Study On Well Integrity Evaluation For Deep Volcanic Gas Reservoir

Most deep volcanic gas reservoirs are low porous and low permeable, with high temperature and high percentage of CO2, resulted in difficulties and complexities of drilling, completion and fracturing,anticorrosion during the lifetime of gas wells. Taking Xushen gas field as an example, since the gas field was put into development in 2005, about 30%of the total wells were found potential damages and undesired performance with abnormal annulus pressure, casing deformation and decreased drift diameter, even ruptured production casing and tubing caused by CO2 corrosion, resulting in safety and well integrity risks of gas field and efficient development.To solve the problems mentioned above, theoretical and experimental study on the wear of casing during drilling, the mechanical property of casings exerted by non-uniform loads in perforated section, the erosion of tubing during large scale fracturing, as well as the corrosion of tubing by CO2 during long time production were carried out in this paper. The study and results achieved in this paper provide key techniques for deep volcanic gas reservoirs well integrity evaluation, include the following research:Study on the wear degree and residual strength of worn casings:To evaluate the wear degree and the residual strength of worn casings accurately, the wear rates of PI 10 casing by S135 drill pipe in the drilling fluids used in XuShen gas field, typical deep volcanic gas reservoirs, were experimented and measured for the first time. The influences of rotating speeds of drill string, contact forces between drill string and casings on wear efficiencies and wear rates were researched. The experiment results provide fundamental data for accurate calculations of casing wear depth by drilling information. The differential equations of radial displacement of worn casings with initial oval were established. And the formulae of casing strain were deduced by small parameter perturbation method to develop the stability criterion of worn casings. The finite element analyses and experiments were carried out to verify the circumferential stress, radial stress and residual collapse strength, residual burst strength of worn casings.Study on the residual strength of perforated casing and safety of perforated casing exerted by non-uniform load:With the consideration of the vertical perforated casing completion and non-uniform compression of casing in typical deep volcanic gas reservoirs, the residual strengths of perforated casings with 90°,45°,30°,0° perforating angle were analyzed systematically, using compressed rod-pipe similarity theory, and iterative formulae for calculating the residual strength of casings with different perforation formats were derived. According to relevant API standard, experiments on collapse strength of perforated casings exerted by non-uniform loads were carried out first time, and the experiments were verified by Finite Element Analyses. Strengths of perforated casings loaded by high perforating pressure were analyzed by Finite Element Method. And, the influence of cemented rocks on the strength safety of perforated casings and well integrity was made clear. By theoretical analyses, experimental analyses and finite element analyses, the method for the evaluating the strength of perforated casings in deep volcanic gas reservoirs was arrived at. The results could also be applied for the optimization of cementing and perforating.Study on the erosion of tubing and down-hole tools during large scale fracturing: According to the characteristics of fracturing fluids used in typical deep volcanic gas reservoirs, such as XuShen gas field, the erosion speeds of P110,13Cr,35CrMo by guanidine gel fracturing fluids were experimented first time. The relationships between erosion rates and velocity of fracturing fluids, contact angles, sand percentages were obtained. Based on the erosion experiments, the prediction models of erosion rates for P110, 13Cr,35CrMo were developed. Also got were flow configuration and erosion rates of down-hole tools using FLUENT software. It was found that there were disturbed flows and vortexes due to geometry change of down hole tools when sand-contained fracture fluids passing by. The erosions near shoulders and inner wall of tools where geometry changed sharply are severer due to solids sucked into the tubing and the inner wall.Study on the corrosion of tubing by CO2:Considering the high percentage and non-uniform the distribution of CO2 in typical deep volcanic gas reservoirs, such as XuShen gas field, the influences of temperature, flow rate, CO2 partial pressure and pH on local corrosion rates were discussed. And the sensitive regions of corrosion were found, providing reference for the prediction of corrosion of tubing in different depth. The 3D model of the local corrosion of production tubing was established. Combined with the model of residual strength of defected tubing and casing, analyses of residual strength of tubular string corrosion and safety of gas wells may be predicted.Based on the studies and results of this paper, the safety and integrity of wells in typical volcanic gas reservoirs, such as XuShen gas field during lifetime could be evaluated. According the evaluations, drilling, completion, perforation, gas production technologies may be optimized, resulting in decreased wear of intermediate casings, optimized loads control of perforation, controlling of erosion and corrosion of tubing in an acceptable safe range. And, the integrity of wells in typical deep volcanic gas reservoirs will thus be ensured.