Effect of temporary plugging agent concentration and fracturing fluid infiltration on initiation and propagation of hydraulic fractures in analogue tight sandstones

Abstract

Temporary plugging agent (TP) fracturing has been widely used in many cases as an efficient technology to improve well production performance. In particular, the initiation and propagation of hydraulic fractures are vital to the evaluation of fracturing effect. Hence, the research of its influencing factors and laws is crucial for successful design of fracturing operation. As a productive research method, TP fracturing laboratory experiments have been broadly applied by innumerable scientists. Despite of the great achievements, those experiments mainly focus on the plugging ability of TP in unconsolidated and high permeability reservoirs in order to promote initiation and refracturing. However, studies are limited in light of tight low permeability reservoirs, in which TP not only function in plugging but also achieves changing the induced stress field. Especially for tight sandstone reservoirs, TP concentration and fracturing fluid infiltration are key factors affecting the initiation and propagation of hydraulic fractures of tight sandstones, and the laws have not been fully mastered. In this work, we poured artificial analogue rock samples (AARS) according to physical and mechanical parameters of tight sandstone reservoir in Shengli Oilfield, China. And true triaxial TP fracturing experiments under different TP concentrations were carried out on AARS. Aided by nano-organoboron crosslinking agent (NBC) and sleeves, different fracturing fluid infiltrations were achieved with the acoustic emission (AE) system monitoring the initiation and propagation of hydraulic fractures. In addition, the injection pressure curves, the geometries, forms and propagation process of hydraulic fractures were investigated in detail. The results show that: TP accretion, fracturing fluid infiltration and hydraulic fractures generation can produce innumerable AE energy that matches with the fluctuating injection pressure. TP fracturing leads to extension pressure higher than initiation pressure and slow down fracturing rate at relatively low TP concentration. TP concentration is positively correlated with both initiation pressure and average pressurization rate, but negatively correlated with the time for initiation, and such regularity is better reflected in weaker infiltration. 1.2% is the optimal TP concentration to increase initiation pressure and shorten the time for initiation. On the middle cross section of AARS, single-wing fractures, bi-wing fractures and multiple fractures are formed. Inside AARS, the main and branch fractures are formed. The synergy of TP, NBC and sleeves generates a complex fracture system composed of main fractures and branch ones, which reduces the influence of sand on fracture propagation. In short, properly weakening fracturing fluid infiltration by using NBC and sleeves, combined with a reasonable selection of TP concentration, have proved to be a productive way of improving hydraulic fracturing effect of tight sandstone. The findings contribute to a more comprehensive understanding of the initiation and propagation laws of hydraulic fractures of tight sandstone and conduce to the optimization of fracturing parameters. • We carried out temporary plugging agent fracturing experiments on artificial analogue rock samples of tight sandstone. • We monitored initiation and propagation of hydraulic fractures with acoustic emission system. • Temporary plugging agent concentration and fracturing fluid infiltration affects initiation and propagation of hydraulic fractures. • We put forward a productive way of improving hydraulic fracturing effect of tight sandstone.