Abstract

A gel system based on the reaction of sulfomethylated resorcinol and formaldehyde has been developed. This gel system, termed SMRF, is tolerant of a wide range of salinities and divalent ion content. It has been tested in laboratory core floods conducted at 41 C(105.8 F) using Berea sandstone cores, Baker dolomite cores and reservoir rock from the Pen Field which produces from the Lansing-Kansas City formation. In these core tests, the gelling system was injected into short cores and allowed to gel in situ. Permeability to brine after in situ gelation was determined for each core as well as permeability after several pore volumes of brine injection. The SMRF gel system reduced the permeabilities of Berea and Baker dolomite cores to values on the order of 8-18 microdarcies. The Lansing-Kansas City reservoir rock was plugged by the gel system.

The new gel system has a viscosity close to water when the reactants are mixed and is easily injected into porous rock. SMRF forms strong and stable gels at temperatures between 25 C and 52 C(126 F) and in the pH range between 5 and 8.0. Gel times are a function of pH, salinity, concentration of the chemical constituents and temperature, and can be controlled by appropriate adjustment of these parameters. Laboratory data describing the characteristics of the SMRF system in beaker tests are presented. Also included are results of extensive core testing.

The SMRF gel system is one of the few gel systems that will form a gel in situ in the presence of the strong buffering that occurs when brine is injected into carbonate rock. The SMRF system has potential for near wellbore treatments of carbonate matrix rock such as found in the Lansing-Kansas City formations of central Kansas as well as the low permeability carbonates found in the San Andres formations in West Texas.

Introduction

The placement of gelant in the targeted zones of a reservoir during permeability reduction treatments is vital to the success of the treatment. Viscosity of the injected fluid plays a significant role in determining where that fluid will travel in the reservoir. In most situations, a less viscous solution will penetrate low permeability zones to a lesser extent than a more viscous solution. Thus, gelants with low, or water-like, viscosity are preferred for permeability reduction treatments. In addition, injectivities are greater for less viscous gelants which enables larger volumes of gelant to be injected.

Most gelants used today contain polymers which increase the viscosity of the aqueous solvent by factors on the order of 5 to 30. Phenolic-aldehyde systems are a class of gels in which the initial viscosity of the gelant (before gelation occurs) is slightly above that of the aqueous solvent. This feature makes phenolic-aldehyde systems attractive for use in permeability-reduction treatments due to the enhanced placement potential. Phenolic-aldehyde systems are composed of water-soluble phenols and aldehydes or dialdehydes. A gel is formed through a step-reaction polymerization. Several studies on the performance of phenolic systems have been reported.

Two phenolic-aldehyde systems were studied. One was composed of resorcinol and formaldehyde (designated the RF system) and the other was composed of sulfomethylatedresorcinol and formaldehyde (designated the SMRF system). Bottle testing to characterize the RF system revealed the gelation was sensitive to pH, salinity and hardness. At conditions where gelation was less sensitive to these parameters, the gel times were relatively short. The RF system was modified to improve the tolerance to chemical composition by sulfomethylation of the resorcinol to produce the SMRF system.

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