Simulating yield stress variation along hydraulic fracture face enhances polymer cleanup modeling in tight gas reservoirs

Abstract

Several measures have been taken to enhance the supply of fossil fuel energy in the United States during the last decades, some of which include the development and exploitation of very low permeability and low porosity reservoirs in challenging environments. These reservoirs usually require enhanced stimulation techniques such as multi-stage hydraulic fracturing and horizontal drilling to increase the contact between the wellbore and the producing formation for a profitable recovery. During the fracturing process, fluids are pumped into the reservoir under high pressure, to create fractures through which gas flows back to the earth's surface during production. However, a layer of concentrated polymer forms on the fracture faces limiting the loss of fluid to the formation during injection while impairing fracture conductivity during production. The inadequacy of the fracture conductivity after a fracture treatment using cross-linked fluids is typically due to poor degradability of our polymers, proppant crushing, clay swelling in the case of incompatible fluids and formation damage. The objective of this work is to develop a fracture cleanup model that simulates the rheological variations of filter cake and degraded fluids inside the fracture by including the effect of breaker and polymer concentration on the yield stress of the fracturing fluid that result in variations in capillary pressure, fracture conductivity, fracture length and formation damage during the cleanup process in unconventional tight gas formations.A dynamic 2-D, three-phase IMPES simulator, incorporating a yield-power-law-rheology (Herschel-Buckley fluids), has been developed in MATLAB to simulate variations in rheological properties of degrading fracturing fluids versus time and investigate the influence of several major parameters on the fracture cleanup process. These parameters include variations in polymer concentration along the fracture length, breaker concentration variations, fracture conductivity, fracture length, capillary pressure and formation damage with a novel correlation between yield stress and breaker concentration, which enhances post – fracture well performance prediction in tight gas reservoirs. The three phases simulated here include water, gas and the gel phases.Simulation of the injection of fracturing fluids and fluid imbibition during the shut-in time indicated that for tight gas formations, fluid recovery increases with increasing shut-in time, increasing fracture conductivity and fracture length irrespective of the yield stress of the fracturing fluid. Simulation of the production phase highlighted that increasing the capillary pressure to a maximum of 350.10 psi resulted in a 10.4% decrease in cumulative gas production. The rate of increase in the yield stress of the fracturing fluid along the fracture face is proportional to the square of the volume of fluid loss to the formation. Production will be enhanced significantly with increasing breaker concentration indicating that simulation of the yield stress variation along the fracture face presents a more realistic scenario of the fracture cleanup process rather than assuming a constant value since the fluid loss to the formation and the polymer concentration distribution decrease with fracture length.