Load Recovery in Hydraulic Fracturing: Insights into Fracture Surface Area

Abstract

During hydraulic fracturing operations, large quantities of water are injected into formations to create new fractures and to stimulate existing fracture networks. Typically, only 10-40% of the injected water flows back during clean-up. The fate of the remaining water in the reservoir is uncertain. Recent experimental and numerical studies suggest that a significant amount of water can imbibe into the rock matrix. Water imbibition is an important factor with both financial and environmental implications, dictating how much of the hydraulic fracturing fluid returns to the well bore. Imbibition is influenced by the suction potential (capillary and osmotic pressures), permeability, and the wettability of the rock. Understanding the physics behind water imbibition in unconventional reservoirs can provide valuable information on fracture surface area.

In this work, we discuss the magnitude of water suction potential in unconventional reservoirs. The validity of mercury injection tests to measure capillary pressure is questioned. We have tested the application of a dew-point psychrometer to measure brine-air suction potential, a method that is largely unknown to the petroleum industry. The results of this application indicate that the suction potential in low-permeability rocks is much higher than previously thought. Suction potential for the Montney siltstone in British Columbia is estimated to be as high as 100 MPa (14,500 psi) or higher at initial reservoir water saturation.

We conducted spontaneous imbibition experiments on several samples from the Montney formation. A new methodology is used to normalize the effects of sample size and porosity on imbibition volumes. The results indicate that permeability is a dominant factor in determining the rate of imbibition. All the samples soak up significant amounts of water, ranging between 30 and 70 percent of pore volume. Based on our estimates of water suction potential, a numerical model is successfully used to simulate spontaneous imbibition behavior of samples under lab conditions. The model can also be used to predict the permeability of the samples by matching the imbibition results. The results of this work provide insights into the low load recovery in unconventional reservoirs. Additionally, successful modelling presented in this work can potentially be used to estimate fracture surface area resulting from hydraulic fracturing.

AAPG Datapages/Search and Discovery Article #90350 © 2019 AAPG Annual Convention and Exhibition, San Antonio, Texas, May 19-22, 2019