Disproportionate Permeability Reduction by Soft Preformed Microgels

Abstract

Abstract Injecting soft, stable and size-controlled microgels to reduce waterproduction is a new concept, introduced some years ago, we refer to as STARPOLprocess. Microgels specifically designed for water shutoff (WSO) treatments donot contain toxic products and can be produced to be fully self-repulsive. Theyadsorb onto rock pore surface by forming soft monolayers with a thickness equalto their size. This size can be adjusted as desired during the manufacturingprocess. As a consequence, water permeability can be reduced as desired.Microgel chemistry is chosen to be insensitive to pH and salinityvariations. This paper reports laboratory results obtained with a new type of microgelexpected to be available at industrial scale in a near future, under bothpowder and liquid form. The results include their characteristics in solution(size, intrinsic viscosity, mutual interactions and rheology), as well as theirperformances in porous media (model granular packs and Berea sandstones). Thesemicrogels were found to reduce water permeability strongly by forming thickadsorbed layers which are so soft that oil permeability is not affected. Asexpected from their crosslinked structure, their mechanical and thermalstability is excellent: shear rates as high as 1.5×10 4s−1 or one month aging at 150°C do not reduce their viscosity.Another very significant advantage of microgels over gelling systems is thatthey can be quasi-ideally placed without need of zonal isolation when properlydesigned according to reservoir conditions: in this case, they penetrate almostexclusively into the highest permeability layers. Introduction Reducing significantly water production is an increasingly important goalfor the oil industry, for both environmental and economical reasons. Among themethods available to reduce water production, injecting a polymer solutiontogether with a crosslinker has been currently used (1–4). In this process, thegel is formed in-situ. Since gelling properties have been found to depend onmany factors (5–10), the gelling time and thus the depth of the gel penetrationis quite difficult to predict. This difficulty results from the uncertaintiesconcerning different factors: shear stresses both in surface facilities and innear-wellbore area and also physico-chemical environment around the well (pH, salinity and temperature). Moreover, both polymer and/or crosslinker adsorptionin the near-wellbore region and the dilution by dispersion during the placementcan also affect the effectiveness of the treatment. For all these reasons, we introduced in 1999 (11) a new concept, we refer toas STARPOL, which consists in injecting non-toxic soft size-controlledmicrogels into the reservoir. This paper reports experimental results concerning the performances of a newtype of soft microgel expected to be available at industrial scale in a nearfuture. In the first section of this paper, the relations between thecharacteristics of microgels and their performances which have beeninvestigated during the last years (11–15) are presented. In the secondsection, the materials and methods used in this investigation are described.Then the main results are presented and discussed, beginning with thecharacteristics of the microgels in solution (size, intrinsic viscosity, interaction properties and rheology), continuing with their mechanical andthermal stability and finishing with their performances in porous media, moreparticularly their Disproportionate Permeability Reduction (DPR) properties.The results are then analyzed in term of self-placement between the differentlayers of a reservoir in the case where injection is carried out without zonalisolation. Properties and Performances of WSO Microgels The knowledge concerning the relations between microgel characteristics andtheir performances as WSO agents has been significantly improved these lastyears (11–17).