Development of a Thermogel for the Treatment of Fractured Reservoir

Abstract

Summary Many fields worldwide suffer of excessive water production and poor sweep efficiency because of conformance problems due to the existence of reservoir heterogeneities and preferential pathways for water flow. When heterogeneity contrasts are high, a permeability barrier has to be placed in water thief zones. In this paper, a new Conformance technology using thermo-associative polymers (TAPs) is presented. TAPs are polymers containing temperature-sensitive chemical moieties, called “LCST moieties” (“LCST” for “Lower Critical Solubility Temperature”). These moieties are totally hydrophilic at low temperatures and becomes hydrophobic above a threshold temperature. They can thus interact with each other, forming a three-dimensional network through hydrophobic associations. This process is thermo-reversible. By adjusting the amount of LCST moieties and the molecular weight of the polymer, strong viscosity contrasts may be obtained between low and high temperature (up to several decades). This strategy was used to develop a new family of conformance products called “thermogel” where a transition from a low viscous solution to a strong gel may be obtained upon heating. The paper presents the work performed to develop a Thermogel for a North Sea fractured chalk application. Since chalk matrix has a very low permeability (K∼1.5mD), strategy of production is to fracture and acid stimulate the formation along the well with water injection support. In this specific field case, one isolated fracture connects injection well to production well inducing a short cut between the wells. The aim of the pilot is to inject a Thermogel treatment to reduce water intake from the fracture. The paper describes the development of the product through laboratory experiments. The evaluation of the properties of different thermogels in bulk rheology and in coreflooding experiments are discussed in view of the pilot application. The main results can be listed as follows: – The new thermogel has a threshold temperature of around 40°C, that enables gelation far from the injection well, deep in the reservoir. – Coreflooding experiments in carbonate pack that mimic fracture permeability (K∼250D) have been performed. A minimum Thermogel concentration of 60 000ppm is necessary to form a gel with high blocking efficiency. – Injectivity in coreflooding experiments depends on Thermogel chemistry and on elasticity properties. – Strong face plugging when Thermogels are injected in pure matrix conditions (K=2mD) ensures that the product cannot propagate inside the matrix and will flow in fracture only. From those laboratory results, two products have been qualified for further field applications.