Abstract

The introduction of water-base fracturing fluids to water-sensitive formations can be detrimental to well performance. The use of aqueous fluids under these conditions should be avoided. The chemistry and application of an improved continuous-mix gelled oil system for use at reservoir temperatures between 75 to 300 F are described. A brief history of oil gelation and phosphate ester chemistry is presented, followed by a discussion of new complexation chemistry. Laboratory data supporting fluid performance are provided.

Introduction

Reservoirs containing swelling or migratory clays may experience significant damage from contact with water-base fluids. Additionally, fluid recovery is often poor in reservoirs treated with water-base fluids, especially if reservoir pressure is low. Formation damage may result from emulsions, swelling clays, and the dislodgment and migration of fines. Damage to the formation can affect changes in relative permeability, thereby restricting the flow of oil and gas. Two possible explanations for the creation of complete water blocks have been suggested. Complete water blocks may occur when formation permeability is reduced to zero or when drawdown cannot compensate for capillary pressure discontinuity resulting from formation damage. Partial water blocks can be created even in fractures of infinite conductivity.

Gelled oil fracturing fluids are appropriate for treating oil wells and condensate-yielding gas wells whose formations are sensitive to water. Treatments performed with gelled oil fluids provide a highly conductive fracture with minimal damage to the proppant pack and fracture face.

Gasoline, diesel, kerosene, and crude oil gelled with Napalm (an aluminum salt of a fatty acid) comprised the first gelled oil fracturing fluids. Soaps were used to impart viscosity to hydrocarbon fluids by micellar aggregation, followed by the use of aluminum carboxylate salts. In the last twenty 25 years, the use of aluminum carboxylate salts as oil gelling agents has been replaced by aluminum phosphate ester technology.

Phosphate Ester Chemistry

The hydroxyl group of a carboxylic acid can be replaced by other groups (including Cl-, NH2, and OR') to produce acid chlorides, amides, and esters. These products are functional derivatives of acids, characterized by the acyl group (Fig. 1). Phosphoric acid contains three hydroxyl groups; when one or more of these groups are replaced by alkoxy groups, phosphate esters are formed (Fig. 2). Many phosphate ester gelling agents are formed as a reaction product of phosphorus pentoxide and selected alcohols forming the dialkyl ester as the major component, with lesser amounts of monoalkyl and trialkyl esters present. Prior work has demonstrated the use of aluminum compounds such as aluminum chloride, aluminum acetate, and aluminum isopropoxide in crosslinking phosphate esters. Iron compounds were used as crosslinking agents for phosphate esters as early as 1970. More recently, ferric sulfate has been suggested as a crosslinking agent used in conjunction with various alkaline activators; however, aluminum compounds are more widely used at this time. Aluminum-crosslinked phosphate ester polymers can be considered tridimensional polymers (Fig. 3). The R groups represent hydrocarbon chains which must remain soluble in the base oil being gelled. They have a high affinity for refined oils such as kerosene and diesel serving in keeping the polymer in solution. The aluminum ion will attract polar species such as water, acids, or bases. The presence of water in the oil may cause excessive viscosity and will adversely affect the thermal stability of the fluid.

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