Well instability mechanism and strong plugging drilling fluid technology of horizontal well in Ma 18 well block

In the drilling process of horizontal well in tight reservoirs in Ma 18 well block of the Mahu Oilfield, wellbore instability happens such as block falling, collapse and hole shrinkage. To solve these problems, X-ray diffraction, scanning electron microscope (SEM), contact angle and rock mechanical experiments were conducted on the cores taken from Keshang Formation and Baikouquan Formation, and the physico-chemical properties, microstructures and mechanical properties of these formations were analyzed. It is experimentally indicated that the structural plane strength reduction caused by clay hydration, quite developed rock pores and fractures and drilling fluid invasion is the main reason for wellbore instability. Considering the actual situations on site, oil-based drilling fluid was adopted to enhance the inhibition to rock hydration. In view that there are more micro- and nano-pores and fractures in the rocks of the target layer, a new type of plugging agent was researched and developed, namely polypropylene glycol graft modified nano SiO₂, whose particle diameter ranges from 17 nm to 197 mn. The compound strong plugging agent of “3% oxidized asphalt+2% superfine calcium carbonate+2% nano plugging agent” was optimized, and the strong-plugging oil based drilling fluid system was prepared, whose high-temperature and high-pressure loss after 16 h of hot aging under 120℃ is only 1.2 mL, recovery rates after two hot agings are over 97%, and permeability plugging loss after 60 min is 1.4 mL, revealing its good rheological, inhibition and plugging behaviors. The research results effectively solve the wellbore instability problem of horizontal well in Ma 18 well block, and have promising promotion and application prospect.