Study and Field Test on Stimulation of Low Permeability Glutenite Reservoir in Mahu Depression

Abstract

The lithology in Mahu depression is mainly composed of sand and conglomerate, and the reservoir is characterized by complex lithology (sandstone and conglomerate reservoir), large buried depth (2750–2900 m), medium pore spaces and low permeability (average porosity is 9.8%, average permeability is 5.4 mD), thin layers distribution and high pressure coefficient (1.3–1.6) et al. This kind of reservoirs has no economical production excepting with well stimulation, while the stimulation operation has faced with difficult such as un-predication of fracture expansion, high cost in stimulation of deep well and unable to develop multi-layers. In order to solve the above problems, we investigated the economic stimulation of glutenite formation. (1) 30 cm × 30 cm × 5 cm indoor experiment revealed that fracture initiation and expansion of the glutenite reservoirs are more complicated than those of the conventional sandstone. Scale of gravels have great influence on fracture propagation: in large size gravel (2–5 mm), fracture mainly extend along the grit surface of gravel and cement; in small size gravel (<2 mm), fracture expansion may occur on gravel, bypass gravel and even result in expansion termination. (2) According to the characteristics of glutenite reservoir, a low concentration fracturing fluid system was developed, in which concentration of guar was reduced from 0.40% to 0.25%. Viscosity of this kind fracturing fluid can maintain 100 mPa.s under condition of 90 °C and 170S-1 after 120 min. By using this fracturing fluid, operation cost can be reduced by 39 Y/m3, in addition, residue amount can be reduced by 10%. (3) Field test of separate layer fracturing method and general fracturing method has revealed that separate layer fracturing is much better because it can improve longitudinal usage degree and improving hydrocarbon production. (4) A reasonable fracturing operation has formed after the optimization of different fracturing scale, pad fluid and slick-water ratio. (5) A soluble bridge plug and multi-stage cementing sleeve was formed after the investigation of fracturing completion tools. (6) A factory fracturing model has formed in which operating time can be reduced by 20%. In the test of 42 wells and 122 layers, the average daily oil production is 12 t/d, exceeding the design of 9 t/d. Application of new technology can be references for efficiency development of low permeability glutenite reservoirs.