Acid Fracturing Technique for CarbonateReservoirs Using Nitric Acid Powder

摘要

The length of the etched fracture is rather limitedutilizing traditional acid fracturing techniques,especially in a high-temperature carbonate reservoir.Although the propped fractures may have a deeperpenetration, they have such drawbacks such as lowfracture conductivity, unintended proppant bridging andsubsequent proppant flow back.This paper presents the development of a new acidfracturing technique, Nitric Acid Powder (NAP) acidfracturing, to improve the acid penetration and fractureconductivity. The NAP acid-fracturing technique hasbeen applied in several oil fields in China. It has beenshown that NAP acid-fracturing technique has theadvantages of both hydraulic fracturing and acidfracturing such as long effective penetration, highfracture conductivity, low cost and easy field operation.The general process of this technique includes the following steps. First, the nitric acid is made into a solidpowder. Second, the non-reactive fluid (oil-basedfracturing fluid, diesel oil, etc.) is applied to create thefractures and carry the NAP into the fractures. Third,hydrochloric acid is injected at a low rate, and the NAPdecomposes into nitric acid to etch the fracture surfacestogether with hydrochloric acid.We have developed a comprehensive mathematicalmodel for the NAP acid-fracturing technique to facilitatethe optimization of the field treatment design. The modelpresented considers fracture growth, acid transport andreaction, leakoff, etched width of the fracture and so on.The study has shown that the NAP acid-fracturingtechnique could reach a very high stimulation ratio evenin the high-temperature carbonate reservoir. Thereforeit is an innovative and promising technique for wellstimulation in carbonate reservoirs.