Investigating effects of matrix acidizing and cryogenic treatments on physical and mechanical properties of Marcellus shale formation

The objective of this research is to stimulate near-wellbore and fracture-face zones by implementing two techniques: matrix acidizing and cryogenic treatment. The most common technique to remove or bypass near-wellbore damage is matrix acidizing. This experimental study investigated the effect of matrix acidizing injection pressure on rock properties including porosity, permeability, solubility, reaction rate, and acid penetration rate. To achieve a successful acid treatment, acid type and concentration must be selected carefully. The results of the XRD test revealed that core samples, from Marcellus formation, are mainly composed of calcite (69.8%), which led to selecting hydrochloric acid (HCl). Also, the results of the solubility tests demonstrated that the optimum acid concentration is 15%. Matrix acidizing treatments were implemented on nine core samples from Marcellus (shale gas reservoir) at the reservoir temperature (150˚F) and three different acid injection pressures (250, 500, and 750 psi). The results of porosity, permeability, solubility, reaction rate, and acid penetration rate were used to evaluate the efficacy of the acid treatment and determine the optimum acid injection pressure. The results showed that performing acid treatments on the core samples, containing continuous carbonate layers, created highly permeable channels (wormholes) with a permeability of 2.3 Darcy. The results of the rock properties revealed increasing the acid injection pressure resulted in increases in porosity, permeability, solubility, reaction rate, and acid penetration rate of the core samples. However, any increase in the pressure beyond 500 psi did not demonstrate any significant improvements in the sample rock properties. The results of the XRD analysis revealed that matrix-acidizing treatments dissolved 23.2% of calcite and 0.4% of dolomite existed in the core samples. This is the first research to study the efficiency of matrix acidizing treatments on shale core samples at a confining pressure of 1,500 psi and reservoir temperature (150°F). The other method to stimulate shale reservoirs is to inject cryogenic fluids such as liquid nitrogen (LN2) into the reservoirs to initiate and propagate fractures and to connect existing natural fracture networks. This research aims to investigate the efficacy of the cryogenic treatments on the porosity, permeability, and mechanical properties (Young’s modulus, Poisson’s ratio, brittleness, bulk compressibility, and unconfined compressive strength (UCS)) of Marcellus outcrop and downhole core samples. Ten outcrop and three downhole core samples, taken from 1,700 m depth, were heated up to the reservoir temperature (150˚F). LN2 was then injected into a core holder hosting the core samples. Then, the samples were pressurized at three different pressures of 200, 400, and 600 psi. Computed tomography (CT) scan, porosity, permeability, and ultrasonic velocity tests were conducted on the core samples prior to and after implementing the cryogenic treatments. The results of the CT-scan analyses, porosity, and permeability measurements revealed that implementing the cryogenic treatments resulted in creating new cracks in all the core samples, and increasing the treatment injection pressure increased the length, height, and width of these fractures. The results demonstrated that implementing the cryogenic treatments increased the porosities of the core samples up to 5.05% and significantly increased the permeability from nanodarcies up to 0.131 millidarcies owing to the creation of new cracks inside the core samples. Also, the results of the mechanical properties measurements proved that cryogenic treatments formed new cracks in the samples and increasing the cryogenic treatment pressure resulted in more significant effects on the mechanical properties of the core samples. The results revealed that the cryogenic treatments could be potentially implemented in the shale fields to increase hydrocarbon production rates.