Research On Key Technologies For Casing Drilling

Casing drilling technology is that the standard casing is used to drill and pack off hole. The casing that plays the role of the drill string, will transmit mechanical and hydraulic power to drill bit through BHA which connects to the lower part of casing. It can remain drilling and circulating of drilling fluid. BHA will be hoisted and cementing will start after the wellhole is drilled to the design depth. The drilling and running casing are combined into an integral operation in casing drilling. The bit and BHA will be triped in casing with the cable and the trip operation in conventional drilling does not require any longer.Compared with conventional drilling, casing drilling has obvious advantages which mainly represent some aspects following. It can shorten the drilling time, reduce down hole accidents, improve the well-controlled conditions and cause drill fluid continuous to cycle while tripping. It also can decrease drilling rig size and the cost of later stage operating surface equipment and reduce the pollution on the layer(reservoir) and surface environment during the drilling. This not only protects the environment, but also increases the single well production, which causes people's awareness and general application ever.Compared with conventional drilling process, casing string serve as drill string. This leads to obvious difference in straining complexity, flowing state and pressure distribution in annular space, eccentric wear and damage, and so on. Technological difficulties includes string bending in wellbore, lager twisting stress amplitude, definite prediction of annular fluid parameters, load on thread, fatigue and damage of casing. Focusing on these difficulties, the article builds up a set of mechanics model of whole casing string combining mechanics of elasticity with finite element method. The model can be used to analyze casing string mechanics, annular hydraulics and thread mechanics. Based on the above model, drilling hydraulics and thread can be optimized, and casing string failure can be prevented or decreased. Through overall and systematic research and application, the article gets main conclusions as follows:(1) Casing dynamics and rotating mechanics are jointly researched. String friction force of whole casing string is analyzed considering deviation and azimuth. Residual strength of casing string is calculated in axial tensile stress condition. Critical buckling load is defined by analyzing casing string buckling, and casing string is optimized.(2) The model of annular pressure loss calculation and momentum equation of axial flowing are built based on drilling hydraulics and application of basic hydromechanics equation. So flowing regularity of drill fluid is obtained. Based on calculating pressure drop of bit jet and fluctuating pressure, interrelationship between flowing pattern and velocity and pressure distribution in annular are presented accurately in casing and annular space, further hydraulics of casing drilling is optimized.(3) Elasticity mechanics theory is applied to research thread load. The model is built by analyzing strain and deformation of casing string thread. Thread teeth's bearing limit is got, and the strength of sub thread in casing string is checked. Furthermore, technical measures of thread's reinforcement is presented, which provides theoretical foundation for optimizing sub and special thread design.(4) Considering of the two processes in casing drilling and completion after drilling, the fatigue and damage types of the casing string are obtained respectively from the mechanics and chemical views. We obtain the fatigue strength and residual intensity in different stress loading and present further the technical measures of preventing casing damage.(5) By applying the theoretical models established in the article, many parameters are calculated respectively, which are friction and torque of casing string, bit hydraulics, thread strength of joint, buckling and residual intensity of casing string in the casing string in Fuyu region. By comparing with actual drilling data, the error of hydraulics parameters is less than 10% and meets the requirement of the construction work.