Criteria For Fracturing Low-permeability Water-sensitive Hydrocarbon Producing Formations: A Review
Authors: Black H.N. 
Published: March 2010

Many sandstone and limestone formations contain large amounts of water-sensitive clays. These clays may swell and reduce the size of flow channels, causing a decrease in permeability when contacted by untreated water. Fine particles may be released which could migrate and cause plugging of the small pore channels. Water may also soften the formation and cause a reduction in flow capacity of the created propped fracture.

The fracturing water may be treated with certain salts to reduce clay swelling. Organic polymers have also been successfully used in water for this purpose. Foam, alcohol and oil-base fluids are used in severe cases of extremely water-sensitive formation. Examples of cases where these chemicals and base fluids have been successfully used to fracture water-sensitive formations are given.

Selection of the base fluid formation should be based on laboratory tests on formation cores, when available, and field results.

Selection of the amount of fluid-loss additive may be important, even in low-permeability formations. Comparison of fluid loss tests on formation cores both with and without fluid-loss additives reveal the importance of using additives in these formations. Some comparisons are given in the paper.

Selection of the proppant and its required concentration can be determined by fracture flow capacity data and calculated propped fracture geometry. A computer program can be used to calculate the propped geometry.

This paper provides insight to usage of suitable processes. materials and techniques to obtain better stimulation results for fracturing low-permeability, water-sensitive formations.

Introduction

Clays are usually present in most oil and gas-bearing formations and their presence can cause problems in the production of hydrocarbons, particular where stimulation processes are used. The clays most commonly present in producing formations are smectite, illite, a mixed layer (illite-smectite, chloritesmectite), kaolinite and chlorite. These clays exhibit a negative charge on the surfaces and are attracted by the positive charge of the hydrogen ion in water. Smectite has the greatest attraction to water and the highest cation-exchange capacity of these clays. The cation-exchange capacity is an indication of the degree or" clay swelling which occurs in the presence of water. especially untreated fresh water. The cation-exchange capacities of several clays are listed in Table 1(1). It depends on the substitution of magnesium and or ferrous iron for aluminum in the clay structure. This substitution results in a net negative charge on the faces of the clay mineral. The broken bonds on the edges of the clay crystals develop a small net positive charge.

Swelling of clay in water is due to hydration of the cation attached to the clay and to hydrogen bonding. The degree of swelling depends on the cation attached to the clay and the amount of salts dissolved in the water contacting the clay. The hydration number represents the number of water molecules that can be associated with each cation. Table 2(1) lists the hydration numbers of several cations.

Aqueous Treating Fluid Requirements for Fracturing

Most formations have interstitial water that contains appreciable amounts of dissolved salts.