Abstract

To combat scaling problems in the course of production, pollution control and water disposals through analyses of formation waters, their chemical activities and scale forming tendencies, are necessary. In this regard, a large number of formation water analyses of different giant oil field fractured carbonate reservoirs in Iranian Onshore Offshore fields, many close to saturated conditions, are obtained, processed in standard forms and graphical presentations. computational study of the acquired data lead to development of a simple empirical correlation capable of predicting scale tendencies of the formation waters in the subject fields Well established models verified the validity of the simple model in different parts of an oil desalting unit under operation The paper & scribes in details how the simple empirical correlation was developed.

Introduction

Very frequently, the results of tests on commercial oil and gas wells show good production potentials. Immediately after being put on stream or at some time during their productive life a decline in production occurs, which could well be due to problems in mineral scale in the form of calcium carbonate, bark sulfate and calcium sulfate, deposits occurring at some point in the producing system and restricting the flow of production. This deposit can occur at any point from the time the produced fluids, which always contain water with dissolved mineral matter, begin to move along minute channels into larger channels and eventually into the wellbore, up the tubing string, and through well head assembly. At any point in this route, the minerals have the opportunity to come out of solution and deposit The deep high pressure wells typical of Iranian oil fields (carbonate reservoirs) are particularly subject to scale formation (CaCO3) because of high CO2 partial pressure due to action of bacteria, abundance of organic sources at high pressure to form soluble calcium bicarbonate.

Scale causes a large number of costly pulling jobs, perforation reamings, redrilling, stimulation and other remedial works periodically. Thus, through understanding of the mechanism of scale formation in Iranian oil fields, identification of the techniques of their prevention and remedial actions, are indispensible.

CAUSES OF SCALE DEPOSITION

The causes of scale and other fouling deposits related to saline waters are many, varied and extremely complex. Numerous investigators have studied scale and deposit mechanism in an effort to understand, quantify and develop remedial or preventative treatments. For example, Spiegler(1)) point out that crystallization from solution of a material directly on site of scale formation requires three simultaneous factors:

  1. supersaturation;

  2. nucleation; and

  3. adequate contact time.

Supersaturation are solutions that contain higher concentrations of dissolved solute than their equilibrium concentration. The causes of supersaturation may be due to: temperature fluctuation, pH alteration, addition of seeding material, commingling of two incompatible waters, pressure reduction, agitation, evaporation, etc. Nucleation or the initial formation of a precipitate can occur for many reasons, e.g. dust particles, vibration, etc. Two basic nucleation mechanisms are recognized:

  • homogeneous nucleation and

  • heterogeneous nucleation which opposite to the homogeneous case requires a pre-existing foreign crystalline substance.

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