Influence of Initial Saturation and Organic Matter Distribution on Petrophysical Changes Induced by Stimulation Fluid Reaction with Baxter Shale

Abstract:

The analysis of rock matrix alteration upon hydraulically fracturing shale reservoirs has been of special interest over the last decade. Although it is the most cost-effective technique to produce hydrocarbons, fracture stimulation in shale can change petrophysical properties that are often detrimental to long-term hydrocarbon recovery. Most laboratory experiments proposed to emulate these reactive changes do not consider the presence of formation water in the pores. The native fluid is always present in the rock, so it is important to find out how this fluid interferes with components of the stimulation fluid diffusivity. Additional complexity arises from the distribution of organic matter in the rock. To evaluate potential combined effects, two reactive experiments were performed. The first experiment was conducted at reservoir conditions (125^oC, 45 MPa) and the second at atmospheric conditions (25^oC, 0.1 MPa). In both experiments, samples of Baxter Shale were initially saturated and aged with formation water for 30 days prior to addition of stimulation fluid (pH=1.8, I= 85 m); experiments continued for 30 days after injection. Petrophysical changes were investigated using Time-Domain NMR and gas adsorption isotherm analysis. The effect of the organic matter was analyzed after its removal. The reaction of rock components with the stimulation fluid is constrained by the presence of formation water in the pore space prior to injecting stimulation fluid. Poor diffusive mixing between the two aqueous phases limits changes of macropores; organic matter acts as a blocking agent, compromising accessibility to micropores. Saturation of the pores with formation water and the presence of organic matter significantly reduce alteration in the micro- and mesopores.