Abstract

The X Field steam-injection project in Sultanate of Oman is the world's first full-field steam-injection project based on thermally assisted gas/oil gravity drainage (TAGOGD) in a fractured carbonate field. The project scope includes drilling some wells and installing facilities to treat water and generate around 18,000 tonnes per day of steam, the plant targeted to be started up in around year 2010. Additional facilities will be built to process the incremental oil and gas produced at the field as well as disposing of excess produced water in deep reservoirs. The X field, which was discovered around year 1970 contains a moderately viscous crude in rock that is rather impervious to its flow. The EOR recovery process being applied - TAGOGD - is based on injecting steam into the formation's fractures to heat the low-permeability oil-bearing rock. As the rock is heated, gas is liberated and the viscosity of the oil is reduced, flowing much more easily into the fractures under the action of gravity. This feature of the project allows the number of wells, and hence development costs, to be kept to a minimum. The paper explains the selection of green chemicals as an opportunity for water treatment facilities of Steam EOR process. Green chemicals are a class of compounds that are biodegradable and less toxic and the selection started with facilities expectation. For most water treatment and boiler applications, the majority of the chemical compounds used tend to be inorganic and non-biodegradable. This is primarily because facilities needs a strongly alkaline source to prevent corrosion. As a result, most of the chemicals have a pH of 8.5 or above depending upon inhibition. In order to applied green chemicals, we need to develop a matrix and inspect industrial applicable green chemicals based on MSDS, technical expectation and OPEX. Toxicity and biodegradability are listed on the sheets. One final thing is the attempt to be green is more of an attitude and application of existing technology.

Introduction

X Field is located in central Sultanate of Oman south of the western Hajar Mountains. This large oil accumulation is trapped in shallow Cretaceous limestone units at a depth of around 200–400m subsea. The anti-clinal structure is a result of a deep salt diaper, with significant crestal faulting and fracturing. The field was discovered in around 1970 and contained 16° API oil with a viscosity of 220cP has been produced from the 29% porosity, low permeability (5–14mD) limestone. During the primary production the first year showed a large peak in oil mainly from emptying of the fracture network with a minor contribution from fluid expansion due to pressure reduction. At the end of the first year, production had declined to a very low sustainable rate interpreted to be from gravity drainage, from a combination of gas-oil (GOGD) from the secondary gas cap and oil-water (OWGD) below the fracture gas-oil contact (FGOC). The reservoir then consists of a matrix with very little drainage and a fracture network with a thin oil rim below the secondary gas cap and above the fracture oil-water contact (FOWC), Figure 1.

Primary production performance such as that of X Field is only expected to recover some 3–5% of the oil in place over any reasonable time frame due to low matrix permeability and high oil viscosity on gravity drainage rates. Recoveries via steam were discounted as development options due to the pervasive fracturing observed in the field which would encourage the flooding agents to completely bypass the matrix.

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