Horizontal Wells Rejuvenate the Mature Yibal Field
Authors: Al-Shidhani S. , Al-Bimani A. , van der Bok J. , Al-Saadi K. , Schoepfer P. , Verwey P. 
Published: December 2007

Abstract
The Yibal Shuaiba field, Oman, has been developed by pattern water flood and infill drilling. The field has produced 185 million m3 (30% of STOIIP) with 1996 average production at 34,000 m3/d, 60% from horizontal wells. This domal structure carbonate reservoir is characterized by complex faulting systems and limited lateral fluid movements. This and the significant reservoir offtake have developed complex reservoir water movements which has a major impact on future development opportunities in addition to the surface congestion and risks associated with the overburden formations. Horizontal wells with 3D seismic assistance have offered, compared to vertical wells, more effective means for addressing such opportunities and managing the field. The experience build-up coupled with active technological trials have resulted in:–Improved productivity (PIF of 3) through effective stimulations, zonal isolation and increased drainage area,–Improved recovery through the ability to look for and target un-swept oil, delayed coning and more effective high density infill development.–Optimised water injection through better distribution.–Improved economics through cheaper (sidetracking) and less wells and higher production rates.–Enhanced safety and environmental protection through less overburden penetration and limited surface locations.
The above has resulted in a complete turn to horizontal producing wells coupled with a change in the development approach to target remaining oil and provide the injection where needed rather than following the equally-spaced pattern infilling. The Horizontal well development is a building block to the previous vertical pattern development with limited expansion. The equally-spaced vertical pattern development have been utilised to achieve lower well costs through sidetracking of watered-out wells and also used as a guide to target the un-swept areas between the vertical producers. Similarly, horizontal wells will form the basis for future developments through lateral sidetracks from existing horizontal holes.
This paper highlights the benefits of horizontal wells in a mature field development and the potential benefits of further advancements.

Introduction
The Yibal field is situated in the North of Oman, 300 km southwest of Muscat, on stream in 1969. The main producing reservoir is the Shuaiba carbonate with a STOIIP of 600 million m3. The reservoir, the Upper Shuaiba formation, is composed of Lower Cretaceous carbonates, deposited on a shallow marine shelf. It forms a low relief domal structure with an areal extent of some 70 km2 at ca. 1400 m beneath the surface. This structure is transected by mainly NE-SW and NW-SE faults and fractures which are the result of deep salt movements and the collision of the Arabian and Eurasian continental plates. A significant amount of these faults and fractures are open and conductive to water flow. This created the challenge to successfully complete horizontal wells penetrating such features. The reservoir permeability is low (1 – 100 mD) with low viscosity (0.6 cp) and light (40 API) oil.
The field was initially produced under depletion drive. In 1972, a five-spot lateral water flooding (injecting into the oil column) pattern development was started. In the early 1980's, simulation studies led to a shift to bottom water flooding (injecting below the 50% water saturation level). Further field development was based on infill drilling (Fig. 1) supplemented by pattern bottom water injection to increase recovery. Inverted nine-spot (500 m well spacing) infilling was started in the 1980s, followed by quarter pattern infill wells (between the corner producers and the centre pattern injectors).
P. 223