Conventional linear gels are very simple to use and can be formulated with a wide array of different polymers and fluids. Common polymer sources used with the linear gels are guar, Hydroxypropyl Guar (HPG), Hydroxyethyl Cellulose (HEC), Carboxymethyl hydroxypropyl guar (CMHPG), and carboxymethyl hydroxyethyl cellulose (CMHEC).Previous studies with these fluids have indicated that gel residue from guar fluids can be as high as 8% to 10% by weight. The high residue content of guar gels can cause permeability reduction in the proppant pack of the fracture, if further cleanup measures are not applied.
Similar problems have been observed with linear HPG and CMHPG, though the resultant damage is not as extreme with this type of fluid system. In both HPG and CMHPG fluids, the residue content can be from 1% to 3% by weight. HEC fluid sytems are virtually residue free and provide the best proppant-pack permeability.
The general characteristics of linear gels are poor proppant transport and low fluid viscosity. In lower-permeability formations (less than 0.1 md), linear gels control fluid loss very well, whereas in higher-permeability formations fluid loss can be excessive. Linear gels tend to form thick filter cakes on the face of lower-permeability formations, which can adversely affect fracture conductivity. The performance of linear gels in higher-permeability formations is just the opposite, since it forms no filter cake on the formation wall. Much higher volumes of fluid are lost due to viscous invasion of the gel into the formation. Fracture conductivity can be much higher when linear gels such as HEC are used.New biopolymer gel systems have been recently added to the selection of gravel pack fluids. These fluids feature clean, controllable breaks that result in excellent regained permeability and fracture conductivity.