Research On Back Pressure Control Technology Of Underbalanced Drilling Wellhead

During the course of drilling gas reservoir by the UBD (Unbalance drilling) technology, back pressure from wellhead is often created to make the unbalanced value constant, keeping the designed value. However, when a thick and most permeable zone is encountered, with exposure area of gas reservoir increasing, the amount of invading gas may exceed the bearing capacity of equipment, which may result in drilling problems. Thus the variable unbalanced value is needed to keep the amount of invading gas constant, which means to keep the gas invading rate constant. In order to ensure the safety of UBD site, it’s important to grasp the theory of wellhead back pressure control in UBD so as to control two-phase fluid type and the amount of invading gas to satisfy cuttings-carrying and well-control equipment. Based on liquid UBD for vertical well, the contents and conclusion of the paper are listed below:(1) Building a gas-liquid fluid steady-state equation, analyse each parameter characteristic of two-phase fluid in annular influenced by back pressure and gas invading rate, through numerical solution, and flow pattern in annular also is studied;(2) Building a gas-liquid fluid transient-state equation and dynamic-state gas invading rate calculation model, to simulate the course of gas-kicking, and using the parameters solved in the gas-kicking simulation as initial conditions to caculate the variation of wellhead back pressure with the exposure area of gas resevoir increasing during which the gas invading rate is constant, and comparing the results solved by simulation with the data collected from UBD site. The variation tendency of back pressure solved through simulation agrees with that collected from well-site. And it is found that back pressure variation tendency is sensitive to permeability, rate of penetration, gas invading rate and so on.(3) Writing software program in VB language to create UBD back pressure calculation and analysis soft, which is used to analyse the two-phase flowing law and pattern in annular and to simulate the gas-kicking course calculating the back pressure during the course of reservoir exposure area increasing, and during the course gas invading rate is constant.