Abstract

The placement of polymer gels in water producing reservoir layers is one ofthe chemical options to reduce or totally shutoff water production. A correctdetermination of the optimum placement depth of the polymer gel treatmentrequires understanding of gel strength behavior when it is subjected to apressure difference during production.

This paper reports a study on the behavior of an organically crosslinkedpolymer system in porous media. The work includes experiments and amathematical model. The experiments were performed using natural sandstonecores at temperatures ranging from 80°C to 120°C. The gel strength wasdetermined under different conditions (gel composition, permeability, temperature and treatment volume). Also the behavior of the gel before andafter gel failure was studied.

The data was successfully interpreted considering three main effects:

  • gel compressibility,

  • micro-flow through the porous gel and porous mediumand

  • post-failure macro-flow. The model takes into account the above threeeffects and is solved analytically to yield an expression of the evolution ofthe pressure, which is in excellent agreement with the experiments.

Introduction

Water production can cause technical, economical and environmentalproblems1,2. One of the solutions to reduce the production ofunwanted water is the placement of full blocking gel systems in reservoirlayers that produce the water3. Most polymer gel systems availablein the industry are placed in the near wellbore before crosslinking takesplace. After the injection of the polymer system in the matrix of a targetzone, the components react and form a three dimensional polymer structure. Thisthree dimensional structure is referred to as a gel. This gel can reduce orfully block water flow through the porous medium.

When a certain full-blocking gel system is present in a target zone duringproduction, it will be subjected to a drawdown pressure. The main technicalquestion will then be whether the gel in the porous medium is strong enough towithstand this drawdown pressure. If the gel can not hold the pressuredifference it can fail to block the flow of water efficiently.

To guarantee a full blocking shut-off, the gel system will have to be placeddeep enough in the target zone. The current recommendation of treatment volumesis base on experience and rules of thumb. There are very few solutions for thecalculations of the required placement depth under different conditions thatare based on technical justification. A scientific based calculation methodwill require the understanding of the mechanism that determines the in situstrength of a specific gel system4,5. The first step to obtain moreunderstanding is the determination of the key parameters.

In this paper, the authors would like to present the results of a study onthe parameters that determine the in situ strength of a recently developedorganically crosslinked polymer (OCP) gel system6. During the studya mathematical model was developed. Also experiments were carried out in whichsandstone cores were treated. The temperature, permeability, crosslinkerconcentration and treatment volume were varied. The results can be used todevelop calculation methods for treatment volume recommendations of actualwater shut-off designs.

Gel Strength Model
Physical description.

A core treated with a polymer solution is considered. After curing, threephases are present in the pores. These are residual oil, connate water and gel. It is assumed that the oil phase is immobile and this is expressed in thereduced porosity fr wherefr=f(1-Sor).

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