Optimization of Acid Fracturing Treatment Parameters in Ultra-Deep Siliceous Dolomite Reservoir

The Deng-4 member of Dengying Formation in Penglai gas block is a typical siliceous dolomite rock reservoir. The difficulties on acid fracturing stimulation include: ① Lower siliceous reservoir and upper reservoir have strong longitudinal heterogeneity; ② High friction and acid-rock reaction rate caused by high temperature and buried depth. Taking the PS101 horizontal well as an example, the well trajectory repeatedly traverses a laterally continuous siliceous interval at ~7200 m true vertical depth (corresponding to ~7380–7500 m measured depth along the deviated trajectory), which introduces considerable uncertainty into the acid fracturing design. For the above question, the temperature at the fracture entrance is modified by coupling the wellbore temperature model with the acid-etching model. With the idea of “integration of geology and engineering, centralized stimulation of advantageous reservoirs, differentiated design of acid fracturing parameters, and improvement of stimulation degree and drainage area”, two stages alternating acid fracturing is adopted and the main acid system is selected as time-temperature compound variable-viscosity gelled acid to reduce wellbore friction and acid-rock reaction rate at the same time. According to the geological characteristics of different stimulation sections, the treatment parameters are designed pertinently to improve the effective drainage area based on the established model, and the injection rate and acid volume of each stimulation interval were selected through a marginal-benefit criterion subject to the 125 MPa wellhead-pressure limit. Results from field tests in Well PS101 results verify the applicability of the concept of “integration of geology and engineering and differentiated design of acid fracturing parameters” in siliceous dolomite, which provides a basis for the efficient development of carbonate rocks composed of such complex minerals.