The Time-Dependent Permeability Damage Caused by Fracture Fluid

Abstract

Abstract Hydraulic fracturing causes a large amount of fluid to leak off into unconventional reservoirs and the fluid is not recovered through flowback. The fluids enter the pore structure of the formation, and the presence of the invading phase blocks the in-situ phase. This event reduces permeability, particularly in unconventional reservoirs. Operators shut-in the well for an arbitrary amount of time after fracturing. The time could allow the phase blocks to dissipate, increasing permeability. Conventional core flow tests and relative permeability evaluations using centrifuge drainage tests are unreliable—if not impossible—in these ultralow-permeability unconventional rocks. We present a method to measure the gas permeability on core plugs of mudstones at reservoir conditions, both before and periodically after the plug was exposed to fracture fluid. The change in gas permeability over time allows optimization of the shut-in time. In addition, the combined rate of leakoff and imbibition is measured while the material is exposed to fluid. At the conclusion of the test, the axial water profile is determined by measuring unconfined compressive strength using the mechanical scratch machine and water saturation with Karl Fisher titration. Initially, the samples are saturated with gas and have the highest permeability. As the plug is then exposed to water, water imbibes into the core forming a water block. The water is removed from the face of the plug. Immediately after the core was exposed to water, the permeability drops below the detection limit. In most core tested, the water block dissipates over time increasing the permeability. Experimental results on core plugs from two major shale plays show that imbibition and fluid loss into ultralow-permeability rock can be substantial, but these processes are also highly variable. We developed a measure of the phase block dissipation. The information gives operators a quantitative measure to determine the length of shut-in periods and for the development of other methods to minimize the damage from water imbibition.