Development And Application Of Evaluation System For Borehole Stability Of Shale In HTHP

As one of complex question during well drilling and completion, wellbore instability refers to formation caving, the change of hole diameter, formation breakdown and other conditions that may occur under the shaft, which have serious effects on the acquisition of geological information, hinder the exploration and exploiting velocity and quality, contaminate reservoir and therefore influence well testing and oilfield development. In addition, the target beds would not be gat in some new exploration areas due to wellbore instability, and that could cause heavy economical loss because of the delay time of exploration development. According to conservation estimates, wellbore instability can give the global oil industry of a loss up to1billion, and the processing time of it accounts for5%-6%of drilling time. Besides, various downhole accidents caused by wellbore instability might result in personal injury or death. The formations encountered during drilling include sandstone formation, silty sandstone formation, clayey sandstone formation, coal formation, conglomerate formation, carbonate formation and shale formation, and wellbore instability can happen to anyone. Approximately75%of encountered formations is shale formation that over90%of it prone to wellbore instability. Therefore, the researches on wellbore stability are run around instability of shale, and the results will contribute to keep hole stable, protect reservoir, accelerate the speed of completion period and reduce drilling cost, all that can improve core competency of Chinese drilling technology.The dissertation’s main purpose is to simulate operation conditions of drilling fluid in bottom and evaluate its effects on wellbore stability. By the analysis of drilling fluid during drilling, it can be known that besides fracturing rock and carrying that to well head, drilling fluid also can wash the wall which would aggravate as velocity in annular increasing. Reduction of well diameter would happen to some formations during washing, having characteristics of being easy to hydrate, low diagenetic strength and weak cementation. In order to avoid hole shrinkage and pipe sticking, we can increase velocity in annular large enough to wear cuttings away, and keep well diameter not less than bit diameter. But if the washing action is larger than rock strength, it would cause formation caving. To shale formation, it can react with drilling fluid being different from interstitial fluid, that would change strength and stress state of shale and have a further effect on shale stability. The process includes two stages:(1) drilling fluid filtrate would spread to shale formation under the hydraulic differential pressure and chemical differential potential;(2) filtrate would react with hydrosialite. And the hydraulic differential pressure come from the difference between liquid head in hole and pore pressure, the chemical differential potential grow out of the differences in the concentration of ions in drilling fluid and interstitial water.In order to evaluate the effect of drilling fluid on wellbore stability, we prepared artificial borehole wall after having gotten information of mineral composition and physical and chemical properties, then simulated the downhole conditions, such as temperature, pressure and cyclical and dynamic process of drilling fluid, and make an damage on borehole wall. Meanwhile, we put the core with the same mineral composition and physical and chemical properties in cell which also can simulate downhole conditions, build different hydrodynamic pressure and chemical potential at both ends of core, and then make drilling fluid throughout the core.According to current conditions of evaluation devices for wellbore stability and the analysis mentioned above, and combined with ultrasonic imaging techniques, transmission of pressure and chemosmosis, the dissertation developed a new apparatus and its evaluation methods——evaluation equipment for shale borehole stability in HTHP. It consisted of simulation setup of artificial borehole well, HTHP triaxial well, control system of temperature, pressure and confining pressures, dynamic circulation system of drilling fluid and data acquisition and processing system. The device has the following functions:(1) The downhole temperatures and pressures and dynamic circulate process of drilling fluids can be simulated by the evaluation apparatus;(2) It can simulate the damage of artificial wellbore caused by drilling fluid, and has the characteristics of recoding data, ultrasound imaging of artificial wellbore, determining filtration velocity;(3) Two models were established——pressure transmission and chemosmosis, and it can be used to measure low permeability and membrane efficiency of shale.(4) The data acquisition and processing system fulfill functions of recording data, then showing it real-time, saving and playing back. The imaging of artificial borehole well, chemical osmotic pressure, low permeability and membrane efficiency also can be obtained real-time.(5) The effects of fluids into the wells on wellbore stability, such as drilling and completion fluids, can be evaluated comprehensively, and that would provide references to optimize fluids which are helpful to wellbore stability.On the testing equipment, eroding process of artificial wellbore, pressure transmission and chemosmosis can be accomplished. The results showed that the evaluation equipment and experimental program could be improved. The main improvement points are as follow:reducing the thickness of artificial wellbore, implementing automation of ultrasonic ranging, improving the voltage regulation of precision plunger pump, perfecting the operational software. In addition, the influences of downhole temperatures and pressures, core compositions, filtrate properties and the annular velocity of drilling fluids on wellbore stability should be evaluated based on one factor tests. In addition, it was known that hydraulic pressure difference and chemical potential difference between drilling fluid and formation water become driving force of invading formation. If keeping shale wellbore stability, it was necessary to control the pressure transmission and keep filtrate from the layers, and that be achieved by improving the effects of inhibited performance, plugging performance and film forming ability of drilling fluids.According to the principal of determining permeability, hydraulic pressure transmission function, the equipment can be used to determine ultra-low permeability of shale cores, to evaluate the impacts of drilling and completion fluid on core’s permeability by testing how much time the hydraulic pressure transmit to the bottom of core, and to assess plugging and film-forming effect additives. In conclusion, the evaluation equipment will be widely used in oil industry.