Fracturing Water Sensitive Tuffaceous Reservoirs

Abstract

Abstract We present successful hydraulic fracture treatment practices in water sensitive tuffaceous reservoirs in Hailar Basin, China. The reservoirs are of too low permeability to be placed onto production without massive hydraulic fracture treatments. The article describes the geological engineering framework, mineralogy investigation, hydrolytic weakening experiments, as well as application and results of new kinds of fracturing fluids. Rock mineralogy investigations indicated that rocks in the reservoir have a strong water sensitive property and a strong component of plastic behavior. Some tuffaceous rocks are rich in alkali minerals and become soft when exposed to aqueous fracture fluids. Although the fracture opening mechanism is the same as for normal sandstone, fracture extension is relatively suppressed and the fracture width is different from elastic predictions for normal sandstone. The analysis led to the changes in treatment stratages. The key difference from previous treatment is that clay content and concentration of different minerals were taken into consideration of fracture fluid design. Different kinds of emulsified fracturing fluid were designed used to mitigate swelling and hydrolytic weakening effects. By the end of 2005, 163 intervals in 66 wells have been treated using these new kinds of fracturing fluids, with a success rate of 97%. Introduction Massive hydraulic fracturing treatments in low-permeability sandstone reservoirs are common practices; wherease, stimulating tight, a volcanic-origin reservoir is far less common. Several reports of successful hydraulic fracturing treatments in volcanic reservoir rocks can be found; e.g., Weijers et al. (2002) reported fracturing practices in volcanic tight gas reservoirs in the Minami-Nagaoka gas field in Japan. Also, Antoci & Anaya (2001) discussed massive fracture treatments in tight gas zones in the Neuquen Basin (Argentina) where the lower parts of the oil zone are tuffaceous porphyrites. In recent years, tight tuffaceous reservoirs were discovered through exploration activities by Daqing Oilfield Ltd. in the Hailaer Basin. The Hailaer basin, together with the East Gobi Basin, and Tamsag Basin, are of a series of basins that formed in the China-Mongolia border region during a period of Late Jurassic-Early Cretaceous (Tse 2003, Johnson & Graham 2004). They have samiliar geological sedimentary structure and lithology. The basins are dominantaly nonmarine synrift sediment and volcanic flows fillings. The reduced porosity and permeability may be a concern because of rift-related volcanism and zeolite cements associated with volcanic input into saline-alkaline lakes. This system was eventually overwhelmed by volcanic debris during eruptions (Johnson & Graham 2004). Core analysis of the tuffaceous rock in Hailaer basin gave a porosity range of 5.6 to 21.7% (average 15.1%) and a permeability range of 0.03×10–3 to 27.4×10–3 µm2 (average 1.17×10–3 µm2). The reservoirs are of too low permeability to be placed onto production without massive hydraulic fracturing treatments. The preliminary two field experiments of fracturing in the tuffaceous rocks (carried out in 2002) were failed. Figure 1 illustrates a standard pressure-time treatment curve for one layer in one well. In this case, after fracture initiation, the stable fracture fluid injection pressure was ~26 MPa at the pump. Proppants experienced sudden tip screen-out when the concentration reached 200 kg/m3, preventing further fracture propagation into the reservoir.