Research On Mechanism Of Sustained Casing Pressure In Gas Wells

As the exploration and development progress of petroleum and natural gas constantly goes deeper, the environments oil and gas wells facing become more and more severe, such as high temperature, high pressure and high corrosive. A number of oil and gas wells present annulus pressure phenomenon during the development process. The sustained casing pressure (SCP) raises severe challenges on the safety exploration and wellbore integrity of gas wells.Based on statistical analysis of abnormal SCP in gas wells, the paper conducted theoretical studies and laboratory experiments in aspects of pressure diagnosis, percolation mechanism of gas channelling in annulus, sealing ability of oil tubing thread, allowable annulus pressure, corrosion of oil country tubular goods (OCTG), rubber materials of packer and cement. A risk evaluation method and control measures for SCP of gas wells have been developed, which can provide technical support for safety production of gas wells.(1) Through studying the situation of annulus pressure in gas wells and collecting annulus pressure field test data, the most dangerous situation of annulus pressure is that caused by gas channelling which may continue to exist permanently after being bleed-off. And according field operation, the main reasons causing annulus pressure include failures of oil and casing strings, downhole safety valve, packer and cement.(2) In view of the problem of annulus pressure caused by gas channeling, based on the non-linear Darcy percolation, multiphase fluid dynamics and gas thermal expansion theory, two gas channeling models under the conditions of cement returned to the ground and did not return to the ground were established respectively. The models could be used to analyze the influence factors of annulus pressure, predict final value and when the time the annulus pressure reaches a stable state. A temperature and pressure calculation model of gas wellbore and the A annulus pressure prediction model were established to analyze the relationship of the equivalent leaking size and annulus pressure.(3) A finite element model of oil tubing thread was established. The sealing properties under make-up, tensile, compression and compound conditions were evaluated, and the changing rule of the maximum contact stress was obtained. Results showed that the most severe condition was in tensile situation after making up, the internal pressure could improve the sealing ability and the SCP would decrease the sealing ability.(4) Based on comprehensive consideration of steel grades of oil and casing strings and the actual status of SCP in gas wells, the API RP90standard was developed and perfected, and the maximum allowable annulus pressure and minimum control pressure calculation method was established. The method considered the influences of wear and corrosion, and the dynamic range of allowable annulus pressure was obtained through analysis of the security of oil and casing strings, casing head and packer. In addition, the range of allowable annulus pressure has great significance in guiding for the design of casing string strength, choice of annulus protection fluid density in well drilling and completion design program.(5) Corrosion rate of different steel materials was evaluated under different conditions using weight loss method. Results showed that corrosion rate of J55, N80and P110steels reached the maximum value under the temperature90℃, and then decreased when the temperature increased. The corrosion rate of13Cr, S13Cr steels increased when the temperature increased. The corrosion rate of the five kinds of steels increased first and then decreased when the CO2partial pressure increased. The selectivity of ion penetration of amorphous Cr(OH)3in corrosion scale of steels that contain element Cr is the main reason for their better corrosion resistance.(6) According to load and corrosion environment, experiments were designed for rubber materials of packer under pressure-bearing state in CO2corrosion environment. Mechanical parameters such as hardness and compression deformation were tested before and after corrosion to evaluate the corrosion behaviour of rubber materials under simulated environment. The hardness reduced and the compression deformation increased after corrosion. Dissolution and plastic deformation were the main failure forms, and the damage to rubber after corrosion in liquid phase was larger than that in gaseous phase.(7) The conventional test method of cement corrosion was improved. Dynamic corrosion experiments were conducted for three different systems of cement. The corrosion mechanism of CO2to cement was analyzed and the corrosion resisting properties of different filling materials were evaluated.(8) Based on the well structure and annulus allowable pressure calculation, a safety assessing method was proposed. One gas wells was selected to compare the methods in API RP90and the one established in the paper. A set of recommendations were formulated aiming to management of sustained casing pressure in gas wells.The theoretical model of sustained casing pressure and pressure prediction method established in the paper had important theoretical value, which could be combined with the actual measured SCP data to evaluate the safety situation, and provide references for setting reasonable working parameters. The annulus allowable pressure calculation method and safety assessing method can be used for other gas wells in management of sustained casing pressure.