Method and Acidizing Tool for Deep Acid Stimulation Using Ultrasound

Abstract

A method of deep acid stimulation for a zone to be treated in an underground formation using an acidizing tool, the method including the steps of introducing the acidizing tool into the well bore, introducing the acid formulation onto the well bore wall at the treatment zone and introducing ultrasound energy into the underground formation at the treatment zone. The subsequent acid penetration depth is deeper than the initial acid penetration depth. A method of stress fracturing a portion of an underground formation includes the steps of introducing the acidizing tool into a well bore and introducing the acid formulation and the ultrasound energy at the focused treatment point. The weakened acidized spots in combination with the stress on the underground formation causes oriented stress-induced fractures to form that are fluidly coupled with the well bore. An acidizing tool includes an acid delivery system and an ultrasonic transmitter.

We claim: 
     
         1 . A method of deep acid stimulation for a zone to be treated in an underground formation using an acidizing tool, the method comprising the steps of:
 introducing the acidizing tool into a well bore, the well bore operable to access the underground formation and defined by a well bore wall, and the acidizing tool operable both to introduce an acid formulation onto the well bore wall and to introduce ultrasound energy into the underground formation;   introducing an acid formulation comprising an acid onto the well bore wall at the treatment zone such that the acid diffuses into the underground formation at the treatment zone to an initial acid penetration depth; and   introducing ultrasound energy into the underground formation at the treatment zone such that the acid diffuses into the underground formation at the treatment zone to a subsequent acid penetration depth, the subsequent acid penetration depth deeper in the underground formation than the initial acid penetration depth.   
     
     
         2 . The method of deep acid stimulation of  claim 1  where the treatment zone of the underground formation comprises carbonate rock. 
     
     
         3 . The method of deep acid stimulation of  claim 1  where the acid formulation is a gel operable to physically adhere to the well bore wall. 
     
     
         4 . The method of deep acid stimulation of  claim 1  where the acid formulation is a foam operable to physically adhere to the well bore wall. 
     
     
         5 . The method of deep acid stimulation of  claim 1  where the acid is a weak acid. 
     
     
         6 . The method of deep acid stimulation of  claim 1  where the acid has a pH value in a range of from about 2 to about 5. 
     
     
         7 . The method of deep acid stimulation of  claim 1  where the ultrasound energy is introduced at a frequency in a range of from about 10 KHz to about 1 MHz. 
     
     
         8 . The method of deep acid stimulation of  claim 1  where the ultrasound energy is introduced with an intensity of sonication in a range of from about 1 and about 10 W/cm 2 . 
     
     
         9 . The method of deep acid stimulation of  claim 1  where the subsequent acid penetration depth is at least 50% deeper than the initial acid penetration depth. 
     
     
         10 . The method of deep acid stimulation of  claim 1  where both the acid formulation and the ultrasound energy are introduced radially from the acidizing tool. 
     
     
         11 . The method of deep acid stimulation of  claim 1  where a significant portion of the acid does not react with the underground formation until the acid is diffused into the underground formation by the introduction of ultrasonic energy. 
     
     
         12 . The method of deep acid stimulation of  claim 1  where the underground formation at the treatment zone has a fracture gradient pressure value and the acid formulation is introduced at a pressure less than the fracture gradient pressure value. 
     
     
         13 . The method of deep acid stimulation of  claim 1  where both the introduction of acid formulation and the introduction of ultrasonic energy occur simultaneously and directed both towards a focused treatment point. 
     
     
         14 . A method of stress fracturing a portion of an underground formation, the method comprising the steps of:
 introducing the acidizing tool into a well bore such that it is positioned proximate to a focused treatment point, the focused treatment point associated with a portion of the underground formation under stress, the acidizing tool operable to direct an acid formulation and an ultrasound energy at the focused treatment point;   introducing simultaneously the acid formulation and the ultrasound energy at the focused treatment point to diffuse acid from the acid formulation into the portion of the underground formation under stress, the acid formulation introduced at a pressure less than the fracture gradient of the portion of the underground formation under stress,   
       where the acid diffused into the portion of underground formation under stress forms weakened acidized spots, the weakened acidized spots in combination with the stress in the portion of the underground formation causes oriented stress-induced fractures to form that fluidly couple to the well bore. 
     
     
         15 . The method of stress fracturing of  claim 14  further comprising the step of introducing a hydraulic fracturing fluid into the oriented stress-induced fractures at a pressure greater than the fracture gradient of the portion of the underground formation under stress. 
     
     
         16 . An acidizing tool for use in a well bore traversing through an underground formation, the acidizing tool comprising:
 an acid delivery system operable to introduce an acid formulation onto a well bore wall of the well bore, and   an ultrasonic transmitter operable to introduce ultrasound energy into the underground formation.   
     
     
         17 . The acidizing tool of  claim 16  where the acid delivery system is operable to introduce a gel acid formulation. 
     
     
         18 . The acidizing tool of  claim 16  where the acid delivery system is operable to introduce a foamed acid formulation. 
     
     
         19 . The acidizing tool of  claim 16  where the acid delivery system is operable to introduce acid formulation radially. 
     
     
         20 . The acidizing tool of  claim 16  where the ultrasonic transmitter is operable to introduce ultrasound energy at a frequency in a range of from about 10 KHz to about 1 MHz. 
     
     
         21 . The acidizing tool of  claim 16  where the ultrasonic transmitter is operable to introduce ultrasound energy with an intensity of sonication in a range of from about 1 and about 10 W/cm 2 . 
     
     
         22 . The acidizing tool of  claim 16  where the acid delivery system is operable to introduce acid formulation and the ultrasonic transmitter is operable to introduce ultrasonic energy to a focused treatment point.