Research On The Flow Behavior Of Foam Drilling Fluid In Horizontal Well Section

Because of the advantages in oil and gas reservoir protection, drilling risk diminution, drilling cost reduction and individual well production improvement,Underbalanced Drilling and Managed Pressure Drilling have been attracting more and more attention from oil companies all through the world. Foamed horizontal drilling, as an important branch of the technique, is proved to be one of the effective methods that can be predicted to improve the overall exploratory development benefit. Though UBD and MPD have been applied extensively in field abroad, related research is still on a preliminary stage in China. Therefore, research on the flow behavior of foam fluid in horizontal well section is of practical importance in improving the narrow-window safe drilling technology and promoting the scientific drilling proceeding in China.Based on extensive literature review, using flow similarity theory and dimensional analysis, a new multi-functional wellbore flow simulation loop was successfully designed and developed. The flow loop can be used to visually measure the flow behavior of compressible or incompressible Newtonian or non-Newtonian fluid. It simultaneously allows the temperature and pressure elevation, sand injection with constant speed, inner pipe rotation, eccentricity, inclination, and real-time control and indicating of experimental parameters. A new approach to studying the wellbore flow performance of gasified drilling fluid was finally obtained. Then the rheological property and cuttings transport capability of foam fluid were studied respectively using the pipe flow viscometer and annular test section of the flow loop. On the basis of Oldroyd-Jastrzebski slip correction method and volume equalized principle, the slip corrected and non-corrected correlations for rheological parameters of power low model of foam fluid were established by means of curve fitting and nonlinear regression, and the effects of polymer concentration, temperature and pressure on rheological parameters were studied. Meanwhile, using quantitative cuttings bed method and constant-speed synchronous sand injection method, the cuttings bed transport and erosion mode were observed, and the influence of different flow parameter on critical annular flow velocity and cuttings concentration were experimentally studied.Foam fluid was considered as a special non-Newtonian compressible fluid. The governing equations for foam flow in concentric annulus and drill pipe were derived by means of finite element and mechnical analysis, and the pressure gradient and flow velocity were calculated under different operational conditiond. The wellbore pressure calculation method in foamed horizontal drilling was then proposed using constant back prssure and reversal circulation iteration, and relevant application program was developed. Simulation results on a in-situ horizontal well agree well with field practice, hence the proposed calculation method was proved to be correct and reliable.A particle grading model for cuttings size distribution was established using fractal geometry theory. The concepts of relative submergence and equivalent diameter were proposed. Taking the location randomness and geometry irregularity into consideration, the particle starting mechanism on the cuttings bed surface was studied in the light of rolling moment balance, and relevant correlation between critical flow velocity and particle starting criterion was obtained. Comparison between theoretical predictions and test results shows that the quantitative starting criterion is proved to be about 40% that is appropriate for correspondent critical starting velocity for uniform particles. For non-uniform particles with starting velocity gradation, compared with uniform particles of the same diameter, it is more difficult for fine particles to start moving but easier for the coarse particles.According to the concept of critical inclination angles, the horizontal well was divided into three segments, i.e. the horizontal and near horizontal section, vertical and near vertical section and transitional section. A one-dimensional stable theoretical model for cuttings transport with foam, appropriate for the entire horizontal wellbore, was established based on the conservation law and mechanical analysis. The progradational iteration method with variable step length was used for numerical calculation for the proposed model. Meanwhile, considering the interlayer mass transmission and formation fluid infiltration, a one- dimensional transient two-layer flow model was proposed for cuttings transport with foam in horizontal section. The model was numerically calculated with the corrected SIMPLE algorithm. Then, an optimized design method for critical flow velocity (CFV) and optimum gas/liquid flow rate in horizontal well section was established by multivariate nonlinear regression. Theoretical predictions by numerical simulation agree well with the experimental results in this dissertation, which validates the correctness and reliability of the proposed models for cuttings transport with foam fluid. Research results offer theoretical foundations for wellbore pressure control and hole cleaning technology in foamed horizontal drilling.