Study On Key Technology Of Horizontal Gas Drilling

Drilling horizontal well with gas as the circulating medium (horizontal gas drilling) provides a way to develop low-porosity, low-permeability and low-pressure reservoir. This technique is an important technique to apply in future drilling industry. Horizontal gas drilling has some distinct advantages over traditional drilling mud. It not only can discover and protect pay zone effectively, but also increase rotary and rate of penetration (ROP), recovery ratio and productivity. However, drilling horizontal well with gas is still in developing stage and there are some technical issues need to be address.This dissertation systemically studied key issues in drilling horizontal well with gas, which includes calculation of minimum amount of injection gas, formation water production during drilling and its complex situations, characterization of drilling bit flow regimes, trajectory control, cutting bed cleaning technique and theory of rock burst occurrence.(1) Based on the different butting carrying characterizations, a critical gas-flow velocity model was created, which is suitable for different cutting carrying requirements utilizing geomechanical analysis. Proposed an evaluation method and formula for the minimum amount of gas injection along wellbore. The results were verified as reasonable and feasible using ChuanZhong G-002well as a practical example. A sensitivity analysis was conducted then to evaluate the factors that affect the minimum gas injection. Factors that affect the minimum gas injection, such as cuttings particle diameter, rate of penetration, trajectory angle, coarseness of wellbore, friction coefficient of drill cutting, area coefficient of windward, are evaluated and summarized.(2) Evaluated and analyzed the characterization of formation water production, set up prediction model for formation water production. Numerical simulation was conducted to evaluate the mechanism of formation water production during gas drilling process. The results show that the permeability is the main factor that affects the formation water production, along with capillary, chemical potential energy and annular return rate.(3) Analyzed and described characterization of formation water flow in annular for each stages. Proposed a method and formula to calculate the critical formation water production quantitatively. Programmed computer code to calculate practical case and conducted sensitivity analysis. A relation was drawn between gas injected amount, well depth, trajectory angle and critical formation water production. The different factors that affect the critical formation water production in different stages were also evaluated. The formation water production from ChuanZhong H-1well and Tarim LX-1well were close to the calculation results, with an error in5%, which is good for engineering project requirement. Analyzed the complex mechanism of formation water production during horizontal gas drilling, and the process was described. The complex relation between horizontal gas drilling and wellbore complex was experimentally evaluated.(4) Set up mathematical model for flow regime of bottom hole based on CFD technique. Numerical simulation was conducted to evaluate the fluid flow behaviour in bottom hole. The results were then evaluated and the effects of trajectory angle, bit nozzle angle (PDC bit), and inclination angle (tricone bit), were carried out. It provides fundamental theory for bit optimization design.(5) Developed PDC bit with stabilizer. Conducted mechanical analysis and built up mechanical model. Evaluated and calculated the deviation ability of PDC bit with stabilizer. The results show the effect of length of stabilizer and installation position on deviation rate and BHA torque. Optimized design the PDC bit with stabilizer. Manufactured6-5/8" PDC bit with stabilizer and practically tested it. Pilot test shows that PDC bit with stabilizer has higher deviation in directional drilling process,1.5degree screw can reach1.75degree deviation or even higher. It can also resolve the problems of high WOB and friction.(6) Proposed an gas motored mechanical rotary directing BHA. Analyzed and described its structure, working mechanism and implement method. Analyzed mechanics of the rotary shaft under biasing effect. Created the mechanical model and optimized solution. Conducted simulation utilizing finite element method to evaluate biasing effect. The results proved the rationality and feasibility, and provide fundamental theory for future R&D.(7) A new device was developed for cutting bed cleaning. Analyzed its structure and working mechanism, proposed its implementation method. Optimized a angle of bypass hole axis and shell axis utilizing flow regime simulation. Results show that the new device and technique resolved the key issue of cutting bed during horizontal gas drilling process. It effectively break and remove the cutting bed, and easy to implement. Optimized a angle is between30-60degree.(8) Analyzed the rock burst mechanism utilizing stress concentration theory, damage theory and catastrophe theory. Analyzed and explained the process regarding the energy depletion and release theory. Using finite element method to simulate the geomechanical behaviour before and after drilling different high-pressure formations so that the theory of rock burst occurrence was carried out. Based on this theory, propose a prevention scheme to prevent the rock burst occurs during horizontal gas drilling.This dissertation proposes the theory, model, method, design and technical imple-mentation scheme. It provides fundamental theory and technical support to resolve the issue during the horizontal gas drilling. It systematically enrich current fundamental the ory of horizontal gas drilling, and provide a path to research and develop this techni-que in greater depth both theoretically and practically.