Wellbore storage effects in the transient flow-testing of gas wells

The primary focus of this study is towards understanding better and being able to predict, a priori, from fundamental principles, the early-time transient-flow effects encountered in gas wells. Specifically, the initial pressure transient generated in the wellbore, the temperature and pressure transients in the well and the reservoir, and the wellbore storage and unloading effects are analysed by simulating the fluid flow behavior within the wellbore and coupling this with the flow from the surrounding reservoir formation. Thus, the three time frames involved are those for the sonic transients, the unloading and storage effects, and the approach to pseudo steady-state.; Emphasis is placed on two different approaches to tackling this problem, which thus comprise the two major parts of this dissertation.; In the first approach, gravitational, frictional, compressibility, variable gas property effects, turbulence, and heat-transfer aspects are all taken into account. The method of characteristics is used in conjunction with a finite-difference spatial discretization within the well; and finite-element procedures are employed in the reservoir to converge simultaneously for the pressure, temperature and velocity of the gas at each progressive time step. Care is taken to ensure numerical stability and accuracy throughout.; The second approach taken is in developing a simplified well model, which replaces the detailed well simulator, and coupling it to the reservoir simulator to make the prediction of the wellbore storage duration more easily obtainable. Inertial and frictional effects are shown to be insignificant compared to the capacitance of the well, which is found to be the dominating effect controlling the duration of well-storage.; Comparison of both the developed models with field data show excellent agreement. In particular, the durations of the transient effects as well as measureable temperature and pressure profiles compare rather well with the field test. The relative merits of the simple model are explored, with the conclusion that it is, in fact, a simplistic yet accurate approach to the prediction of the wellbore storage duration. The analysis and practicality of this approach make it a very convenient and useful tool for the field engineer.