Experimental and Simulation Approaches on Evaluating an Improved Hydraulic Fracturing Treatment for Stimulating Tight Carbonate Reservoirs

摘要

Hydraulic fracturing has been widely used in stimulating tight carbonate reservoirs to improve oil and gas production.Improving and maintaining the connectivity between the natural and induced microfractures in the far-field and the primary fracture networks are essential to enhance the well production rate since these natural and induced unpropped microfractures tend to close after the release of hydraulic pressure during production.This article provides a conceptual approach of an improved hydraulic fracturing treatment to enhance the well's productivity by minimizing the closure of the microfractures in tight carbonate reservoirs and enlarging the fracture aperture.The proposed improved fracturing treatment was to use the mixture of the delayed acid generating materials along with microproppants in the pad/pre-pad fluids during the engineering process.The proof-of-concept laboratory coreflood experiments were conducted under elevated temperature on a core plug assembly,which was comprised of a one-half core Eagle Ford sample,another half core of hastelloy core plug,and a mixture of delayed acid generating materials along with small-sized proppants sandwiched by two half cores.Simulations using the discrete element method(DEM)and the lattice Boltzmann method(LBM)for three different scenarios-14 cases in total-were conducted to further demonstrate the feasibility of the concept.The delayed acid generating materials used in this strategy not only create voids to enlarge the aperture in flow pathways within microfractures owing to degradation introduced by elevated temperatures and the interaction with the treatment fluids,but also generate more microfractures through the fluid's chemical reaction with the calcite formation.The microproppants were used to support the opening of natural or newly induced microfractures.A surface profilometer was used to quantify the surface etched profile before and after coreflood experiments.Simulations of fluid flow through proppant assembly(inside of the microfractures)were conducted to estimate the permeability of the fractures under three different scenarios:(1)fluid flow in a microfracture without proppant,(2)fluid flow in a microfracture with small-sized proppants,and(3)fluid flow in a microfracture supported by small-sized proppants and generated voids on the fracture walls.The simulation results show that with proppant support(Scenario 2),the microfracture permeability can be increased by tens to hundreds of times in comparison to Scenario 1,depending on the gaps between proppant particles.The permeability of the proppant supported microfracture(Scenario 3)can be further enhanced by the cavities created by the reactions between the generated acid and calcite formation.This work provides experimental evidence that the use of the mixture of the delayed acid generating materials along with microproppants or small-sized proppants in stimulating tight carbonate reservoirs in the pad/pre-pad fluids during the engineering process might be able to effectively improve and sustain permeability of flow pathways from microfractures-either natural or induced.These findings will be beneficial to the development of an improved hydraulic fracturing treatment for stimulating carbonate-rich tight gas reservoirs.