A study is presented on the effectiveness of different combinations of fluid and fluid-loss additives to control fluid loss in high-permeability formations under high shear rates. The impact on matrix damage and proppant-pack damage is also studied. Borate-crosslinked guars, hydroxyethylcellulose (HEC) and a surfactant water-base gravel packing fluid were investigated. The fluid-loss additive considered was silica flour. All fluid-loss tests were run in dynamic fluid-loss cells. To properly test high-permeability cores, new long core dynamic fluid-loss cells were used. The matrix damage caused by the invasion of the fluid was determined using pressure taps along the core. Conductivity tests were also run to determine the damage to the proppant pack.
Results show that the effectiveness of particulate fluid-loss additives under dynamic conditions is strongly dependent on the initial leakoff rate, which depends on the pressure gradient across the core, permeability of the core and viscosity of the invading fluid. The use of silica flour helps matrix flowback and it has a minimal effect on proppant-pack conductivity in clean fluids (e.g., surfactant water-base gravel packing fluid). with the exception of the borate-crosslinked guar with no fluid-loss additive, the variety of fluids used in these tests (with and without silica flour) have a negligible effect on postproduction.
During hydraulic fracturing of high-permeability formations, a large fraction of the fluid pumped to propagate the fracture permeates into the rock matrix through the fracture wall during the early leakoff times. This volume of fluid, called spurt, is not used to create fracture volume and is lost into the matrix lowering the fluid efficiency and potentially damaging production afterward. Fracturing treatments of high-permeability formations tend to be short jobs and require high pumping rates due to the low fluid efficiencies. Therefore, it is important to be able to adequately control spurt loss in these types of formations to reduce pumping costs and improve production.
Spurt loss can be reduced by either using more viscous fluids (such as crosslinked guars instead of hydroxyethylcellulose [HEC]) or by using fluid-loss additives.