Research On Diverted Acidizing With Nitrogen Foam Fluid

Acidizing is a kind of stimulating methods for oil and gas well or injection well, through dissolution of plug in formation matrix. There are some problems in conventional acidizing technology, such as uneven distribution of acid, unthorough flowback of reacted acid, bad acidizing effect, and so on. Foam fluid has selection of different formations with different permeability, and oil and water. Diverted acidizing with foam fluid can improve acidizing distribution, and acidizing effect.Through displacement experiments for single and parallel core, theory for diversion of foam fluid in cores with different permeability was gained. Capillary tube pressure is higher in low permeable core. The bubble breaks easily, which causing gas cross flow and low flow pressure loss in low permeable core. The situation is opposite in high permeable core. When permeability ratio of high and low permeable formations is less than 10, diverted acidizing with foam plug is suitable; When permeability ratio lies in 10 to 15, diverted acidizing with foamed acid is suitable. Corrosion of foamed acid to formation is meaning to site application. Corrosion of foamed acid, base fluid, and mud acid to quartz sand were researched by experimental method. Corrosion weight loss of N80 steel was tested. The affect of foam quality and temperature to corrosion were also analysised. The results show that corrosion of foamed acid is much less than that of mud acid. The lower the foam quality is, and the higher the temperature is, the faster the reaction is. Foamed acid has good corrosion resistant, which can realize deep acidizing in formation.The concept of thermal resistance was introduced in pressure reduction calculation of foam pipe flow. Thermal resistance coefficient was deduced according mass conversation, momentum conversation and energy conversation equations. The results of example calculation show that the larger the non-dimensional heating number is, the larger the thermal resistance is. Thermal resistance is compared with fraction resistance. When non-dimensional heating number is small, thermal resistance can be ignored. When non-dimensional heating number is larger than 0.25, thermal resistance is about 10% of fraction resistance, which must be considered during pressure reduction calculation in pipeline. Based on gas trapping theory and mass conservation equation, mathematical models were developed for foam diverted acidizing, which can be achieved by foam slug followed by acid injection or continuous injection of foamed acid, respectively. The mathematical models were solved by a computer program. Computed results show that the total formation skin factor, and wellhead pressure and bottomhole pressure increase with foam plug injection, but decrease with acid plug injection. Flow rate of high-permeability layer decreases. It can divert acid into low-permeability layer from high-permeability layer. It can be seen from the results that concentration of HF decreases quickly during sandstone-matrix acidizing in radical direction, and operating range of HF is less than 0.5 m. Dissolving capacity of H2SiF6 is equal to HF, which increases penetration depth. Deposition of Si(OH)4 increases with time. Reacted acid must be blowed out thoroughly after some time, otherwise secondary precipitate will be produced, which affects acidizing effect. Consumption of HF increases with formation temperature. High injection rate of acid can increase penetration depth. Differential equations for temperature and pressure distributions of foam fluid in the wellbore were established in consideration of the heat exchange between the foam fluid in the annular space and the injected fluid from tube. Coupled solution of temperature and pressure distributions using numerical method was conducted. Distributions of foam temperature, foam density, foam quality, pressure and foam velocity in wellbore were obtained.Well selection methods and operating technique were provided for diverted acidizing with foam fluid. Field applications show that diverted acidizing with foam fluid can effectively block high permeability layer, and improve acidizing effect and oil production. It is fit to heterogenous formation wells and old wells with acidizing for many times.