Multilateral Wells Improve Development of Heavy Crude Production in the Mukhaizna Field, Sultanate of Oman.
Authors: Al-Azkawi Ahmed , Taylor Gary , Chadwick Robert , McGarian Bruce 
Published: December 2007

The latest development of a cost effective level 4 multilateral junction wasused to complete dual-lateral wells in Mukhaizna and commingle production fromthe Upper Gharif 2 (UG2) and Middle Gharif (MG) reservoirs. The junction hasthe ability to be cemented in place and allows selective access to both legs.This is in line with the functional requirement of a dual-lateral well inMukhaizna that calls for isolating the MGR in case of water breakthrough. Thecemented junction also provides a sand barrier around the milled window of the9.?" production casing. To date one level 4 duallateral well using a cementedjunction was successfully completed in Mukhaizna. Two previous trials havefailed due to operational and design reasons. This paper presents the evolutionof well design in Mukhaizna and the benefits of utilising horizontal wells anddownhole junctions. It also contains a description of the junction system andsome of the problems that were encountered with it and how they were resolvedin subsequent trials.

Introduction

Mukhaizna is the third largest oil field in south Oman, figure 1. The fieldwas discovered in 1975 and was found to contain heavy, viscous oil (14–16° API,1500–2000 mPa.s in-situ viscosity). The UG2 and the deeper MG reservoirs arethe two oil bearing sandstone reservoirs found in the field. They contain 374MMm3 of STOIIP and are separated by laterally extensive Middle Gharif shale(MGS). Technical difficulties posed by the heavy oil, unconsolidated reservoirsand and potential early water breakthrough resulted in high development cost.This precluded early development of the field. The project was turned into anattractive investment opportunity by dedicated field appraisal, drillinghorizontal and dual-lateral wells and the application of new technology.Mukhaizna has been on stream since 2000 and is currently producing 2500 m3/dnet oil from 49 horizontal wells and 1 duallateral well. The well designcomprises of a surface casing to isolate a loss zone and allow the productioncasing to be cemented to surface. Downhole sand exclusion in the 800 m openhole section is provided by slotted liners. The wells are lifted withElectrical Submersible Progressing Cavity Pumps (ES-PCP) that are installeddeep into the well (900 mah) and are tolerant to sand production. Productionprimarily comes from the UG2 reservoir at the crestal part of the field withonly 3 existing MG producers. Developing the MGR was delayed due to itsrelatively small size and concerns regarding early water breakthrough from anunderlying aquifer.

Geological overview

The targeted reservoirs are the UG2 and MG sandstone reservoirs. The top ofthe UG2 is found at 700 mss. These are unconsolidated, laterally continuoussands of the Permian age. Red clay and tight carbonates of the Khuff formationseal the reservoir, see figure 2.

The two reservoirs share a common Oil Water Contact, which has been observedat a depth of 754 mss. The middle gharif development was delayed for tworeasons,Concerns regarding early water breakthrough in the MG reservoir and hencejeopardising further cold development of the UG2.Oil volume in the Middle Gharif (MG) is not large enough to risk UG2development.

Based on this the ability to shut of the MG in case of water breakthroughwas seen as a key functionality of a duallateral well.