Abstract

Guar and its derivatives, hydroxyl propyl Guar (HPG), carboxy methyl propyl Guar (CMPG), are currently the major gelling agents used for fracturing. However, there is the problem of insoluble residues in Guar after gel breaks. Such residues cause formation damage and low conductivity. Conventional cellulose fluid has problems such as difficult preparation, low temperature limit and high formation damage. This paper presents a novel cellulose derivative of low molecular weight, instant dissolution and free from residue. The new cellulose has better hydrophilic and thickening properties through chemical modification of its structure. The strong hydrogen bonding in cellulose is reduced by alkalization and etherification of substituent group. The viscosity of the new cellulose at 1% (w/v) is 505 mPa·s while the viscosity of HPG or CMPG is 280~320 mPa·s. Its molecular weight is 6×105, which is 1/4 Guar. An organometallic crosslinker was also developed for the new cellulose. Fluid was crosslinked at pH 5-7. Its upper temperature limit was 158 °C. The viscosity of fluid remained 75 mPa·s for 120 minutes at 170 S-1 while that of broken fluid decreased to 1.76mPa·s. No residue was found after the gel was broken, and its conductivity improved. Formation damage is less than 13% in the core test. The new fluid was tested in 18 wells in the tight gas and oil reservoir in western China. Results showed that the new fluid was easy to prepare and rapidly dissolved with no "fish eye". The crosslink of the fluid can be delayed. It has excellent properties of proppant suspension. Oil production after stimulation using the new fluid increased 2-5 times compared with wells in similar locations. Gas production in multiple wells of the tight gas reservoir increased to 1.78×105m3/d after stimulation with the new fluid, 3.38 times higher than similar groups of wells using Guar as stimulation.

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