Ultra deep well fracturing technologies applied in Well Yutan 1 in the tight oil reservoir

Well Yutan 1 is a vertical wildcat well in the Tuha Basin, and its fracturing target layer has buried depth of 6 056-6 063 m, temperature of 150 ℃, porosity of 8.42%, permeability of 0.495×10⁻³ μm², pressure coefficient of 1.26 and minimum principal stress of 118-124 MPa. Therefore, it is a typical tight reservoir with ultra depth, ultra high temperature and ultra high pressure. The successful development of Well Yutan 1 means the great exploration breakthrough in Taibei sag. However, it is currently the deepest well in Tuha Oilfield, and its fracturing stimulation faces a series of difficulties. For example, the construction pressure is extremely high, the performance of fracturing fluid is highly required, pipe string is of great risk and even the reservoir cannot be fractured. To solve these difficulties, mini fracturing test was carried out before the initial fracturing of Well Yutan 1. Friction, permeability and fracture propagation pressure were calculated based on drawdown test data and stepped displacement reduction analysis. In the process of initial fracturing, ultra high temperature (150 ℃) organic boron delayed crosslinking fracturing fluid, displacement control, dual-balance pressure based casing protection and slug proppant pumping mode were adopted. However, the wellhead pressure during the construction was too high, so the expected proppant volume was not completed. In the process of refracturing, the artificial barrier technology was introduced on the basis of the initial fracturing technologies and the ultra high temperature delayed weighted fracturing fluid with the density of 1.12 g/cm³ was adopted. As a result, the wellhead pressure was reduced by 7-9 MPa and the expected proppant volume was completed successfully. The successful fracturing test of Well Yutan 1 marks that the fracturing technologies step to the new stage of three-ultra (ultra depth, ultra high temperature and ultra high pressure) well fracturing.