Fahud Field Review: A Switch From Water to Gas Injection
Authors: O&apos;Neill Niel 
Published: December 2007

The water injection schemes implemented in the Fahud field during the early 1970's led to poor recoveries because the reservoirs were both fractured and oil-wet. On the basis of the results of a thorough performance review, it was decided in 1983 to promote gas/oil gravity performance review, it was decided in 1983 to promote gas/oil gravity drainage fully by drilling rows of downdip producers and switching completely from water to gas injection. This paper investigates the reasons behind each stage of development and reviews recent efforts to evaluate the future production potential through the use of dual-porosity simulators.

Introduction

The Fahud field (Fig. 1), which has a stock-tank oil initially in place (STOIIP) of 1000 × 10(6) stock-tank m3 [6,300 × 10(6) STB], is an elongated monocline with an eroded fault escarpment. The field is divided by a permeability barrier into two accumulations, Fahud North-West (NW) and South-East (SE), each of which contains three fractured Natih limestone reservoirs. The three Fahud NW reservoirs (NW-A, NW-C/D, and NW-E/F/G) are separated by continuous shales but are in communication with each other across the eroded fault escarpment (Figs. 2 and 3). The same applies to the three Fahud SE reservoirs (SE-A, SE-C/D, and SE-E). The oil has a density of 870 kg/m3 [31 deg.API] with initial reservoir fluid properties as presented in Table 1. The field was discovered in 196 properties as presented in Table 1. The field was discovered in 196 and started production in 1967. To date, 12 % of the field's STOIIP has been produced. This paper reviews the field's performance during four distinct phases of development: initial, waterflood, reassessment, and present development.

Initial Development 1967–71. The field was initially produced under natural depletion supplemented in 1968 by gas injection. The high offtake rates led to a rapid displacement of the gas/oil contact (GOC), which resulted in the early gassing out of a number of relatively downdip completions. Simulation studies and field trials were conducted, which concluded that better recoveries would be obtained through waterflooding.

Waterflood Period-1972–80. Following the recommendations, water injection schemes were implemented in four of the six Fahud reservoirs. However, they failed to arrest either the pressure or oil production decline.

Reassessment Period-1981–83. The poor performance of the waterflood led to a thorough review during which a number of thermal decay time (TDT) logs and tracer tests were conducted. It was concluded that because the reservoir rock was both fractured and oil-wet, recovery factors from waterflooding were low and could be substantially improved by reverting to full-scale gas/oil gravity drainage.

Present Development-From 1984. The present policy is to promote gas/oil gravity drainage fully. This ongoing development promote gas/oil gravity drainage fully. This ongoing development requires the phasing out of water injection, the completion of downdip grids of producers, and the full replacement of voidage by gas injection. Dual-porosity simulation studies are currently being conducted to optimize the remaining development and to predict the future field performance.