Abstract

To increase the efficiency of cyclic-steam treatment of high-viscosity oil pools we propose to use thermoreversible gel-forming polymer systems with lower critical dissolution temperature (LCDT). Presented are the results of laboratory research of thermoreversible gel-forming systems with LCDT: cellulose ether - aqueous phase in the temperature range of 20 - 250 ??. The studies carried out on kinetics of gelation and rheological properties demonstrated repeatability of gel rheological parameters at cyclic reversal of temperature conditions. The action of additions increasing temperature of gelation was determined to be additive. The temperature of gel conversion into liquid is 30–50 degrees below than that of gelation. The indicated temperatures linearly increase with additive concentrations and for all reagents under study except carbamide these dependencies are cymbate.

Gel-forming systems (GFS) proved high efficiency on heterogeneous reservoir models, where seam permeability differed 3-5 times, under the conditions simulating operation of a cyclic steam well. We can recommend GFS application to regulate filtration flows of reservoir fluids and to increase conformance of cyclic-steam treatment of high-viscosity oil pools. In October 2005 GFS were successfully injected into two cyclic steam wells in high-viscosity oil pool in Lyaokhe oil field (China).

Introduction

Cyclic-steam treatment - a method of cyclic steam injection at 200–350 ??into producing wells followed by oil production - is used toenhanceoilrecoveryfromhigh-viscosityoilpools. The first cycle is the most effective, while the efficiency of the subsequent cycles is significantly reduced [1].

Using thermoreversible gel-formingsystems (GFS) one can increase the efficiency of cyclic-steam treatment. Gel-forming solution is injected into a well. At high temperature caused by steam injection GFS solution is converted into gel, which blocks the most permeable channels and as a result increases conformance of thermal-steam treatment. In the process of oil production reservoir temperature decreases and as a consequence the gel is converted into liquid and clears way to oil recovery. The gels, based on polymers with lower critical dissolution temperature (LCDT), in particular, cellulose ethers, have such unique property [2–4].

Kinetics of gelation and rheological properties of solutions and gels

Previously kinetics of gelation and rheological properties were studied in the system cellulose ether (CE) - aqueous phase at temperature ranging from 20 to 100??, as a result we revealed series of substances increasing temperature of gelation [3–6]. Toelucidateavailability ofthesystemsatcyclic-steamtreatment we have studied kinetics of gelation at temperatures higher 100 ?? and also the dependence of gelation temperature on the concentration of reagents increasingtemperatureofgelationindynamicandstaticmodes.

The studies carried out on kinetics of gelation and rheological properties in the system cellulose ether - aqueous phase at temperature range of 20 - 250 ?? proved that on reaching temperature of gelation (phase transfer) one could observe practically momentary and reversible increase in shear stress and viscosity (from 8 - 30 to 100 - 90000 mPa×s), while at cooling the gel was again converted into liquid. Cyclic reversal of temperature conditions - alternation of heating and cooling processes - testified repeatability of gels rheological parameters.

Rheological studies carried out using a rotary viscometer Haake RheoStress 600 at temperatures ranging from 20 to 150 ?? and at pressures up to 50 atm demonstrated that the addition of carbamide and ammonium thiocyanate to CE solution increased temperature of gelation, at the same time the additions acted additively: at combined introduction of carbamide and ammonium thiocyanate the temperature of gelation was higher 100 ?? (Fig. 1). The gels were stable and retained their rheological properties at high temperatures - up to 150–200 °?. In the range of shear rate 0.5 - 5 s-1 the gel was a solid-like body and mode of rheological dependencies indicated its viscoelastic properties. Therefore one can use GFS to regulate profile of steam injection in cyclic steam wells.

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