lengths! The new design will facilitate some 25% reduction in field development costs, 17% reduction in torque and drag, as well as 42% reduction in environmental footprint.

Introduction

Production from the Shuaiba reservoir commenced in 1984 with vertical wells. The first successful horizontal producer, a sidetrack, was completed in 1991. Up to August 1997 some 40 horizontal wells including 11 multilaterals have been drilled.

Initial well rates from long horizontals were up to 750 m3/d. However, severe pressure depletion was observed in the areas of the high productivity horizontal producers.

Consequently, a line-drive waterflooding was started in 1995: produced water is injected through horizontal wells placed below the OWC to provide pressure support. The oil is produced through horizontal wells drilled along the top of the low relief structure. Earlier designs consist of single horizontal producers and dual lateral water injectors, drilled in symmetrical, 'tuning fork shape' with injector-producer lateral spacing of 125m (i.e., injector-injector spacing of 250m). After drilling 5 symmetrical fork dual lateral injectors, it was established that drags in the symmetrical-fork wells were constraining drilling progress and lateral reach. Other well profiles were then analysed for torque and drag and the asymmetric-fork profile selected. Six asymmetric-fork wells (3 dual- and 3 triple-laterals) have since been drilled, two of them to world record total horizontal lengths.

Geology

The overburden consists of Tertiary and Cretaceous deposits. Some 80 m of Nahr Umr shale are the top seal for the Early Aptian Shuaiba limestone reservoir.

The Shuaiba structure is a low-relief dome, truncated on its southern side by a WNW-ESE trending fault. With the exception of this boundary fault, no faults with a vertical throw of more than 3 m have so far been detected.

The dominant lithology in the Shuaiba formation is porous lime wackestone typically with 25% porosity and 2 mD permeability. The best permeability and porosity are found in a reasonably thick layer at the top.

Core data suggests that vertical permeability in the reservoir is good, with kv being nearly equal to kh. This has been confirmed from well performance modeling and RFT pressure data. The absence of barriers to the vertical movement of fluid is of particular importance for the recovery process. Barefoot completion: The Shuaiba structural homogeneity and the absence of water fingers dictate that the wells be completed barefoot.

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