Abstract

This paper presents an analysis and design study of field scale inhibitor squeeze treatments. Simulators of varying levels of complexity have been used to model squeeze data from several wells in North Sea fields. The modelling has been carried out both for simple single sandbody squeezes and for inhibitor treatments in heterogeneous systems where there is free crossflow between strata. The appropriately matched results are then used in the design of improved inhibitor squeeze strategies for the fields studied.

The main results from this study relate to the field design criteria which should be applied to both adsorption/desorption and "precipitation" scale inhibitor squeeze treatments. Using quite simple models to simulate the field results, a wide range of sensitivities in adsorption/desorption squeeze treatments has been investigated (e.g. injection concentration, overflush size, shut-in time etc). A more complex model was used to simulate an adsorption squeeze carried out in a multi-layer, near-well formation. Results show the significant effect of reservoir heterogeneity on inhibitor returns and, for such cases, that the placement strategy for inhibitor slug injection should be taken into consideration. A field "precipitation" squeeze has also been modelled which highlights the effect of local formation temperature. The results demonstrate the importance of selecting appropriate preflush and overflush volumes for creating the correct thermal conditions for in-situ inhibitor "precipitation".

A number of generally applicable conclusions arise from our study and some example economic results are presented. In particular, for precipitation type processes, some field application guidelines are given for the first time in the literature.

Introduction

Downhole scale inhibitor "squeeze" treatments provide the most common and effective means of preventing formation of oilfield scale deposits. There are two main types of the squeeze process which are currently applied in the field, depending on the inhibitor retention mechanism within the formation, as follows:

  • inhibitor adsorption/desorption squeeze treatments

  • inhibitor "precipitation" or phase separation squeeze treatments.

The precipitation squeeze processes are often accompanied by some degree of additional or enhanced adsorption of inhibitor molecules onto the rock surface.

Inhibitor adsorption squeeze processes have been studied quite extensively in the laboratory using static beaker tests and inhibitor core floods. The case histories of many field squeeze treatments, based on the adsorption/desorption of inhibitor, have also been studied. Based on both laboratory evaluation and field experience, squeeze treatments have then been designed and implemented. In recent years, however, mathematical simulation of inhibitor adsorption/desorption processes in both inhibitor core floods and in field squeeze treatments has been applied in order to understand the fundamental mechanisms governing inhibitor retention in porous media and to improve squeeze treatment design in a more quantitative manner. Using this type of modelling approach, inhibitor properties (e.g. adsorption isotherms and adsorption kinetics) and the squeeze operational parameters (e.g. inhibitor injection concentration, brine overflush volume and well shut-in time) have been systematically studied. As a result, a much clearer view has emerged on how to utilise inhibitor properties and operational parameters to improve adsorption squeeze treatments. In addition, a methodology for using laboratory inhibitor core flood effluent data to derive inhibitor "dynamic" adsorption isotherms has been developed and the use of such data for designing field squeeze applications has been described.

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