Analysis of fluid flow to horizontal and slant wells

Abstract

The applications of horizontal and slant wells in the oil industry have increased rapidly for the last decade after successful improvements in horizontal drilling technology. Compared with conventional vertical wells, horizontal or slant wells can significantly improve the per-well productivity because of the longer contact length of the well with the reservoir and a smaller pressure gradient. Horizontal wells are also more effective in controlling problems such as gas or water coning and solids production. Horizontal wells have also been found more effective in phase displacement in cold or hot injection assisted production.

Productivity from a horizontal or slant well is a function of many factors such as well length, orientation and elevation, reservoir thickness, anisotropy, porosity, compressibility, and boundary conditions, both lateral and vertical. To find the relationship between these factors and the productivity of a well is an important task in choosing the proper well pattern in reservoir exploitation. For this purpose, the focus of this thesis is on the analysis of fluid flow to horizontal and slant wells. A new more general solution is developed in this thesis for evaluating per-well productivity of a horizontal or slant well. Compared with the solutions available in the literature, the new solution considers an arbitrarily oriented slant well in a three dimensional anisotropic reservoir. By using the new solution, the direction along which a well obtains the optimum production can be determined.

As we know, reservoir parameters are the most important factors in well productivity prediction. There are several methods (transient solutions) available in the literature for reservoir parameter estimation. However, the parameters estimated from these solutions can deviate from the real field parameters because of poor assumptions, particularly in high anisotropic reservoirs. A new transient solution is developed considering an arbitrarily oriented well in an anisotropic reservoir, which is not considered in the solutions available. In the new solution, we have presented detailed analysis on the two characteristic times, the upper and lower impermeable boundary effect time tb and the well length effect time tw from a horizontal well. We found these two parameters to be a s important as the others such as permeability and storativity in characterizing a well-reservoir interaction model.

Wellbore storage and formation alteration are another two important aspects in a horizontal well test and productivity analysis. A brief analysis of the wellbore storage effect on horizontal well test interpretation is discussed. The storage effect on a horizontal well test interpretation is discussed. The storage effect on a horizontal well test is similar to that of a vertical well test, but can mask one or more flow regimes which can destroy possibilities to estimate the parameters which are characterized only by these regimes, such as the vertical permeability characterized in the early time flow regime.

Formation alteration effect has often been simulated by a skin term for both vertical and horizontal well tests. However, the skin term does not always work, particularly when a large size of the formation is altered (improved or damaged). In order to simulate the alteration effect in such situations, a new model is developed in this thesis. The new model considers a more realistic permeability variation model in the altered zone, a continuous variation model. The permeability in the new model is assumed to satisfy a power relationship with radial distance from the well. Experiments indicate that this permeability model can more closely model the field alteration than the step permeability model widely discussed in the literature. By using the new model, the alteration characteristics such as alteration degree and alternation size can be evaluated. Because of the more general and detailed analysis on fluid flow to a horizontal or slant well, we believe the conclusions derived from this research will be beneficial to the further applications of horizontal and slant wells in the petroleum and other industries.