Permeability damage micro-mechanisms and stimulation of low-permeability sandstone reservoirs: A case study from Jiyang Depression, Bohai Bay Basin, China

According to the characteristics of structural elements (framework grain, interstitial material and pore throat structure) of low-permeability sandstone reservoir, the step by step dissolution and separation acidification and acid fracturing technology has been developed and tested in field. There are three main mechanisms affecting permeability of low-permeability sandstone reservoir: (1) The mud fillings between the framework grains block the seepage channels. (2) In the process of burial, the products from crystallization caused by changes in salinity and solubility and uneven migration and variation of the syn-sedimentary formation water occupy the pores and throat between grains. (3) Under the action of gradual increase of overburden pressure, the framework grains of the rock is compacted tighter, making the seepage channels turn narrower. The step by step dissolution and separation acidification (acid fracturing) technology uses sustained release acid as main acidizing fluid, supramolecular solvent instead of hydrochloric acid to dissolve carbonate, and a composite system of ammonium hydrogen fluoride, fluoroboric acid, and fluorophosphoric acid to dissolve silicate, and dissolving and implementing step by step, finally reaching the goal of increasing porosity and permeability. By using the technology, the main blocking interstitial material can be dissolved effectively and the dissolution residual can be removed from the rock frame, thus expanding the effective drainage radius and increasing production and injection of single well. This technology has been proved effective by field test.