Hydraulic fracturing treatment of unconventional reservoirs requires large quantities of fluids. These fluids often damage the rock. Fluid not recovered during flowback can cause loss of relative permeability to hydrocarbons in the rock. It disrupts the chemical equilibrium of the rock/fluid system leading to a physically and chemically altered zone of rock adjacent to the fracture face. Loss of rock strength due to fluid invasion can lead to increased proppant embedment, a reduction in fracture conductivity and fines generation. It is important to understand the rate of combined fluid leak-off and imbibition, the amount of fluid invading the rock matrix, the depth of the altered zone, and the loss of rock strength in this zone in order to achieve optimal production.
A laboratory method to quantitatively measure combined fluid leak-off and imbibition into ultra-low permeability rocks, such as shales, under reservoir conditions was developed. Dissecting the plug with a mechanical scratch testing machine and precise determination of the water content as a function of plug-depth results in water saturation and rock strength profiles throughout the plug. This enables one to correlate loss of rock strength with induced water saturation and thus quantify damage associated with fracturing fluids.
Experimental results on core plugs from several major shale plays show that fluid loss into ultralow permeability rock can be substantial, but is also highly variable and dependent on the texture and composition of the rock. The rock strength decreases with increasing exposure time to the fracturing fluid and correlates with the invading water saturation. This suggests that fluid leak-off/imbibition, rock weakening and fracture conductivity are strongly linked. Therefore, knowing the variation of leak-off and imbibition into the different rock types encountered by the fluid enables operators to predict where the treatment fluid goes, how it is distributed in the rock matrix behind the fracture face, and ultimately how best to mitigate the effects by choosing the fluid and procedure that causes the least damage to the rock and ultimately fracture conductivity.