A Novel Diverting Acid Stimulation Treatment Technique for Carbonate Reservoirs in China

Abstract

Abstract Carbonate reservoirs are wildly distributed in China, and the reserve is very large. Bulk of big and middle oil fields were discovered in Sichuan Basin, BoHai Gulf Basin, Ordos Basin, Tarim Basin and so on. The cumulative demonstrated geological oil reserve reaches 1.62 billion ton, and the gas reserve is 1.42 trillion. Formations of the newly discovered carbonate oil/gas fields have the following features: deep buried (>5000m); serious heterogeneity; full of pores, caves and cracks; long well section. So, it's difficult for acid stimulation treatment. And the treatment effect using traditional polymer-based acid fluid system is not very good. Based on surfactant, this paper develops a novel diverting acid stimulation treatment for carbonate formation. This kind of self-diverting acid system contains a special visco-elastic surfactant. The viscosity of fresh acid is very low. However, when it enters into the pores or cracks of carbonate formation and reacts with rocks, the viscosity will increase rapidly as the concentration of acid decreases, because the surfactant molecules will aggregate to baculate micelle along with the consuming of acid. The structure will increase the viscosity of system, and prevent the fluid from leaking off into wormholes and cracks which have been etched. The micelle structure of surfactant can be used as a diverting agent in the process of acid treatment, and can also be used to control the fluid leaking off in acid fracturing stimulation. This technique has been used in 5 carbonate oil fields on land in China. 38 wells were acid fractured and 25 wells were treated. The effect is very well. Introduction The heterogeneity of the carbonate formation in China is very serious. The proportion of low permeability carbonate formations is very high. This type of reservoir is deep buried, and its stretch is wide. In addition, the damage in the process of drilling, cementing, completing is serious. Because of above reasons, the carbonate reservoirs which need to be treated take up a high percentage. However, the effect of (repeat) acid (fracture) treatment becomes weaker and weaker year by year(Xugang Wang, 2003; F.Zhou et al., 2006; M.A.Buijse et al., 1998). In the process of traditional acid treatment, most of the acid will flow into the high permeability zone or lightly damaged zone, and cracks, leaving other zones of interest unstimulated, such as, low permeability zone or seriously damaged zone, and the basement of porous-fractured reservoir. This problem brings a poor effect to the entire acid treatment. So, it is difficult to carry out an acid treatment (fracture) in carbonate formation. The study and application of self-diverting acid treatment technique began in 1950s (Houchin, et al., 1986; Strassner, et al., 1990; Olivier Lietard, 1997; A.Saxon et al., 1997; S.C.Zeilinger et al., 1995; M.Buijse et al., 2000). This type of technique can divert acid into the low permeability zone and achieve a uniform treatment to the whole formation through temporarily blocking to the high permeability zone. So far, the commonly used diverting techniques include packer diverting, seal ball diverting, solid particle diverting, foam diverting, cross-linking polymer gel diverting. These techniques can implement limited diverting acid treatment, but packer and seal ball can't be used in the screen well and open hole; solid particle and cross-linking polymer gel diverting agent don't suit deep acid treatment because they will cause serious damage to formation; foam diverting is a complex technique and can't be used in deep well; oil soluble resin diverting agent can't be used in gas well and water well. So, all of the above techniques are limited.